Brian - you seem to have totally missed my point. My point was NOT that
"nothing exists" near 10khz - my point IS that in designing a receiver
for maximum fidelity one must consider that between the audio "source"
and the received signal - there are a lot of factors to consider - one
such is that there IS a roll-off as one approaches 10khz.

I never suggested that there was "nothing there" - only that if one were
to try to "reconstruct" the audio as it existed from the original source
-- processing needs to be taken into consideration - including the NRSC
- and any roll-off approaching the "brick wall".

How do you know that the 10khz signal in your last picture wasn't
originally 20db higher than shown here - before being "processed"?

(at this point Jeff adds):

Jeffie hands you another bucket of pearls to cast before the swine.

Oh, give 'em time - they'll learn - you didn't "pop-out" spouting this
stuff either... well- come to think of it --- never mind.

One more time - without knowing exactly what was going into the
transmission chain - your pictures are annecdotal at best. While they
show conformance with NRSC-1 - they on NO WAY tell us anything about
what indignities the audio suffered on it's way through the chain. You
are assuming that something that "looks flat" is. Let me remind you -
if it's flat after being pre-compensated 75us (10db at 10Khz)- SOMETHING
ate some of it!!!!!

To answer your last question (bandwidth to fully recover modulation)

Go look at John Byrns recent post where he shows the curve comparison
between two IFTs. Note that even in this very broad filter - there is
STILL some loss at Fc +/- 5khz. (You're the one that said "FULLY" - i.e.
total - no loss). Even these won't do that. And in fact - no circuit is
that ideal - too many trade offs - so one settles for practical.

To accomplish "usual" standards of fidelity - Johns numbers show that
these particular IFTs would have a "passband" of 40khz. Can you
determine why that is?

The problem (in this case using IFTs - implying a hetrodyne system) - is
that with a 40Khz passband (+/- 3db points - ooops gave away the answer
to the above question)- unwanted "stuff" pours through on unwanted
hetrodynes as well. But that's another issue.

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

Patrick Turner wrote:

Would not the use of pink noise through a low pass filter
and used as the carrier signal modulation be a better way to see the
frequency contour on an analyser, why noise + piano?

Absolutely - then you "know" what you're looking at.

Actually the NRSC (I know Patrick -you guys don't have to fool with
such) specifies a white noise source (equal energy at all frequencies)
filtered* then gated at 2.5hz with a 12.5% duty cycle. This is felt to
best simulate "real world" broadcasting. Again see the NRSC-1 spec I
noted yesterday.

*filter is 100hz high pass 6db/octave and 320hz low pass 6db/octave.
(yes that's -36db @ 10.24khz)

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

the J.W. Miller
TRF receiver



I did a cursory check on the Internet, but did not yet find any
schematics for the mentioned receivers. Are they online somewhere?
Anyone?



Just posted a schematic of the Miller TRF receiver, with the "secret"
inductance values filled
in. a.b.p.radio

RDH4 says most AM BCB radio makers tried for a final IF bandwidth response of
3.5 kHz
That was in 1955/
Since then, the BW has shrunk in many sets to even less than 2 kHz, especially
in solid
state gear, giving horrid state AM listening.
No good turning up the treble control knob, there is no treble there to boost.


I modified a fairly generic Radio Shack Optimus receiver's AM section by
removing the narrow
bandwidth ceramic filter and using a set of overcoupled IFs in its
place. See:
http://pw2.netcom.com/~wa2ise/radios...tml#solidstate
Sounds a lot better on local stations, though DX will have a lot of
monkey chatter.




The FCC limits interference only partly by bandwidth restrictions. Mostly,
it uses geographic seperation and power restrictions.

By ear, I think most stations go to about 7 or 8 kHz audio. Many of the AM
stations are talkers, but the ads can really sparkle. There's one I hear
which sounds like it goes to the 10 kHz audio max.



Much AM is talkback from mobile telephones, and its pretty dreadful....






Of course talk shows using telephone lines will be limited by the
quality of the phone system.
But you should hear better quality from the talk show host and
commercials played on station
equipment (or via satellite), as mentioned above. Audio from digitally
compressed cell phones sounds the worst.
If someone uses a cell phone to do a remote (like a high school football
game) be sure to
use an old analog cell phone (the kind that one could easedrop on with
an FM scanner radio).
But that assumes that the phone system doesn't do compression at the
cell tower site to send
it down the landlines.

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are.

Sure. But the same Q would give wider BW at 2 MHz.
I have not ever done this, so I guess at what the final response could be.

But so what, I thought we were talking about IFs for audio here, not video
IFs? For an audio receiver I would think at the most we would want a 40
kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC
effectively limits the audio bandwidth to 10 kHz? What exactly do you see
as the advantage of a 2.0 MHz IF in an AM broadcast receiver?

The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the
bandwidth necessary for High Fidelity AM audio reception. The stage gains
will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of
similar bandwidth, with the exact stage gain depending somewhat on design
choices and practicalities. The wideband 455 kHz IF will have lower stage
gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from
lower stage gain. The wideband 455 kHz IF has the advantage that standard
RF front-end components like tuning capacitors and oscillator coils can be
used, while the 2.0 MHz IF will require special RF components.

What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver? Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios. I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

The equality in performance depends on a large Q difference, with
544 kHz Q much lower than 2MHz Q to get the same BW.

Yes, although I have some reservations about the use of the term "Q", that
is obvious, but so what, what difference does it make?

The Q of a typical 455 kHz IFT is higher than you have indicated, because
the impedance of the LC circuit at Fo is required to be high to suit
pentode loading, and to get high gain.

You also are going to sacrifice stage gain in the same way with a 2.0 MHz
IF, so this is no more of a problem for the wideband 455 kHz IF than for
the 2.0 MHz IF.

If the Q was real low, and hence the Fo impedance, you
would probably need 3 IFTs.

This is a consequence of the wide bandwidth, not the IF frequency, the
problem is identical at 2.0 MHz.

I have never tried 3 very damped IFTs.

The fact that you haven't tried something doesn't prove anything one way
or the other. Also, what does "damped" mean in this context? I would
have to do some research, but I suspect that "damping" is more related to
filter bandwidth than to the center frequency, and both filters are aiming
for the same bandwidth.

What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

Build a radio with 2MHz and measure it, maybe it works better.

You are the 2.0 MHz IF advocate not me, you still haven't suggested any
reason why it might work better from a bandwidth/selectivity standpoint?

Just don't knock the idea before trying it, or condemn the idea
with postulations about what might be.

I'm not, I know it would work, what I don't understand is what the
advantages are over a 455 kHz IF of the same bandwidth? You are not
explaining yourself, cite some concrete facts.

These things must be tried and measured, to really know.

While I can't claim to have designed the filter I used, I have actually
built a transistor superhetrodyne AM tuner using a 455 kHz block filter
with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this?
Have you tried a properly designed wideband 455 kHz IF filter to see how
it worked? The filter I used came out of a 2-way land mobile radio and I
think it was about an 8 pole filter. Back in the old days of land mobile
here in the US, wider channels with greater bandwidth were used than are
used today. Over time the channels were squeezed down to accommodate
additional channels in the same space, and block filters of several
different bandwidths were available to suit the changing allocations and
operating frequencies.

I have also built wideband single frequency TRF receivers using modified
double tuned IF transformers.

So what it boils down to is that you haven't tried a wideband 455 kHz
filter while I have, and I haven't tried a 2.0 MHz IF filter, which you
may or may not have done. I at least have cited some concrete facts about
IF filters, while you have only muttered about Q, without indicating how
it actually relates to the problem. I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.


Regards,

John Byrns


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

John Byrns wrote:
What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

Both pros and cons to this John.

Since the bandwidth is a percentage of the center frequency,
the shape of the bandwidth will change based on the distance
from the center frequency (as a percentage.)

Assuming just for the moment +/- 5 KHz.
At 455 KHz that's about 1% above and below.
At 2 MHz, that's now only .25% above and below.

As you get further from the center frequency, percentage
wise, the shape of the curve as it transitions from inside
to outside of the band pass is going to look different at
the upper frequency than it does at the lower frequency.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar.

With the notable exception of the difference in shape of the
roll off above and below the center frequency.

In the world of designing filters (and overall system performance)
this is called group delay. A shorter, perhaps more recognizable
term would be linear phase shift over the entire band pass of the
filter.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed

They were under the same constraints as the Weekly World News.
"If it wasn't true, they couldn't print it." Note smiley face
here. ;-)

Yes, although I have some reservations about the use of the term "Q", that
is obvious, but so what, what difference does it make?

Back to the original comments about Q. In a perfect world, it
would only be a matter of the LC ratio setting the bandwidth
of a tuned circuit. Of courses, there are other things that
get in the way to reduce the overall Q of a circuit. Nasty
little things like the series resistance of the coils, dielectric
losses in both the coil forms and capacitor insulation material.

Back to the original "ideal" values of Q. 15 at 455 KHz and
67 at 2 MHz. It is physically "more challenging" to get higher
Q at a higher frequency. All of the various losses of the
components tend to get in the way. Wire losses, dielectric
losses and any losses of the ferrite used in the core materials.

Also, what does "damped" mean in this context? I would
have to do some research, but I suspect that "damping" is more related to
filter bandwidth than to the center frequency, and both filters are aiming
for the same bandwidth.

"Damped" means adding some form of resistance across the reactive
components of a circuit. As an example, if you were to assemble a
nice 455 KHz IF transformer and found that the bandwidth was too
narrow, a fast method of widening it would be to place parallel
resistors across the windings.

Another point about "damped" is that if a tuned circuit has too
high a Q, a sudden transient will tend to make it oscillate.
In communications receivers, this is obvious that a signal sounds
more like you're ringing a bell, than simply turning a tone on
and off. (Kind of like using the sustain pedal on a piano.)

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

Lets not go in that direction.

An IF transformer is simply a two pole butter worth filter.
That it can have different input and output impedance just
makes it really convenient for taking the source from a plate
and connecting it to a grid for a load.

By definition, a butter worth filter has a smooth curve with
only one peak (in the middle.) And the shape (steepness) of
the band pass is related to the overall Q of the circuit.

The next type of filter, would be Chebychev, This is no more
than a "predistorted" butter worth filter network. By allowing
a certain amount of ripple in the pass band, the shape of the
rejection can be made sharper. The obvious trade off is the
amount of distortion to the signal within the pass band.

A simple example of this would be stagger tuned IF coils.
Two or more peaks, and a dip (or dips), ripple, in the middle.

While I can't claim to have designed the filter I used, I have actually
built a transistor superhetrodyne AM tuner using a 455 kHz block filter
with a 30 kHz IF bandwidth. Will the 2.0 MHz IF work better than this?
Have you tried a properly designed wideband 455 kHz IF filter to see how
it worked? The filter I used came out of a 2-way land mobile radio and I
think it was about an 8 pole filter.

The point you've probably overlooked in land mobile operations is that
it was NEVER designed as a "hi-fi" system. There's a reason for the
term "voice grade." Having as much a 3 dB of ripple in a band pass
filter is meaningless especially when the filter is in the midst of
a limiting IF strip for FM recovery, and on AM demodulation. What
really matters here is limiting the bandwidth of the received signal to
ONLY include that of the wanted (in channel) information and none of
the unwanted (adjacent channel) information to get to the discriminator.

So what it boils down to is that you haven't tried a wideband 455 kHz
filter while I have, and I haven't tried a 2.0 MHz IF filter, which you
may or may not have done. I at least have cited some concrete facts about
IF filters, while you have only muttered about Q, without indicating how
it actually relates to the problem. I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

You should take the time to read up on "filter jockeying" John.
You're making a lot of incorrect assumptions on how they work.

The primary requirement on the Q of individual components in
filter design is only such that their value of Q be high enough
to not materially effect the overall Q of the circuit. As an
example, (and without getting into cryogenic treatments and
styrofoam cups) A speaker system sounds better through 25 feet
of #12 AWG wire than it does through 25 feet of #18 AWG wire.
And that's strictly due to the resistive loss of the wire in
comparison to the losses in the actually speaker design and
implementation.

I had a electronics instructor in college that would show you
"The secret of electronics" that he kept hidden, and locked,
inside a small jewelry box if you "caught on" during his course.

With some fanfare, he would slowly open the box and you would
see an inductor, a resistor and a capacitor.

And it's really just that simple. What gets complicated is when
you forget that all three items have hidden values of the others
contained within them. (I.e. the difference between practical
and theoretical parts.)

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"

Randy and/or Sherry wrote:

Patrick Turner wrote:

Would not the use of pink noise through a low pass filter
and used as the carrier signal modulation be a better way to see the
frequency contour on an analyser, why noise + piano?

Absolutely - then you "know" what you're looking at.

Actually the NRSC (I know Patrick -you guys don't have to fool with
such) specifies a white noise source (equal energy at all frequencies)
filtered* then gated at 2.5hz with a 12.5% duty cycle. This is felt to
best simulate "real world" broadcasting. Again see the NRSC-1 spec I
noted yesterday.

I thought white noise had a rising amplitude as F rose.
Pink noise is white noise filtered at a slope of 3 dB/octave,
and thus giving a flat average level amplitude response for any single F
filtered
out of the pink noise, and is thus used for speaker testing etc....

The pink noise gene I made has such a filter applied to a white noise
source.
My bandpass filter for speaker tests has a Q of 12 for any part of the
audio band, and the amplitudes
of the noise bands filterd out from the noise signal is the same between
20 Hz and 20 kHz.

Patrick Turner.



*filter is 100hz high pass 6db/octave and 320hz low pass 6db/octave.
(yes that's -36db @ 10.24khz)

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

In article , Patrick Turner
wrote:

John Byrns wrote:

Patrick, you are missing the point, the issue was the merits of a 2.0 MHz
IF frequency vs. a 455 kHz IF frequency with respect to
bandwidth/selectivity, my point was that for the sort of bandwidths we are
talking about for audio, a 455 kHz IF can provide virtually identical
"pass band and attenuation out of band" with exactly the same number of
IFTs as a 2.0 MHz IF frequency. The loaded Qs result from the design
specifications in both cases, and are what they are.

Sure. But the same Q would give wider BW at 2 MHz.
I have not ever done this, so I guess at what the final response could be.

But so what, I thought we were talking about IFs for audio here, not video
IFs? For an audio receiver I would think at the most we would want a 40
kHz bandwidth, more likely 30 kHz, or even 20 kHz in the US where the FCC
effectively limits the audio bandwidth to 10 kHz? What exactly do you see
as the advantage of a 2.0 MHz IF in an AM broadcast receiver?

Its a lot easier to get a wider pass band of 30 kHz with 2MHz IFTs than with
455 kHz IFTs.
Try it some time, and then you'll know.



The way I see it both 455 kHz IFs and 2.0 MHz IFs can be built with the
bandwidth necessary for High Fidelity AM audio reception. The stage gains
will be virtually identical for both the 455 kHz IFs and 2.0 MHz IFs of
similar bandwidth, with the exact stage gain depending somewhat on design
choices and practicalities. The wideband 455 kHz IF will have lower stage
gain than a normal narrow 455 kHz IF, but the 2.0 MHz IF also suffers from
lower stage gain. The wideband 455 kHz IF has the advantage that standard
RF front-end components like tuning capacitors and oscillator coils can be
used, while the 2.0 MHz IF will require special RF components.

What exactly are the advantages of a 2.0 MHz IF from a
selectivity/bandwidth point of view?

I suspect 3 x 2MHz IFTs would be easier to get a flat topped
pass band and sufficient steep roll off just outside the band.

I also suspect any old 455 kHz IFTs could easily
have about 3/4 of their turns removed, and retain the same
caps of 250pF.

For 250pF, to get 455 kHz, one needs 0.48 mH

For 250pF, and 2 MHz, one needs 0.025 mH.

To reduce L by 20 times, the turns would need reducing
by a factor of 1/4.47.

Thus the DCR would fall, and Q could rise.

I have used ex IFT windings with turns removed for
high Q RF input coils on my reciever, to get the range of
tuning required between 500 and 1750 kHz with a
20 pF to 360 pF tuning gang.
The ferrite slug is retained.

The wire is litz wire, with low RF resistance, hence it gives a high Q,
but for 2 mHz, solid round wire would probably be OK, like in 4.5 MHz
TV IFTs and 10.7 MHz FM IFTs.

HF IFTs are easier to wind than 455 kHz.



There may be
architectural advantages to using one or the other IF frequency in a
radio, but so far only the bandwidth/selectivity has been mentioned and in
that regard an IF of 2.0 MHz offers no significant advantage over a 455
kHz IF for the reception of the full audio bandwidth.

I supect it might, and one article in Wireless World refered to using
10.7 MHz.

Certainly a high IF frequency will have advantages in image response, but
if the bandwidth is the same, the audio quality should be similar. What
exactly did Wireless World say was so great about using a 10.7 MHz IF for
a MW AM receiver?

Wide AF response was easily achieved.

Wireless World is a hobbyist magazine and all their
authors are not necessarily up to speed, although in the old days they
often did have articles by people who knew what they were talking about
with respect to radios.

I differ. WW and what it became, Electronics World wasn't just an
amateur's magazine. It had cutting edge articles about electronics
from 1917 onwards, and I suggest you park yourself beside a
pile of all the old copies and have a good read.
Most of the info was only comprehensible by very well university educated
professionals, or intellectuals, and most ideas were backed up with mathematical
proofs which nearly all the general public couldn't understand.



I suspect that the reason Wireless World might
have used a 10.7 MHz IF in a MW AM broadcast receiver is because it was an
easy way for a hobbyist, who both doesn't have a clue what he is doing,
and doesn't have the necessary test equipment, to get a super wide
bandwidth.

I leave you to your suppositions.



To illustrate this consider the example of the following calculated
response curves for both a 455 kHz IFT and a 2.0 MHz IFT:

The only advantage the 2.0 MHz IFT shows is marginally better symetry of
responce about the ceter frequency, the response of the two IFTs is
virtually identical.

The equality in performance depends on a large Q difference, with
544 kHz Q much lower than 2MHz Q to get the same BW.

Yes, although I have some reservations about the use of the term "Q", that
is obvious, but so what, what difference does it make?

Build a receiver, and find out.



The Q of a typical 455 kHz IFT is higher than you have indicated, because
the impedance of the LC circuit at Fo is required to be high to suit
pentode loading, and to get high gain.

You also are going to sacrifice stage gain in the same way with a 2.0 MHz
IF, so this is no more of a problem for the wideband 455 kHz IF than for
the 2.0 MHz IF.

Use more stages if stage gain is low.
The EA design used 3 IFTs, with two j-fet IF amps, with quite
heavily damped 455 kHz IF coils.




If the Q was real low, and hence the Fo impedance, you
would probably need 3 IFTs.

This is a consequence of the wide bandwidth, not the IF frequency, the
problem is identical at 2.0 MHz.

I have never tried 3 very damped IFTs.

The fact that you haven't tried something doesn't prove anything one way
or the other.

It means that what you or I am saying may not include all the facts about
the subject. Build and measure will give the facts.

Also, what does "damped" mean in this context?

Strapping resistance across the LC tuned circuit to reduce the Q.
The rate of attenuation just either side of the pass band becomes
much less, so more IF stages must be used.

I would

have to do some research, but I suspect that "damping" is more related to
filter bandwidth than to the center frequency, and both filters are aiming
for the same bandwidth.

Damping reduces Q, and increases BW.
But it also reduces Z at Fo, thus reducing gain in an amp
which must be a current source, like a pentode or j-fet,
to realise the best selectivity for the LC circuit.



What I said was what I said.
You are confused.

Maybe, in what way are you suggesting I am confused? I would suggest to
you that you don't understand how to design an IF filter, and don't
understand what can be done at 455 kHz.

I know enough about IFT design, after having built my own radio.


Build a radio with 2MHz and measure it, maybe it works better.

You are the 2.0 MHz IF advocate not me, you still haven't suggested any
reason why it might work better from a bandwidth/selectivity standpoint?

I refuse to repeat myself any further.



Just don't knock the idea before trying it, or condemn the idea
with postulations about what might be.

I'm not, I know it would work, what I don't understand is what the
advantages are over a 455 kHz IF of the same bandwidth? You are not
explaining yourself, cite some concrete facts.

I have already stated that for a given Q, the pass band for a 2MHz IFT
is naturally a lot wider than for a 455 kHz IFT.

Put it this way, if you make IFTs of 100 kHz, then its all the harder to get
a flat topped bandpass response which is 20 kHz wide, with
high sloped skirt response each side.



These things must be tried and measured, to really know.

While I can't claim to have designed the filter I used, I have actually
built a transistor superhetrodyne AM tuner using a 455 kHz block filter
with a 30 kHz IF bandwidth.

Ceramic filters are another way to achieve the same bandpass filter
that the IFT could do.
But they were never used in tube sets for the BCB.


Will the 2.0 MHz IF work better than this?

I suspect yes, but getting a 2 MHz cermic filter with 30 kHz of BW might be
unobtainium.


Have you tried a properly designed wideband 455 kHz IF filter to see how
it worked?

Yes, and trying to squeeze 20 kHz of flat topped BW was difficult with stock IFTs.

I have already said what my solution was, to use a variable distance coils and
some damping
on IFT no1, which allowed me to have only 2 IFTs, and 1 IF amp, a 6BX6, fixed
bias,
for low thd IF amplification.

The filter I used came out of a 2-way land mobile radio and I
think it was about an 8 pole filter. Back in the old days of land mobile
here in the US, wider channels with greater bandwidth were used than are
used today. Over time the channels were squeezed down to accommodate
additional channels in the same space, and block filters of several
different bandwidths were available to suit the changing allocations and
operating frequencies.

I have also built wideband single frequency TRF receivers using modified
double tuned IF transformers.

One of the Electronics Australia kit designs I have used
a two stage TRF design with highish Q LC, with stagger tuning
at the low F part of the band. This utilised having mutual capacitive
coupling of the Ls in their earthy ends to ground via one common 0.1 uF.
I couldn't easily reproduce the nice response curves of the kit set,
and it was not good enough to give selectivity between locals
here where I wanted to hear a 300 watt station which was only
45 kHz away from a 5,000 watt station.
But otherwise, the TRF was a fine performer.



So what it boils down to is that you haven't tried a wideband 455 kHz
filter while I have, and I haven't tried a 2.0 MHz IF filter, which you
may or may not have done.

I have tried getting 455 kHz IFTs to go wider, but I was dissapointed with
overall results, because I'd have needed 3 IFTs, and lots of damping.

I got 10 kHz of audio BW at low thd using simple methods of damping, sliding IFT1
coils closer,
and some RC boosting of audio HF.
I thus achieved the use of tubes, good AF BW, and excellent local station
selectivity, which allowed me to hear my wanted 300 watt station without
the 5,000 watt station able to be heard even though it is only 45 kHz away.

I at least have cited some concrete facts about
IF filters, while you have only muttered about Q, without indicating how
it actually relates to the problem. I am not a "filter jock" (tm) but I
think it is generally desirable that the Q of the components used in a
filter be high, especially when we get beyond simple double tuned
transformers. What you are calling Q is more related to how the filter is
terminated, which is a different matter than the Q of the components that
make up the filter.

I leave you to wonder the full content of my mutterings,
and I do hope you spend some time soon in your shack with a soldering iron
and response meter.

The other advantage of a 2 MHz IF is that the filtering of RF from the recovered
audio is easier, because the C value is less, and the filter used has less effect
on recovered audio at 10 kHz, and at high amplitudes.

But don't let me mention it, I know you'd be aware of it already.

Patrick Turner.



Regards,

John Byrns

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

In article , Robert Casey
wrote:

Just posted a schematic of the Miller TRF receiver, with the "secret"
inductance values filled
in. a.b.p.radio


Hi Robert, that's certainly a cute little radio you have there. An
interesting point is that the separate "negative mutual coupling"
inductance, the one with the "secret" value, isn't even necessary and the
part can often be eliminated from the circuit. All you need to do is wind
L1 and L2 like a typical double tuned IF transformer, and if the coupling
coefficient is correctly chosen to yield the required value of mutual
inductance, and if the two windings are phased correctly to make the
mutual inductance "negative", then the separate coil like you used isn't
necessary, although you must retain the capacitor in the common lead of
"L1" And "L2", since that is part of the "secret". This scheme will work
in a circuit like the Miller "High Fidelity" Crystal Tuner where L1 and L2
are just single winding coils, there would obviously be problems applying
the idea to your circuit because of the extra winding you put on L1,
making it into a transformer by itself.

I listened to the WABC "jpg" you posted, and the tuner certainly has a
good bandwidth, although I wonder how much pre emphasis WABC might have
been using and how well your receiver matches it, I will have to listen to
it again to see how correct the de emphasis seems to me, there was also
some background noise at several points, I will have to listen again to
see if it was part of the audio at points, or if it was interference of
some sort. The thing I didn't like about it was that it had pretty
horrible levels of distortion, and while this could be WABC's fault, I
have found that it is typical of these so called "High Fidelity" crystal
receivers. I have a couple of J.W. Miller "High Fidelity" crystal tuners,
and they have the same distorted sound. I think this is because the
crystal detectors produce really horrendous distortion, unless you have a
big enough antenna to get the audio output level up to at least the 2
volts RMS that Patrick recommends. I would try for another 10 dB or so of
audio output above that level before being completely happy myself. But
no way do these simple crystal sets sound "High Fidelity" to me because of
all the nonlinear distortion. The distortion probably does help give them
their bright sound though, sort of a psycho acoustical trick if you will,
but it does wear on one.


Regards,

John Byrns


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

Patrick Turner wrote:

I thought white noise had a rising amplitude as F rose.

as "burnt toast" as I am tonight - someone could claim white noise comes
from Procol Harum and pink noise from Pink Floyd - and I'd agree.

Here is the quote from the NRSC-1 spec for bandwidth testing - as it
relates to "source"...

Section: 6.3.2 Use of Standard Test Signal. Audio bandwidth shall be
measured using a test signal consisting of USASI (United States of
America Standards Institute) noise that is pulsed by frequency of 2.5 Hz
at a duty cycle of 12.5%. See Figure 4. USASI noise is intended to
simulate the long-term average spectra of typical audio program
material. Pulsing of the noise is intended to simulate audio transients
found in audio program Material. USASI noise is a white noise source
[note 4](i.e. noise with equal energy at all frequencies) that is
filtered by (1) a 100 Hz, 6 dB per octave high-pass network and (2) a
320 Hz, 6 dB per octave low-pass network. too Figure 4. A pulsed USASI
noise generator is shown in Figures 5 and 6. Using the attenuator pad,
the ratio of peak-to average amplitude shall be 20 db at the audio
output of the pulser. [snip]

Note 4. Acceptable white noise sources include GenRad Models 1382 and
1390B; Bruel & Kjaer Model 1405; and National Semiconductor IC No. MM5837N.

[end NRSC-1 quotes]

If you can find specs on any of those generators or that IC - then
you'll find what they think white noise is.

Right now it's approaching midnight - just went through the emotionally
draining experience of watching a old family friend's funeral on TV...
and Sherry and I are toast - so someone else can look them up.

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com