[Following up on a thread dating back to January, similar to one I
started recently. Responding to Patrick Turner's comments.]


Patrick Turner wrote in January 2004:
Jerry Wang wrote:

1. Even it is a single channel [AM] receiver, I would still suggest
the use of one or two intermediate frequency (IF) stages. Because
to achieve good sensitivity you need to have enough gain.

Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely.

Interesting.

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency. There'd be a switch to select from a number of channels,
each associated with a specific frequency the user wants to listen to
(suggesting a plugin mini-board for each channel, but there are other
possible configurations.)

I infer from what Patrick said that it is unnecessary for a single
frequency AM tuner to be a super-het design, and that (I assume) a
much simpler two RF amp TRF design is sufficient for good to excellent
audio quality and good to excellent sensitivity and selectivity. (John
Byrns implies the same in his various comments on TRF AM tuners.)

So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

Obviously, architecturally implementing this in a practical AM tuner
design is not trivial (we do benefit by throwing away the multigang
air capacitor.) However, several ideas suggest themselves. For
example, we can imagine having multiple plugin slots, where we plug
into each slot a PCB mini-board specific to a particular frequency.
The board will contain the few components whose values *independently*
change as a function of frequency. They probably will have trimmers
for fine calibration of the center frequency and other bandpass filter
characteristics. We may need multiple mini-boards for each channel
(one for each tuning stage) if necessary for shielding purposes (to
prevent oscillation by stage-to-stage interference if that is a
problem.) And if higher frequency channel boards require some minor
changes in the circuitry configuration, and not just component value
changes, that can easily be done, too. In principle, this tuner might
even be able to extend a little beyond (on both sides) the 500-1800
khz MW band -- just plugin the right mini-board circuitry for the
frequency desired.

Of course, others here will probably have much better ideas as to
how to implement the channel approach.

Thoughts? Comments? Criticisms?

Jon Noring


(It's interesting to think of doing the same "channel" approach for
an FM tube tuner. Will that also confer several advantages in
simplifying the circuit design for the same overall performance
level?)

Jon Noring wrote:

[Following up on a thread dating back to January, similar to one I
started recently. Responding to Patrick Turner's comments.]

Patrick Turner wrote in January 2004:
Jerry Wang wrote:

1. Even it is a single channel [AM] receiver, I would still suggest
the use of one or two intermediate frequency (IF) stages. Because
to achieve good sensitivity you need to have enough gain.

Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely.

Interesting.

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency. There'd be a switch to select from a number of channels,
each associated with a specific frequency the user wants to listen to
(suggesting a plugin mini-board for each channel, but there are other
possible configurations.)

The problem is that if you want a channel at 9 kHz intervals to
choose from across the band, you need around 12 perfectly set up
tuning circuits all with multiple LC circuits.
Then you need sitable switching.
Far better is to forget all that BS and use a PC to decode the antenna
signal.



I infer from what Patrick said that it is unnecessary for a single
frequency AM tuner to be a super-het design, and that (I assume) a
much simpler two RF amp TRF design is sufficient for good to excellent
audio quality and good to excellent sensitivity and selectivity. (John
Byrns implies the same in his various comments on TRF AM tuners.)

But you won't sell many kits set up optimally for just one F.
As soon as the owner moves to another area, the radio becomes useless.



So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

This has al been investigated before, and the conclusions were about as
simple
as possible by about 1927.
Try studying basic L,C, & R theory, and work all this out for yourself.

I once fixed a 1932 TRF Radiola with only two single tuned circuits.
It gave OK local reception with about 5k of audio BW
where the stations were 100 kHz or more apart.
It used the then high tech new fangled type 22 tetrode.



[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

1925 TRFs had 3 or 4 separate tuning gangs, each set to a certain numbered
position
for reception of a given station. Finding stations was exciting.
Try studying the history of radio, and you won't need to ask such
questions here.



Obviously, architecturally implementing this in a practical AM tuner
design is not trivial (we do benefit by throwing away the multigang
air capacitor.) However, several ideas suggest themselves.

The 1932 Radiola did have its two single capacitor gangs connected by
cables, which had corroded,
so I used builder's line. It worked OK.

For
example, we can imagine having multiple plugin slots, where we plug
into each slot a PCB mini-board specific to a particular frequency.

?


The board will contain the few components whose values *independently*
change as a function of frequency. They probably will have trimmers
for fine calibration of the center frequency and other bandpass filter
characteristics. We may need multiple mini-boards for each channel
(one for each tuning stage) if necessary for shielding purposes (to
prevent oscillation by stage-to-stage interference if that is a
problem.) And if higher frequency channel boards require some minor
changes in the circuitry configuration, and not just component value
changes, that can easily be done, too. In principle, this tuner might
even be able to extend a little beyond (on both sides) the 500-1800
khz MW band -- just plugin the right mini-board circuitry for the
frequency desired.

This idea is totally impractical for 120 different stations,
and plug ins get lost or broken, or worn out.



Of course, others here will probably have much better ideas as to
how to implement the channel approach.

You bet there are, and only possible with chip technology,
with press button station selection, and digital station F read out, with
digitally generated oscillator frequency
for the F converter of a superhet, with ceramic filter IF.
Grundig have been multiband radios for about 20 years +.
Not a tube in sight inh these lightweight plastic radios bought cheaply
by the masses to allow connection to the world's AM, FM, and HF bands,
and even amateur SSB stations.

But how to improve such designs to make wider AF BW is unknown to me.

Try examining the history of Yeasu.



Thoughts? Comments? Criticisms?

Jon Noring

(It's interesting to think of doing the same "channel" approach for
an FM tube tuner. Will that also confer several advantages in
simplifying the circuit design for the same overall performance
level?)

Study the way most post 1980 AM/FM tuners are constructed.
Tubes cannot be used with such methods.

I reckon you got a pile of reading to do.

Patrick Turner.

(assorted snips because these threads get sooo long!) see reply below

Jon Noring wrote:

[Following up on a thread dating back to January, similar to one I
started recently. Responding to Patrick Turner's comments.]


Patrick Turner wrote in January 2004:

Jerry Wang wrote:


1. Even it is a single channel [AM] receiver, I would still suggest
the use of one or two intermediate frequency (IF) stages. Because
to achieve good sensitivity you need to have enough gain.


Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely.


Interesting.

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. T
So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope

to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

Obviously, architecturally implementing this in a practical AM tuner
design is not trivial (we do benefit by throwing away the multigang
air capacitor.) However, several ideas suggest themselves. For
example, we can imagine having multiple plugin slots, where we plug
into each slot a PCB mini-board specific to a particular frequency.
Of course, others here will probably have much better ideas as to
how to implement the channel approach.

Thoughts? Comments? Criticisms?

Jon Noring

For a one-channel receiver it makes perfect sense. Beyond that any
advantage is lost.
Why would I say that? You can create a perfectly acceptable single IF
filter with not so much ado. Lets use 455kc as the example. Its
considerably easier to build a single 'custom' IF filter at 455kc to do
what you want to do than it is a bunch of modules at three or four times
that frequency.
Yes, you could do as you suggest but I see no advantage in doing so. It
would be more critical, more expensive and probably not yield as good a
result as a nice 455 filter.

One thing I haven't heard mentioned, and admittedly I have only been
grazing what has been a very windy thread, why not use a WIDE 455kc IF
with tunable traps on either side? You can get a very steep skirt on a
good hi-q trap...likely steeper than in a transformer configuration that
is inherently q-disadvantaged. This would come in handy at night when
dozens of adjacent channels stations will be struggling to find their
way into your wide bandpass - and this scenario alone is a huge negative
about any wideband scheme that needs to be addressed.

A savvy person might be able to 'gang' the two adjacent channel traps
for a single knob "bandwidth" control.

My apologies if I'm missing the point. I'm unsure if the motive of the
discussion is that of a wideband AM radio or a discussion of ways to
reinvent the wheel.


-Bill M

Patrick Turner wrote:
Jon Noring wrote:

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency. There'd be a switch to select from a number of channels,
each associated with a specific frequency the user wants to listen to
(suggesting a plugin mini-board for each channel, but there are other
possible configurations.)

The problem is that if you want a channel at 9 kHz intervals to
choose from across the band, you need around 12 perfectly set up
tuning circuits all with multiple LC circuits.
Then you need sitable switching. Far better is to forget all that BS
and use a PC to decode the antenna signal.

Well, if you recall, I did agree with you that the ultimate AM tuner
will be all PC-based DSP as close to the antenna feed as possible,
along with a true Class D digital amp for final output to the
speakers. Everything inbetween will be only real-time digital signal
processing. No need to sell me on that!

So why are we even bothering talking about tube-based equipment?
smile/

Because there is definitely an interest in tube-based equipment, for
various reasons: nostalgia, the challenge, the aesthetics, and in
some cases (such as high end audiophile amplifiers), The Sound (tm).

When true Class D amplifiers mature, they will supplant tube amps for
pure sonic quality. But that's still a few years off until PWM
switching improves.)

And even then, tube equipment is definitely of interest for
aesthetic and nostalgic reasons.

Regarding the channel TRF receiver being "BS", well that's in the
eye of the beholder. smile/


I infer from what Patrick said that it is unnecessary for a single
frequency AM tuner to be a super-het design, and that (I assume) a
much simpler two RF amp TRF design is sufficient for good to excellent
audio quality and good to excellent sensitivity and selectivity. (John
Byrns implies the same in his various comments on TRF AM tuners.)

But you won't sell many kits set up optimally for just one F.
As soon as the owner moves to another area, the radio becomes useless.

If the owner installed a number of "mini-boards" (or whatever) to
receive stations (both local and DX), then moves, he simply either
swaps mini-boards with new ones, or keeps the ones he has and adds new
ones, so now he has even more channels to "surf". The boards don't
become useless at all, especially if they're interested in casual DX.

The mini-boards can be sold either as kit boards (just add the
components of the right value, calibrate and plug-in) or buy them
already made and calibrated from the kit supplier. For simpler
bandpass tuning filters (not the complex ones like nine order
Chebychev, as an extreme example), the mini-board may only have a few
simple components to add. For example, for a given center frequency
(check the chart) just add a capacitor here of a certain value, a
resistor there of a certain value, an inductor over there of a certain
value, etc. Not a big deal. I envision the mini-boards to maybe be as
small as 1" x 2" in size, more like a stick, with terminals on the
narrow end to plug into a slot connected to the main circuitry of the
tuner (hopefully none of the components will be very large -- thus
the question I asked you about making the critically coupled RF
transformers common to all channels -- we don't want to have any more
than two or three of them!)


So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

This has al been investigated before, and the conclusions were about
as simple as possible by about 1927. Try studying basic L,C, & R
theory, and work all this out for yourself.

I have. This channel concept has nothing to do with "basics". It is a
twist to TRF tuner architecture taking advantage of the fact that AM
BCB is done in specific assigned frequencies, just like FM, like TV,
like the CB band, etc. It will not be practical for general shortwave
listening since that is a huge band (from 1.8 mhz to 30.0 mhz) and
amateurs in particular pick their own frequencies (and over time even
commercial SW broadcasts move around a lot, for those only interested
in listening to the majors like Radio Australia, as I do many evenings
on 15.515 mhz. It comes in loud and clear here in Salt Lake City.)

Back in the late 20's and early 30's, on MW there was clearly a need
for continuous tuning since broadcasts could be anywhere on the band.
(And tubes then had poor gain, among other problems.)

Today, a lot of the issues of building TRF circuitry is trying to
overcome the limitations of one-dimensional tuning using, for example,
a multigang air capacitor -- John Byrns is going through agony trying
to find the magic formula to get what he wants with a multigang air
capacitor. But with the channel TRF concept, the sky's the limit as to
how many components in the bandpass tuning filter can be independently
selected and hardwired for any given frequency. So one can optimally
tune the bandpass characteristics for each and every frequency in the
TRF without worrying how that affects other frequencies, since each
channel frequency tuning circuit is now effectively decoupled (made
independent) from the other channel frequencies.


[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

1925 TRFs had 3 or 4 separate tuning gangs, each set to a certain
numbered position for reception of a given station. Finding stations
was exciting. Try studying the history of radio, and you won't need
to ask such questions here.

With the channel TRF concept, the component values of the bandpass
filter (or parts of the filter circuit) are hardwired on the channel
plug-in board (and trimmed during calibration), so all the person has
to do in listening to the tuner is switch to the channel, and the
radio will be in tune to the desired frequency, with the optimal
bandpass characteristics for that frequency. (There is likely to be a
need for a very fine tuning control, maybe +/- 1 khz, to handle slight
drift, both for tuner warmup, and for the inevitable long-term
drifting of component values.)

I suppose back in 1925 radio stations where in all sorts of weird
locations on the dial, and constantly moving around, so hardwiring
all the tuning components for a particular frequency, and likewise
for other frequencies, was not even an option.


For example, we can imagine having multiple plugin slots, where we
plug into each slot a PCB mini-board specific to a particular
frequency.

?

You probably understand the channel TRF concept, but did not
understand what I wrote the above, so let me restate with an example:

I want my channel TRF tuner to tune in 830 khz (WCCO in Minneapolis),
so I get the mini-board for that frequency already hardwired with the
optimum configuration of the various tuner components, plug it in, and
then listen to that frequency whenever I switch to whatever channel
slot I placed that mini-board in (I am reminded of how components are
plugged into PCs, such as via PCI slots.)

Or, I buy the blank mini-board, check the kit-supplied chart for 830
khz, and then solder in a 50 ohm resistor in this spot, a 200pF
capacitor in that spot, etc. -- probably will take me all of five
minutes. Then calibrate it by tweaking the trimmers. If I instead want
the mini-board to tune 1160 khz (KSL in Salt Lake City), I check the
chart, put in a 75 ohm resistor in this spot, a 150pF capacitor in
that spot, etc. (whatever values are called for.) Then calibrate it.
Plug it in, listen to 1160 khz, knowing that the TRF bandpass tuning
circuitry is now optimized for that frequency, and much better
optimized than could ever be done with the one dimensional limits of a
multigang tuning capacitor.

For those who build tube kits, this will border on the trivial.


And some hobbyists may find the channel TRF AM tube tuner architecture
of real interest, since now they can more easily experiment with new
higher-order bandpass filters of various mathematical functions to see
how they affect TRF performance. This could lead to a revised
mini-board to be issued at some future time based on all this
research,
and the channel tuner owner can, if they so choose, simply buy or
build updated boards for the broadcast stations of interest, and
instantly get better performance. It's possible to mix bandpass
filters for different stations: a third order Butterworth for 1160
khz, and a fifth order Chebychev for 830 khz. The possibilities are
endless.


The board will contain the few components whose values *independently*
change as a function of frequency. They probably will have trimmers
for fine calibration of the center frequency and other bandpass filter
characteristics. We may need multiple mini-boards for each channel
(one for each tuning stage) if necessary for shielding purposes (to
prevent oscillation by stage-to-stage interference if that is a
problem.) And if higher frequency channel boards require some minor
changes in the circuitry configuration, and not just component value
changes, that can easily be done, too. In principle, this tuner might
even be able to extend a little beyond (on both sides) the 500-1800
khz MW band -- just plugin the right mini-board circuitry for the
frequency desired.

This idea is totally impractical for 120 different stations,
and plug ins get lost or broken, or worn out.

I don't believe it is impractical for 120 different stations, for two
reasons:

1) Those tube-o-philes who only want to listen to stronger local
stations, or to particular distant ones, are likely only to want to
have 10-20 stations (with the ability to add more if they want.)

One purpose of picking TRF is its legendary high-fidelity audio
capability which will appeal to audiophiles -- most won't want to
listen to a very weak station 1000 miles away that can only be
picked up some evenings.

And I believe it is easier to sell tube-o-philes on the Channel TRF
concept once it is explained how it maximizes audio performance for
each and every broadcast frequency that cannot be done with a
continuously tuned TRF.

2) Those who would use this for casual DXing (and note the hardcore
MW DXers will use something like a Drake R8B or ICOM R75, or some
digital receiver) will certainly be motivated to add more
mini-boards, and can do so over time. The tuner will work with 1
channel board, or with all 130+ (if enough slots are provided. For
the moment I am imagining the mini-board approach, but the sky's
the limit for other ideas to implement the channel TRF AM tube
tuner.)

3) And as noted above, hobbyists may find the "plugin" bandpass filter
capability of particular interest.


Of course, others here will probably have much better ideas as to
how to implement the channel approach.

You bet there are, and only possible with chip technology,
with press button station selection, and digital station F read out,
with digitally generated oscillator frequency for the F converter of
a superhet, with ceramic filter IF. Grundig have been multiband
radios for about 20 years +.

I have a Radio Shack DX-399 (the Sangean 606A) which is a very good
performer for casual MW (with the Radio Shack MW loop) and shortwave
DXing. So I am very familiar with that hobby, and with the benefits
digital systems bring to tuners. You need not sell me on that!

See my previous note above on "why tubes then?"


Not a tube in sight inh these lightweight plastic radios bought
cheaply by the masses to allow connection to the world's AM, FM, and
HF bands, and even amateur SSB stations.

See my previous note above on "why tubes then?"


(It's interesting to think of doing the same "channel" approach for
an FM tube tuner. Will that also confer several advantages in
simplifying the circuit design for the same overall performance
level?)

Study the way most post 1980 AM/FM tuners are constructed.
Tubes cannot be used with such methods.

O.k. But are you referring to tube-based tuners? Again, if all I
wanted was an audiophile grade AM/FM tuner, and did not care about
what was under the hood, I'd be open to solid state designs, but I'm
specifically looking at tube-based tuners.

I still assume that the channel approach to tube-based FM tuner design
may confer some benefits, but maybe less since the frequency ratio to
tune from the lower to the upper ends of the band (about 1.25) is much
less than that for the AM BCB (a whopping 3.5 or so.) And there are
probably other factors as well specific to frequency modulation.


It's trying to tune the AM band with only one degree of freedom (e.g.,
air tuning capacitor) which is causing all the hassle in tube-based
TRF AM tuner circuit design. One would want to pick a bandpass filter
which is optimally tuned to the specific frequency we want to listen
to, and this involves optimally selecting *several* component values,
not just one as we are limited to by continuous tuning with a
multigang tuning capacitor. The channel TRF approach appears to free
up the TRF designer from the tyranny of having to compromise the
bandpass characteristics over the entire tuning range which only one
degree of freedom allows.

Of course, superheterodyne is one solution to the TRF problem, and
allows for continuous tuning. Note that super-het works because it
uses "one channel" (the IF). So in a sense, superheterodyne supports
the channel TRF approach for those who don't want to build a
super-het, but rather want a pure TRF receiver (e.g., for sound
quality reasons, or whatever.)

*****

Now, I've made the call several times for classic and proven AM tube
tuner designs of the past which have excellent audio quality (and wide
bandwidth capability), are good for casual DX use, and can easily be
"modernized" for a kit. There are no doubt many excellent super-het
designs out there, but I've had very few recommendations. Patrick,
since you appear to much prefer super-het over TRF for AM tube
tuners, which classic super-het tube AM radio designs of the past
would you suggest as candidates to consider?

Anybody?


I reckon you got a pile of reading to do.

Yes, I have been reading. That one-year equivalent of EE training back
in 1974 at the University of Minnesota is slowly coming back to me.
Back then we spent a few weeks on tubes, and only a couple days with
transistors. Things have changed a lot since then.

And it was interesting reading about Chebychev bandpass filters today
since I wrote a lot of Fortran code years ago to do various types of
numerical analytic processing including integration using quadrature
with orthogonal polynomials (mostly Legendre polynomials.) It was
especially cool to see how the higher order Chebychev polynomials U(x)
plot out in the desired shape (well approximately) for a bandpass
filter (but with that slight ripple within the bandwidth.) I'm not
saying all this to brag, but to give a better idea of my background.

Definitely I have a lot to learn, of course, and your posts are
helping me to better understand things. I still believe the channel
TRF concept is viable for those who want to build the best possible
TRF tube tuners where for each frequency the absolute best bandpass
characteristics can be chosen without worrying about how it impacts
the other frequencies since each channel is now largely independent.

The obvious downsides with the tube-based channel TRF concept a

1) The practical, real-world implementation of it (I believe it is
doable, I suggest one approach),

2) Losing the ability to continuously tune, which for BCB is not an
issue as I've noted several times, and

3) Calibration of each mini-board if done by the kit-builder (I think
this is solvable, but it is an issue to consider.)

The upsides are several, as previously noted.


Thanks for your helpful comments!

Jon Noring

Bill (exray) wrote:
Jon Noring wrote:

Nice to hear from you again, Bill! I'm still in the process of
restoring the Philco 37-670 console, and will need your advice on a
couple of issues, such as how to replace the rubber insulators on
the RF chassis and on the back end of the tuning capacitor, which
are all disintegrating due to the radio being exposed to the LA
smog for decades.


For a one-channel receiver it makes perfect sense. Beyond that any
advantage is lost.

Why would I say that? You can create a perfectly acceptable single
IF filter with not so much ado. Lets use 455kc as the example. It's
considerably easier to build a single 'custom' IF filter at 455kc to
do what you want to do than it is a bunch of modules at three or
four times that frequency. Yes, you could do as you suggest but I
see no advantage in doing so. It would be more critical, more
expensive and probably not yield as good a result as a nice 455
filter.

I think the ultimate explanation is the desire for the tube tuner to
remain a pure TRF design, for audio quality purposes -- John Byrns
has discussed this as well (yes, we've hammered to death the poor
quality of most AM broadcasts, but that's been covered elsewhere.)

As soon as one decides the tube tuner is to be a pure TRF, then one is
instantly confronted with the very difficult problem in how to get
optimal bandpass characteristics for all the frequencies from 500khz
to 1800khz. As I read the many messages on this from the Google
archive, it clearly borders on a nightmare to overcome when the only
degree of freedom the TRF designer has to work with is a variable
air capacitor. John Byrns is wrestling with this issue even as I
write, trying to find the magic formula.

When confronted with an intractable problem in design, it is time to
think outside the box. It is obvious we need to have more degrees of
freedom in tuning, but for continuous tuning all this does is add more
knobs to tweak, not unlike the TRF designs of the 1920's. Do we want
to go in that direction?

But since we observe the stations on the BCB are restricted to
specific frequencies, this means we don't *need* to have continuous
tuning, and from this paradigm shift the channel TRF idea springs
forth.

As I noted in a parallel message I just sent out, the channel TRF has
its problems for practical implementation, and it goes against the
almost 100 year paradigm of continuous tuning that is so ingrained in
BCB radio tuner design, but I think it solves that otherwise
intractable problem with TRF tube tuner design. But, if John Byrns
or someone else can discover the magic way to allow one degree of
freedom to give optimal enough bandpass design for a TRF tube tuner,
then that's the direction I'd recommend going, and not the channel
TRF approach, interesting as it is. (Of course, understandably many
still recommend super-het.)

Jon

Jon Noring wrote:

Bill (exray) wrote:

Jon Noring wrote:


Nice to hear from you again, Bill!

Indeed, hi again.

I think the ultimate explanation is the desire for the tube tuner to
remain a pure TRF design, for audio quality purposes -- John Byrns
has discussed this as well

Firstly, I'm not getting the WHY this (TRF idea) is of such great
import. Conceptually its a nice idea to not add 'unnecessary' stages
but if one harkens back to why this was (and still is) the panacea to
overcome the TRF ills then maybe they shouldn't be categorically
discarded as bad things.

As soon as one decides the tube tuner is to be a pure TRF, then one is
instantly confronted with the very difficult problem in how to get
optimal bandpass characteristics for all the frequencies from 500khz
to 1800khz. As I read the many messages on this from the Google
archive, it clearly borders on a nightmare to overcome when the only
degree of freedom the TRF designer has to work with is a variable
air capacitor. John Byrns is wrestling with this issue even as I
write, trying to find the magic formula.

I'm of the mind that going pure TRF is not necessarily the answer to
your original request. But we can run with that for the sake of
discussion. There may well be some magic combination of ganging
inductors and caps but upon finding that we'll still have to weigh in
the cost, complexity, repeatability, performance, etc compared to a
superhet. Radio folk haven't reached that point yet in 80 odd years so
there's no disagreement to be found :-) And don't assume that radio
minds are in a 'box'. The crystal radio fanatics beat this issue to
death on a daily at a very sophisticated level.

When confronted with an intractable problem in design, it is time to
think outside the box. It is obvious we need to have more degrees of
freedom in tuning, but for continuous tuning all this does is add more
knobs to tweak, not unlike the TRF designs of the 1920's. Do we want
to go in that direction?

I don't...at least not for the purpose of hooking up something to my
home stereo for e-z audiophile listening.

But since we observe the stations on the BCB are restricted to
specific frequencies, this means we don't *need* to have continuous
tuning, and from this paradigm shift the channel TRF idea springs
forth.

I disagree 180 degrees. If BCB channels could be counted on as
equivalent building blocks maybe this would apply but we are talking
three octaves of frequency range.

As I noted in a parallel message I just sent out, the channel TRF has
its problems for practical implementation, and it goes against the
almost 100 year paradigm of continuous tuning that is so ingrained in
BCB radio tuner design, but I think it solves that otherwise
intractable problem with TRF tube tuner design. But, if John Byrns
or someone else can discover the magic way to allow one degree of
freedom to give optimal enough bandpass design for a TRF tube tuner,
then that's the direction I'd recommend going, and not the channel
TRF approach, interesting as it is. (Of course, understandably many
still recommend super-het.)

I fully understand what you are suggesting and all I can say is that
we've been there and done that. When I stated that you could build a
nice hi-q BCB circuit that would yield 3kc bandwidth at 550 and 25 kc
width at 1600 I wasn't exaggerating. Intuitively one might think that
hey, I'll twist the LC combo somehow and come back to the same Q across
the band simply doesn't work...either in numbers or worse still in
practice. I'd like to say you can't obtain a sharp 3kc bandwidth at
1600 with a simple LC circuit but thats too open-ended. Suffice it to
say that it ain't easy.

One can visualize some scenarios of mechanical (or electrical) ganging
of components that might approach this goal but that visualization
typically falls in the ditch once one tries to transfer the idea from
the brain to an actual breadboarded version of the concept.

Going back to some of the earlier filter flatness discussion, well toss
that idea into the mix when you think in terms of TRF. Not only do you
want to achieve a specific width but you want it to be flat. My 3/25 kc
TRF scenario isn't flat at all. Its a big peak that just broadens out.
When we say a 'bandwidth'number we are relating to something specific
like 3 or 6 db down from the peak. Its still a peak in this context.
So whats happening at 20 db down? You guessed it, that 25kc number is
150 kc wide. I dunno how you could control the width AND the flatness
AND the skirts.

I'm a fairly recent convert to crystal radios. For the sake of
discussion there's little difference in xtal technology vs trf
technology in that both are non-superhet. I get absolutely glorious
quality audio from my xtal set when fed thru an amp. With 6 or 8 knobs
on the front panel and top notch components I can find a dead spot
between semi-locals on 680 and 690. With a local station on 1370 it
takes traps and VERY hi-q stuff to ferret out semi-locals on 1240, 1290
and 1480. Its as if it were a totally different radio from one end of
the band to the other and this has been the plague of TRF circuits since
day one. If I didn't have the local 1370 I could safely say, hey Jon,
this is the ticket, but there's scarce few of us who don't have a strong
undesired local station to bollox up the works.

Go superhet, my man.

-Bill M

Jon Noring wrote:

Patrick Turner wrote:
Jon Noring wrote:

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency. There'd be a switch to select from a number of channels,
each associated with a specific frequency the user wants to listen to
(suggesting a plugin mini-board for each channel, but there are other
possible configurations.)

The problem is that if you want a channel at 9 kHz intervals to
choose from across the band, you need around 12 perfectly set up
tuning circuits all with multiple LC circuits.
Then you need sitable switching. Far better is to forget all that BS
and use a PC to decode the antenna signal.

Well, if you recall, I did agree with you that the ultimate AM tuner
will be all PC-based DSP as close to the antenna feed as possible,
along with a true Class D digital amp for final output to the
speakers. Everything inbetween will be only real-time digital signal
processing. No need to sell me on that!

So why are we even bothering talking about tube-based equipment?
smile/

Well you are the one wanting an avaliable kit which had everything,
including an ability to glow in the dark ;-)

I already got my answer in my kitchen.

WTF are all you other keen dudes gonna do about getting good AM to listen to?

If I can do it, so can you.



Because there is definitely an interest in tube-based equipment, for
various reasons: nostalgia, the challenge, the aesthetics, and in
some cases (such as high end audiophile amplifiers), The Sound (tm).

Tubes do sound the best when they are good, imho.

They are lousy devices for computers.



When true Class D amplifiers mature, they will supplant tube amps for
pure sonic quality. But that's still a few years off until PWM
switching improves.)

I will believe it when I see it.


And even then, tube equipment is definitely of interest for
aesthetic and nostalgic reasons.

Regarding the channel TRF receiver being "BS", well that's in the
eye of the beholder. smile/

Bold Scheme, perhaps, maybe even Naughty Electronics Endeavours, or NEL.




I infer from what Patrick said that it is unnecessary for a single
frequency AM tuner to be a super-het design, and that (I assume) a
much simpler two RF amp TRF design is sufficient for good to excellent
audio quality and good to excellent sensitivity and selectivity. (John
Byrns implies the same in his various comments on TRF AM tuners.)

But you won't sell many kits set up optimally for just one F.
As soon as the owner moves to another area, the radio becomes useless.

If the owner installed a number of "mini-boards" (or whatever) to
receive stations (both local and DX), then moves, he simply either
swaps mini-boards with new ones, or keeps the ones he has and adds new
ones, so now he has even more channels to "surf". The boards don't
become useless at all, especially if they're interested in casual DX.

I doubt your idea would ever catch on........



The mini-boards can be sold either as kit boards (just add the
components of the right value, calibrate and plug-in) or buy them
already made and calibrated from the kit supplier. For simpler
bandpass tuning filters (not the complex ones like nine order
Chebychev, as an extreme example), the mini-board may only have a few
simple components to add. For example, for a given center frequency
(check the chart) just add a capacitor here of a certain value, a
resistor there of a certain value, an inductor over there of a certain
value, etc. Not a big deal. I envision the mini-boards to maybe be as
small as 1" x 2" in size, more like a stick, with terminals on the
narrow end to plug into a slot connected to the main circuitry of the
tuner (hopefully none of the components will be very large -- thus
the question I asked you about making the critically coupled RF
transformers common to all channels -- we don't want to have any more
than two or three of them!)

Your'e dreamin.....



So, with respect to the channel approach, the next question to ask is
if we can use the same two critically coupled RF transformers (as
Patrick notes), and *independently* vary several of the other smaller
components (e.g., capacitors, resistors, and even inductors) in the
two or three tuning stages (if we include the antenna tuner) so as to
maintain, from channel to channel in the BCB, reasonably optimal
bandwidth and other desirable tuning characteristics?

This has al been investigated before, and the conclusions were about
as simple as possible by about 1927. Try studying basic L,C, & R
theory, and work all this out for yourself.

I have. This channel concept has nothing to do with "basics". It is a
twist to TRF tuner architecture taking advantage of the fact that AM
BCB is done in specific assigned frequencies, just like FM, like TV,
like the CB band, etc. It will not be practical for general shortwave
listening since that is a huge band (from 1.8 mhz to 30.0 mhz) and
amateurs in particular pick their own frequencies (and over time even
commercial SW broadcasts move around a lot, for those only interested
in listening to the majors like Radio Australia, as I do many evenings
on 15.515 mhz. It comes in loud and clear here in Salt Lake City.)

Ah well, long ago I gave up backing ideas in which basics meant SFA.



Back in the late 20's and early 30's, on MW there was clearly a need
for continuous tuning since broadcasts could be anywhere on the band.
(And tubes then had poor gain, among other problems.)

The no 22 had plenty of gain, gm was around 1 mA/v at least,
and plenty by 1930.

Continous tuning kept radios affordable.
Implementing your scheme, whatever it may be, would have never caught on in
1935.



Today, a lot of the issues of building TRF circuitry is trying to
overcome the limitations of one-dimensional tuning using, for example,
a multigang air capacitor -- John Byrns is going through agony trying
to find the magic formula to get what he wants with a multigang air
capacitor. But with the channel TRF concept, the sky's the limit as to
how many components in the bandpass tuning filter can be independently
selected and hardwired for any given frequency. So one can optimally
tune the bandpass characteristics for each and every frequency in the
TRF without worrying how that affects other frequencies, since each
channel frequency tuning circuit is now effectively decoupled (made
independent) from the other channel frequencies.

You just need 120 optimised sets of tuning circuits...

An electronic 120 position switch should be a doddle.

You won't get anyone to finance your endeavour, or pay the patent fees.




[With traditional continuous tuning, achieved with multiganged air
capacitors, we do indeed vary a few capacitors in the tuning
circuitry, but because all of them track each other, in reality we
only have one degree of freedom, leading to circuit design constraints
for continuous "single knob" tuning. Now imagine, for each channel
frequency, to *independently* vary the value of several components at
the same time -- we now have several degrees of freedom to play with
and thereby hope to achieve reasonably constant (as a function of
frequency) bandpass characteristics.

1925 TRFs had 3 or 4 separate tuning gangs, each set to a certain
numbered position for reception of a given station. Finding stations
was exciting. Try studying the history of radio, and you won't need
to ask such questions here.

With the channel TRF concept, the component values of the bandpass
filter (or parts of the filter circuit) are hardwired on the channel
plug-in board (and trimmed during calibration), so all the person has
to do in listening to the tuner is switch to the channel, and the
radio will be in tune to the desired frequency, with the optimal
bandpass characteristics for that frequency. (There is likely to be a
need for a very fine tuning control, maybe +/- 1 khz, to handle slight
drift, both for tuner warmup, and for the inevitable long-term
drifting of component values.)

I suppose back in 1925 radio stations where in all sorts of weird
locations on the dial, and constantly moving around, so hardwiring
all the tuning components for a particular frequency, and likewise
for other frequencies, was not even an option.

That was an age where nations and states on the same continent
built railways mainly with different guages.
It was a natural for man to fight man, and millions were slaughtered
in 20th century wars, and having selectable and agreed radio station Fs
wasn't ever going to prevent all that stupidity.
Billions were wasted keeping lawyers fabulously wealthy.
Many arguments were over radio ideas and patents.

But having channels spaced at 9 or 10 kHz hasn't revolutionised
receivers. press auto tune on many, and they just go searching for what's
there,
and lock ono it, and no drift, and no tubes, just rotten fidelity.



For example, we can imagine having multiple plugin slots, where we
plug into each slot a PCB mini-board specific to a particular
frequency.

?

You probably understand the channel TRF concept, but did not
understand what I wrote the above, so let me restate with an example:

I want my channel TRF tuner to tune in 830 khz (WCCO in Minneapolis),
so I get the mini-board for that frequency already hardwired with the
optimum configuration of the various tuner components, plug it in, and
then listen to that frequency whenever I switch to whatever channel
slot I placed that mini-board in (I am reminded of how components are
plugged into PCs, such as via PCI slots.)

It would peave me if I had to buy seperate plug ins for each station,
and peave me greatly if i had to find the darn plug in after the dog or child
ran off with it,
or buy another after treading on one.
The plug in wears out.
Its ok for plug in coils like in a HRO, for a full band, but
not for one station F.



Or, I buy the blank mini-board, check the kit-supplied chart for 830
khz, and then solder in a 50 ohm resistor in this spot, a 200pF
capacitor in that spot, etc. -- probably will take me all of five
minutes.

Ye are hopeful; such farnarcling around, such skyborne dreams.....

Then calibrate it by tweaking the trimmers.

AHHHHHHH.....

If I instead want
the mini-board to tune 1160 khz (KSL in Salt Lake City), I check the
chart, put in a 75 ohm resistor in this spot, a 150pF capacitor in
that spot, etc. (whatever values are called for.) Then calibrate it.
Plug it in, listen to 1160 khz, knowing that the TRF bandpass tuning
circuitry is now optimized for that frequency, and much better
optimized than could ever be done with the one dimensional limits of a
multigang tuning capacitor.

For those who build tube kits, this will border on the trivial.

Where is my Smith and Western?

I need to put a chronic dreamer out of his misery....



And some hobbyists may find the channel TRF AM tube tuner architecture
of real interest, since now they can more easily experiment with new
higher-order bandpass filters of various mathematical functions to see
how they affect TRF performance. This could lead to a revised
mini-board to be issued at some future time based on all this
research,
and the channel tuner owner can, if they so choose, simply buy or
build updated boards for the broadcast stations of interest, and
instantly get better performance. It's possible to mix bandpass
filters for different stations: a third order Butterworth for 1160
khz, and a fifth order Chebychev for 830 khz. The possibilities are
endless.

The possibilities will end.



The board will contain the few components whose values *independently*
change as a function of frequency. They probably will have trimmers
for fine calibration of the center frequency and other bandpass filter
characteristics. We may need multiple mini-boards for each channel
(one for each tuning stage) if necessary for shielding purposes (to
prevent oscillation by stage-to-stage interference if that is a
problem.) And if higher frequency channel boards require some minor
changes in the circuitry configuration, and not just component value
changes, that can easily be done, too. In principle, this tuner might
even be able to extend a little beyond (on both sides) the 500-1800
khz MW band -- just plugin the right mini-board circuitry for the
frequency desired.

This idea is totally impractical for 120 different stations,
and plug ins get lost or broken, or worn out.

I don't believe it is impractical for 120 different stations, for two
reasons:

1) Those tube-o-philes who only want to listen to stronger local
stations, or to particular distant ones, are likely only to want to
have 10-20 stations (with the ability to add more if they want.)

One purpose of picking TRF is its legendary high-fidelity audio
capability which will appeal to audiophiles -- most won't want to
listen to a very weak station 1000 miles away that can only be
picked up some evenings.

And I believe it is easier to sell tube-o-philes on the Channel TRF
concept once it is explained how it maximizes audio performance for
each and every broadcast frequency that cannot be done with a
continuously tuned TRF.

2) Those who would use this for casual DXing (and note the hardcore
MW DXers will use something like a Drake R8B or ICOM R75, or some
digital receiver) will certainly be motivated to add more
mini-boards, and can do so over time. The tuner will work with 1
channel board, or with all 130+ (if enough slots are provided. For
the moment I am imagining the mini-board approach, but the sky's
the limit for other ideas to implement the channel TRF AM tube
tuner.)

3) And as noted above, hobbyists may find the "plugin" bandpass filter
capability of particular interest.

I leave answering points 1 thru 3 for others more patient than myself...



Of course, others here will probably have much better ideas as to
how to implement the channel approach.

You bet there are, and only possible with chip technology,
with press button station selection, and digital station F read out,
with digitally generated oscillator frequency for the F converter of
a superhet, with ceramic filter IF. Grundig have been multiband
radios for about 20 years +.

I have a Radio Shack DX-399 (the Sangean 606A) which is a very good
performer for casual MW (with the Radio Shack MW loop) and shortwave
DXing. So I am very familiar with that hobby, and with the benefits
digital systems bring to tuners. You need not sell me on that!

See my previous note above on "why tubes then?"

Not a tube in sight inh these lightweight plastic radios bought
cheaply by the masses to allow connection to the world's AM, FM, and
HF bands, and even amateur SSB stations.

See my previous note above on "why tubes then?"

(It's interesting to think of doing the same "channel" approach for
an FM tube tuner. Will that also confer several advantages in
simplifying the circuit design for the same overall performance
level?)

Study the way most post 1980 AM/FM tuners are constructed.
Tubes cannot be used with such methods.

O.k. But are you referring to tube-based tuners? Again, if all I
wanted was an audiophile grade AM/FM tuner, and did not care about
what was under the hood, I'd be open to solid state designs, but I'm
specifically looking at tube-based tuners.

The tubes are nice to use, but any scheme of discrete gain devices limits
your own channel approach, which is so far free of any details, and
probably impossible as it is impractical unless you care to prove otherwise
with a fully made prototype.



I still assume that the channel approach to tube-based FM tuner design
may confer some benefits, but maybe less since the frequency ratio to
tune from the lower to the upper ends of the band (about 1.25) is much
less than that for the AM BCB (a whopping 3.5 or so.) And there are
probably other factors as well specific to frequency modulation.

true.



It's trying to tune the AM band with only one degree of freedom (e.g.,
air tuning capacitor) which is causing all the hassle in tube-based
TRF AM tuner circuit design.

No, its not just the tuning cap.

Its the cost and effectiveness, low drift, and serviceablity and
selectivity of the superhet which makes the TRF look like a dinasoar.

One would want to pick a bandpass filter
which is optimally tuned to the specific frequency we want to listen
to, and this involves optimally selecting *several* component values,
not just one as we are limited to by continuous tuning with a
multigang tuning capacitor. The channel TRF approach appears to free
up the TRF designer from the tyranny of having to compromise the
bandpass characteristics over the entire tuning range which only one
degree of freedom allows.

Of course, superheterodyne is one solution to the TRF problem, and
allows for continuous tuning. Note that super-het works because it
uses "one channel" (the IF). So in a sense, superheterodyne supports
the channel TRF approach for those who don't want to build a
super-het, but rather want a pure TRF receiver (e.g., for sound
quality reasons, or whatever.)

Sound quality don't have to suffer with frequency conversion.

This truth knocks the life out of TRF fanatics.



*****

Now, I've made the call several times for classic and proven AM tube
tuner designs of the past which have excellent audio quality (and wide
bandwidth capability), are good for casual DX use, and can easily be
"modernized" for a kit. There are no doubt many excellent super-het
designs out there, but I've had very few recommendations. Patrick,
since you appear to much prefer super-het over TRF for AM tube
tuners, which classic super-het tube AM radio designs of the past
would you suggest as candidates to consider?

Read all my other recent AM radio posts again and you will see my preferances

repeated.



Anybody?

I reckon you got a pile of reading to do.

Yes, I have been reading. That one-year equivalent of EE training back
in 1974 at the University of Minnesota is slowly coming back to me.
Back then we spent a few weeks on tubes, and only a couple days with
transistors. Things have changed a lot since then.

I doubt they spend more than a single sentence on receiver tubes
in courses today, and all revolves around chips, in which
the inner workings are never to be fully understood,
and only the uses are known.



And it was interesting reading about Chebychev bandpass filters today
since I wrote a lot of Fortran code years ago to do various types of
numerical analytic processing including integration using quadrature
with orthogonal polynomials (mostly Legendre polynomials.) It was
especially cool to see how the higher order Chebychev polynomials U(x)
plot out in the desired shape (well approximately) for a bandpass
filter (but with that slight ripple within the bandwidth.) I'm not
saying all this to brag, but to give a better idea of my background.

Definitely I have a lot to learn, of course, and your posts are
helping me to better understand things. I still believe the channel
TRF concept is viable for those who want to build the best possible
TRF tube tuners where for each frequency the absolute best bandpass
characteristics can be chosen without worrying about how it impacts
the other frequencies since each channel is now largely independent.

The obvious downsides with the tube-based channel TRF concept a

1) The practical, real-world implementation of it (I believe it is
doable, I suggest one approach),

2) Losing the ability to continuously tune, which for BCB is not an
issue as I've noted several times, and

3) Calibration of each mini-board if done by the kit-builder (I think
this is solvable, but it is an issue to consider.)

The upsides are several, as previously noted.

Thanks for your helpful comments!

Jon Noring

I'll delegate you to chief honary prototype developer, and let you spend the
next 20
years building something for AM that nobody else has.

During my wait, I'll live a bit, then I'll die.

Patrick Turner.

Snip,


Going back to some of the earlier filter flatness discussion, well toss
that idea into the mix when you think in terms of TRF. Not only do you
want to achieve a specific width but you want it to be flat. My 3/25 kc
TRF scenario isn't flat at all. Its a big peak that just broadens out.
When we say a 'bandwidth'number we are relating to something specific
like 3 or 6 db down from the peak. Its still a peak in this context.
So whats happening at 20 db down? You guessed it, that 25kc number is
150 kc wide. I dunno how you could control the width AND the flatness
AND the skirts.

I'm a fairly recent convert to crystal radios. For the sake of
discussion there's little difference in xtal technology vs trf
technology in that both are non-superhet. I get absolutely glorious
quality audio from my xtal set when fed thru an amp. With 6 or 8 knobs
on the front panel and top notch components I can find a dead spot
between semi-locals on 680 and 690. With a local station on 1370 it
takes traps and VERY hi-q stuff to ferret out semi-locals on 1240, 1290
and 1480. Its as if it were a totally different radio from one end of
the band to the other and this has been the plague of TRF circuits since
day one. If I didn't have the local 1370 I could safely say, hey Jon,
this is the ticket, but there's scarce few of us who don't have a strong
undesired local station to bollox up the works.

Go superhet, my man.

-Bill M

The only way to gain enough RF bw at any F on the BCB to ensure there is no
sideband cutting
which would restrict the AF bw, you have to use two LC circuits and couple
one to the other,
and I used a 39k resistor.
At the low end of the BCB, the coils are tuned about 10kHz apart, and at the
top end,
they are tuned to the same F.

If you only have two LC circuits, and the bw is 25 kHz for each at 1,500
kHz,
then the Q is 60 only .
Using two LCs with a Q like that in cascade, the bw will be reduced to 19
kHz, or
thereabouts, and the selectivity away from the pass band will be twice that
of a single circuit.

But another powerful station at 50 kHz away will be heard,
although it won't be loud.

Once you are 50 kHz away from say 1,000 kHz, the rate of attenuation is
at 6 dB /octave only for the one tuned circuit.
So a station at 500 kHz of equal strength is only -6 dB below the 1,000 kHz
station.

You need multiple tuned circuits to give decent selectivity, and here the
superhet is king.


But it is possible to series three double LC twin gang stagger tuned stages.

This gives 6 tuned circuits. The final Q has to be 60 to allow full audio
bw,
The initial Q therefore has to be much much lower, maybe 15 only at 1,500
kHz,
allowing 100 kHz of bw.
Such a tuned circuit has a blunt nose, and no advantage can be had as with
with the double tuned IF transformer's flat topped steep sided bandpass
characteristic

The only reason for RF input selectivity with relatively low Q
tuned front ends in AM BCB sets is to make sure the mixer does not
get overloaded by too much signal from a poweful unwanted station
which would then try to cross modulate the mixer tube.

The purpose of the RF front end is spelled out in RDH4.

For those struggling on Q issues, go find out, I'm sick of repeating text
books.

Patrick Turner.

The fixed-step (10 kHz) tuner is what killed music on AM radio. You
can get a much more pleasant sound by detuning a few 100 Hz. This PLL
crap sounds like ****.

On Sun, 13 Jun 2004 00:36:21 GMT, Jon Noring wrote:

[Following up on a thread dating back to January, similar to one I
started recently. Responding to Patrick Turner's comments.]


Patrick Turner wrote in January 2004:
Jerry Wang wrote:

1. Even it is a single channel [AM] receiver, I would still suggest
the use of one or two intermediate frequency (IF) stages. Because
to achieve good sensitivity you need to have enough gain.

Since you only want one channel, there is no need for a frequency
converter or any IFTs or IF amps, and a TRF with four tuned circuits
in the form of two critically coupled RF trannies will do nicely.

Interesting.

As I noted in a recent message, it is very intriguing to build a
modernized, high-performance AM tube tuner using the "channel"
approach. This takes advantage of the fact that licensed broadcasters
today must broadcast on specific frequencies, every 10 khz in North
America and 9 khz in Europe and elsewhere. So, instead of trying to
be able to continuously tune across the BCB spectrum, we can think
outside the box for the moment and consider the alternative of
building reasonably optimized tuning circuits for each listened-to
frequency.

Hi Fi AM--

This was "almost" popular as a design project in the 60s. Even RCA in
their RC-19 Tube Manual have a circuit labelled "TRF AM Tuner-- for High-
Fidelity Local Broadcast Reception." (Circuit 19-8, p. 357)

Fidelity on an AM signal requires that most common circuits used in
radios be eliminated:

1: No AVC. This distorts the low frequency frequency response
2: No cathode bias bypass.
3: No diode detectors, unless the signal feeding them is greater than 10Vrms.
4: No AC coupling if diode detector is used (the "AC-loading" distortion
described in Terman, et al).
5: Speaker resonance 30Hz. Assumes the line out goes to a real "Hi Fi"
system.

All these "don't do" can be found in Terman, the Radiotron Designer's
Handbook and others.

If you are willing to live with about 5-10% THD, then you can use more
common circuits. However, there are dozens of "Hi Fi" AM circuits published
by the hobby magazines, tube vendors and kit makers. Have a look at them.

The RC-19 circuit uses a 6BA6 as an RF amp, followed by a 12AU7 used as a
detector and audio amplifier.

Good luck.

Steve.

--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA