David wrote:

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 ****.

Howcome?

Patrick Turner



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.

Steven Swift wrote:

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.

Let me say a few words.

1. Many pages of RDH4 are devoted to AVC.
The time constant for AVC application is very long,
comprising of 1M and 0.047 uF, and measurement
of bass distortions resulting from well applied AVC is low
enough to be negligible.

2. Nothing wrong with cathode bias, especially nowdays when cheap
large value elcaps are plentiful, and we have better plastic caps.
RDH4 speds a lot of time on cathode bias.

3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.

4. AC coupling is fine from an RC load fed by a diode.
The impedance fed by the audio + RF ripple voltage should be high,
like a cathode follower grid.

5. I have tried my radio with various speakers, and no trouble
making full amplitude signals at 20 Hz.
The LF pole is determined by the audio amp in the radio, but at the detector, the

pole is much lower.

On which pages are RDH4 and Terman "dont's" spelled out?



If you are willing to live with about 5-10% THD, then you can use more
common circuits.

True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.

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.

RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.

But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on the band?

The 6BA6 is a variable U tube, with a non linear
transfer curve.

If a large voltage is detected, there is quite a bit of distortion of the
detected
wave form.

Preferable is a 6AU6, a sharp cut off RF amp, with a more linear transfer curve,
although too much gain could be a problem, if so, use a lower Gm
pentode like a 6J7.
But having a 6AU6, or perhaps a 6BX6 is OK if biased with a cathode
resistance, and this R is left unbypassed to further reduce thd in the envelope
amplification.
With a load on the R amp typically of 25k, and Gm = 3 mA/V,
gain is 75, and if the Rk = 500 ohms, gain is 30, and thd reduced by 6 dB.

In a superhet, the voltage levels of the IF envelope coming from a mixer tube are

usually so low that the mixer don't maul the linearity of the IF signal,
even when the amplification is backed off with AVC.

Patrick Turner.




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

Steven Swift wrote:

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)

John Byrns web site has the circuit diagram for the RCA design you
mention (or a related one if there's more than one of them):

http://users.rcn.com/jbyrns/pics/RC-17-8.jpg


Fidelity on an AM signal requires that most common circuits used in
radios be eliminated: [snip of good list]

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

[snip]

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

As the diagram at John Byrns site shows.


What's intriguing is how simple this design is -- it has one RF stage,
which indicates that a one RF stage TRF for local, high power stations
makes sense when audio fidelity is the overriding criterion.

Now, I wonder how much improvement in the audio quality is possible
if the channel TRF approach is used (which optimizes the bandpass for
each broadcast frequency)? Or does it not make sense by the law of
diminishing returns? Being able to use a higher order plug-in bandpass
filter (such as a constant delay/linear phase one), optimized for each
frequency, is intriguing.

Jon Noring

David wrote:

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 ****.

Assuming that indeed a more pleasant sound is had by detuning off the
center frequency by a few hundred Hz (Patrick was skeptical), this is
not a problem for the Channel TRF tube tuner design since I believe a
very fine tuning control will be necessary, due to both tuner warmup,
and long-term drift as bandpass component values slowly change over
time between "calibrations".

Only guessing, I think the control should vary the center frequency
for a channel by about +/- 1 khz, enough to cover the several hundred
Hz of deviation for those who think this makes the station sound
better.

Jon Noring

Nothing wrong with a channel approach. Simplifies most of the design
tradeoffs.

BTW, I measured all the AM stations at my house (Seattle). Using a
typical AM radio (a Sony SW7600GR with internal antenna), I tuned all
the stations that were easily received without too much QRN/QRM (day
time). I then checked the field strength of each using a spectrum
analyzer and a calibrated antenna with 1 meter effective electrical
length.

I received 15 (reasonably clear) stations. The strongest station,
only 2 miles away, gave -55dBuV (about 1.8mV/m), while the weakest of
the 15, gave -85dBuV (about 56uV/m).

QRN, due to skywave, increased substantially at night. This is "same
channel" interference, which we get from the fact that there are no
"clear channels" anymore. Better antennas actually make the QRN worse
on some channels-- even while helping local daytime reception.

I would design the radio to only work with strong local signals, if
fidelity is the goal.

Again, I say "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

Patrick Turner writes:

Let me say a few words.

1. Many pages of RDH4 are devoted to AVC.
The time constant for AVC application is very long,
comprising of 1M and 0.047 uF, and measurement
of bass distortions resulting from well applied AVC is low
enough to be negligible.

To work properly for fading, the AVC needs to be about 100ms. This causes
significant distortion at bass audio. If a longer time constant is used
since only local (non-fading) stations will be tuned, then you are right.

2. Nothing wrong with cathode bias, especially nowdays when cheap
large value elcaps are plentiful, and we have better plastic caps.
RDH4 speds a lot of time on cathode bias.

Cathode bias is great, but do not bypass the resistor. The degenerative
feedback will improve audio performance, but you lose gain.

3. Diode detectors are quite low distortion detectors even with
very low voltages of 100 mV if there is a constant current trickeled
through the crystal diode to keep them turned on with their
forward conducting voltage.
I gave details yesterday in another post of a detector which will change your
views about
diode detectors.
Diodes can be used with DC shunt feedback around an RF opamp,
and thd is negligible.

I agree that this can be made mostly true using active filters and such, but a
perfect diode, with perfect modulation has lots of distortion. I am willing
to take a look at your analysis, but if you use Volterra series expansion,
you simply can't prove that you'll get better than a few percent distortion.
Somewhere in my old grad school notes, I have a derivation done by Prof.
Meyer (of Gray and Meyer, UC Berkeley) which shows the limits. I'll look for
your other post. Better than a few percent is NOT possible with just an RC
load (diagonal clipping) except for low modulation percentages.

4. AC coupling is fine from an RC load fed by a diode.
The impedance fed by the audio + RF ripple voltage should be high,
like a cathode follower grid.

5. I have tried my radio with various speakers, and no trouble
making full amplitude signals at 20 Hz.
The LF pole is determined by the audio amp in the radio, but at the detector, the

There is a discussion of speaker/cabinet resonances in RDH4 somewhere.
Lots of distortion when you approach resonance.

On which pages are RDH4 and Terman "dont's" spelled out?

Each chapter has "crumbs" of knowledge in it. I have yet to find a nice
do/don't do list anywhere. I may also be "integrating" Terman and Henney.

I think the guy who started the thread should build his idea and try them
on a few "beta" testers. He'll be able to sell enough to pay off his costs.
Maybe pick a few channels in a few big markets (LA, New York, Chicago) to
keep the work load down.

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

Jon Noring writes:

David wrote:

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 ****.

Hmm, and I thought it was a phase noise problem. A lot of these radios
used a 10kHz reference frequency, multiplied up to the LO, giving lots of
close in phase noise. Due to FM/AM conversion, this then showed up in
baseband.

Jon-- are you the originator of this thread?

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

Telamon wrote:
Please don't cross post to rec.radio.shortwave.

Perhaps you would be happier if you just learned to use your
delete key instead of expecting everyone else to conform to
your demands.

Jeff


--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"

In article , Patrick Turner
wrote:

Steven Swift wrote:

If you are willing to live with about 5-10% THD, then you can use more
common circuits.

True, but onje doesn't have to live with 5-10%.
linearize the IF amp and detector, and thd plummets.

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.

RDH4 has the circuit for the Selsted and Smith "infinite impedance "
detector, where a 12AU7 performs as credible detector, and as a diode,
but I think I'll stick with a germanium diode fet by a 12AU7 CF.

The "Selsted and Smith" detector is not the same thing as the so called
"infinite impedance" detector. The "infinite impedance" or "reflex"
detector was designed by RCA, while the "Selsted and Smith" detector was
designed by, well "Selsted and Smith", or at least "Selsted" who is still
around, or was a year or two ago.

The "Selsted and Smith" detector differs from the "infinite impedance"
detector in that it has a diode in series with the grid, and also a diode
load resistor. There is no peak detection capacitor across the diode
load, so the diode does not act as an ordinary diode peak detector, and
the triode doesn't act as a cathode follower. The triode is the actual
detector operating in a fashion similar to the "infinite impedance"
detector, with the diode apparently serving to linearize the "infinite
impedance" detector. The input impedance of the "Selsted and Smith"
detector is not infinite due to the presence of the diode load resistor.
For that matter the input impedance of the so called "infinite impedance"
detector is also not infinite, and can even have a negative resistance
component which can cause stability problems. The negative resistance
effect can occur when circuit conditions are right, similar to the
conditions that can cause oscillation in cathode and emitter follower
circuits if you aren't careful.

But does the RC-19 have enough tuned circuits to give over 70 dB rejection of
signals which are 50 kHz away from the wanted station at any place on
the band?

The 6BA6 is a variable U tube, with a non linear
transfer curve.

There is nothing wrong with the 6BA6, it was specifically designed for
this service and has very low odd order distortion which is all that
matters since the even order distortion products can't get through the
IFT. I hope I got that the right way around, if not it is explained in
some detail in some of the old texts, I think "Radio Receiver Design" by
Sturley is one that explains it. You only get in trouble if you try to
run the tube at a very high signal level, simultaneously with a high AGC
voltage applied for a large gain reduction. This is mainly a problem in
the stage driving the diode detector, so it is best to avoid AGC on that
stage, but in a minimal radio that is of course problematic. This is one
of the many topics that the RDH4 gives short shrift. The 6BA6 is even
usable as a gain control element in audio circuits where even order
distortion does matter. IIRC the peak limiter at a radio station where I
once worked used four 6BA6s in the audio path, where they were connected
in push pull, presumably to cancel the even order nonlinearities which are
inherent in the design of the tube.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/

In article , wrote:

Steven Swift wrote:

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)

John Byrns web site has the circuit diagram for the RCA design you
mention (or a related one if there's more than one of them):

The circuit is the same, with one exception, in all the editions of the
RCA tube manual in which it was contained. The one change that occurred
in later editions was a change from a 5Y3GT rectifier in the power supply,
to a rectifier with an indirectly heated cathode, I think it was a 6X4.
This circuit was only published during the 1950's, it did not appear in
editions before about 1950, and was dropped from later editions after
about 1960. I have often wondered if it was inspired by Williamson's late
1940's design for a radio feeder unit.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/