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AM radio receiver - design

M

Mark Zenier

Jan 1, 1970
0
One of the problems with terminology is what the circuit really does.

You can stick a PLL on the BFO and feed a product detector. (Can growl
at you). Or you can use a narrowband filter and a limiting amplifier
to regenerate the carrier. (Can drop out if the carrier fades).

You can use both sidebands into a plain product detector, or you can use
a image reject a.k.a. I/Q mixer with phasing to cancel out an unwanted
sideband.

Various combinations end up doing different things.

Using both sidebands does a good job cleaning up signals with selective
fading, where chunks of the audio frequencies in the sideband are getting
canceled out. Common in signals coming in over the poles. Aurora flutter.

And using a image reject mixer will cancel out the signals on one side
of the carrier better than the skirt response of the IF filter.

But they all get lumped under the synchronous detector label.
The ones with the best reputation are the Sony receivers that
use a chip designed to demodulate Kahn style AM stereo, using
image reject mixer(s).
Sync sounds good on paper. In reality it is hard to do. You radios
that "growl" at you. If the lock isn't perfect, you get low frequency
grunge. You also can get a slight frequency offset (translation).
You might want to go to rec.radio.shortwave. Lots of medium wave
listeners there too. Pretty much you will get the same answer. There
is some guy there who has multiple Wellbrook loops. He uses a phaser
(MFJ I think) to use the 2nd loop to null transmitters on the same
frequency. However, that doesn't sound like your issue.

Actually, it's a smoking crater infested with right wing zombies. But
there should be a lot of useful stuff in the archive. Ron Hardin? is
(I think) the guy with a yard full of fancy antennas and noise phaser
boxes. Another poster to look up is Pete Gianopolus [or something like
that] who was working on a very high performance medium wave receiver
last year or two.

Mark Zenier [email protected]
Googleproofaddress(account:mzenier provider:eskimo domain:com)
 
Here is the paper:http://www.kongsfjord.no/dl/Audio/ELPAFs.pdf [....]
Then look at that filter. I mean serious, discrete LCR with
questionable source and load impedance?

Wow, he is putting a large capacitor across the audio amplifier
output.  That should sound *interesting*

I know. He should have used the dual of the circuit. Series L rather
than parallel C. Even then, the source impedance isn't known. I never
bothered to see if the component values are correct. The whole
implementation is stupid enough that I didn't believe checking the
design was worth the effort.

I have little faith in Mr. Lankford's theories, but at least it keeps
him off the streets. Granted, it sounds like he did use some cheap
synchronous demod radios since he complained about the growl. As I
said, sync is hard to do well in practice. Drake wouldn't have made
the B rev if the sync in the older model didn't suck. The AR7030
probably had the best off the shelf synchro for a consumer radio, but
even the designer complained about it.

See the bottom of this page:
http://www.aoruk.com/archive/pdf/sync.pdf

Some other interesting reading about the design:
http://www.aoruk.com/archive/comments.htm
http://www.aoruk.com/archive/7030bulletin.htm

I never own a Sherwood sync demod, so I can't comment on the quality.
In fact, I never met anyone that owned one. However, Sherwood does
test a lot of radios and is considered well in the industry.
 
M

MooseFET

Jan 1, 1970
0
Here is the paper:http://www.kongsfjord.no/dl/Audio/ELPAFs.pdf [....]
Then look at that filter. I mean serious, discrete LCR with
questionable source and load impedance?
Wow, he is putting a large capacitor across the audio amplifier
output.  That should sound *interesting*

I know. He should have used the dual of the circuit. Series L rather
than parallel C. Even then, the source impedance isn't known.

A simple buffer amplifier would make the impedance known. If I was
betting on what it is, I'd bet under and Ohm for the impedance of even
a modest amplifier.

I never
bothered to see if the component values are correct.

I stopped at the first part I saw.
 
M

MooseFET

Jan 1, 1970
0
I suggest that you do the same.  Consider making one frequency in one
side band:

  Y = sin(At) * sin(wt) + cos(At) * cos(st)

This does one frequency.  I can now do two frequencies in the same
side band as:

Y = (sin(At) + sin(Bt))*sin(wt) + (cos(At) + cos(Bt))*cos(wt)

@@ Then each Fourier component of noise would be defined by a single
variable, A, B etc. in turn.

You are missing the point. You can do one frequency, two frequencies,
three frequencies ... an infinite number of frequencies.

 But the components of additive white gaussian
noise (AWGN) each have a random amplitude _and_ a random >phase.

I am keeping it simple. To get the phases just insert another
constant in each component to adjust the phase. I thought that this
would be obvious but I guess not so here it is with the objection
fixed.

Y = (sin(At+C) + sin(Bt+D))*sin(wt) + (cos(At+C) + cos(Bt+D))*cos(wt)

Do you now agree that any collection of side bands can be made as I
suggested?

Once you see that point, you then need to rearrange the terms. The
fn1 and fn2 were not for the two side bands as you seem to keep
assuming. They are two function that together compose the total
noise.
 
One of the problems with terminology is what the circuit really does.

You can stick a PLL on the BFO and feed a product detector.  (Can growl
at you).  Or you can use a narrowband filter and a limiting amplifier
to regenerate the carrier.  (Can drop out if the carrier fades).

You can use both sidebands into a plain product detector, or you can use
a image reject a.k.a. I/Q mixer with phasing to cancel out an unwanted
sideband.

Various combinations end up doing different things.

Using both sidebands does a good job cleaning up signals with selective
fading, where chunks of the audio frequencies in the sideband are getting
canceled out.  Common in signals coming in over the poles.  Aurora flutter.

Here is a spot where I clearly disagree. Using one sideband narrows
the RF bandwidth while leaving the recovered bandwidth alone. The
narrow RF bandwidth has a smaller "window" where direct and reflect
signals can mix, which is the cause of the fading.
And using a image reject mixer will cancel out  the signals on one side
of the carrier better than the skirt response of the IF filter.

But they all get lumped under the synchronous detector label.
The ones with the best reputation are the Sony receivers that
use a chip designed to demodulate Kahn style AM stereo, using
image reject mixer(s).

I have a Sony with the chip. It growls. It doesn't hold the lock very
well. The AR7030, granted at over 10x the cost, does a better job.
Sync sounds good on paper. In reality it is hard to do. You radios
that "growl" at you. If the lock isn't perfect, you get low frequency
grunge. You also can get a slight frequency offset (translation).
You might want to go to rec.radio.shortwave. Lots of medium wave
listeners there too. Pretty much you will get the same answer. There
is some guy there who has multiple Wellbrook loops. He uses a phaser
(MFJ I think) to use the 2nd loop to null transmitters on the same
frequency. However, that doesn't sound like your issue.

Actually, it's a smoking crater infested with right wing zombies.  But
there should be a lot of useful stuff in the archive.  Ron Hardin? is
(I think) the guy with a yard full of fancy antennas and noise phaser
boxes.  Another poster to look up is Pete Gianopolus [or something like
that] who was working on a very high performance medium wave receiver
last year or two.

Mark Zenier  [email protected]  
Googleproofaddress(account:mzenier provider:eskimo domain:com)
 
[....]
Here is the paper:http://www.kongsfjord.no/dl/Audio/ELPAFs.pdf
[....]
Then look at that filter. I mean serious, discrete LCR with
questionable source and load impedance?
Wow, he is putting a large capacitor across the audio amplifier
output.  That should sound *interesting*
I know. He should have used the dual of the circuit. Series L rather
than parallel C. Even then, the source impedance isn't known.

A simple buffer amplifier would make the impedance known.  If I was
betting on what it is, I'd bet under and Ohm for the impedance of even
a modest amplifier.
I never
bothered to see if the component values are correct.

I stopped at the first part I saw.
The whole
implementation is stupid enough that I didn't believe checking the
design was worth the effort.
I have little faith in Mr. Lankford's theories, but at least it keeps
him off the streets. Granted, it sounds like he did use some cheap
synchronous demod radios since he complained about the growl. As I
said,  sync is hard to do well in practice. Drake wouldn't have made
the B rev if the sync in the older model didn't suck. The AR7030
probably had the best off the shelf synchro for a consumer radio, but
even the designer complained about it.
I never own a Sherwood sync demod, so I can't comment on the quality.
In fact, I never met anyone that owned one. However, Sherwood does
test a lot of radios and is considered well in the industry.

It begs the question why he didn't use an active filter. At those
frequencies, the quality would be better. In any event, the DSP boxes
can bandlimit just fine. Clearly, this paper is Lankford at his worse.
 
M

MooseFET

Jan 1, 1970
0
[....]
Here is the paper:http://www.kongsfjord.no/dl/Audio/ELPAFs.pdf
[....]
Then look at that filter. I mean serious, discrete LCR with
questionable source and load impedance?
Wow, he is putting a large capacitor across the audio amplifier
output.  That should sound *interesting*
I know. He should have used the dual of the circuit. Series L rather
than parallel C. Even then, the source impedance isn't known.
A simple buffer amplifier would make the impedance known.  If I was
betting on what it is, I'd bet under and Ohm for the impedance of even
a modest amplifier.
I stopped at the first part I saw.

It begs the question why he didn't use an active filter.

I forget who said it but: When stupidity will serve as an explanation,
you need look no further.
At those
frequencies, the quality would be better. In any event, the DSP boxes
can bandlimit just fine. Clearly, this paper is Lankford >at his worse.

At communications quality sound, a micro controller can make the
filter if you want a low cost DSP.

If you have signal with all these problems we aren't talking HiFi.

You could also build a radio with a very narrow IF strip and clean the
signal up a lot before you have to demodulate it.
 
J

JosephKK

Jan 1, 1970
0
You are missing the point. You can do one frequency, two frequencies,
three frequencies ... an infinite number of frequencies.



I am keeping it simple. To get the phases just insert another
constant in each component to adjust the phase. I thought that this
would be obvious but I guess not so here it is with the objection
fixed.

Y = (sin(At+C) + sin(Bt+D))*sin(wt) + (cos(At+C) + cos(Bt+D))*cos(wt)

Do you now agree that any collection of side bands can be made as I
suggested?

Once you see that point, you then need to rearrange the terms. The
fn1 and fn2 were not for the two side bands as you seem to keep
assuming. They are two function that together compose the total
noise.

Both of you seem to be talking as the noise arriving before the
modulator. It is not. It is between the tx antenna and the rx
antenna. It is not just Gaussian white noise, it include many impulse
sources, single frequency transients, harmonic series from many
sources, and even other tx on the same (and adjacent) frequency
sources. There are frequency domain models for each of these sources.
There is even a tractable model for the typical aggregate spectrum for
medium wave and short wave for 500 kHz to 30 MHz and slices thereof.
 
[....]
Here is the paper:http://www.kongsfjord.no/dl/Audio/ELPAFs.pdf
[....]
Then look at that filter. I mean serious, discrete LCR with
questionable source and load impedance?
Wow, he is putting a large capacitor across the audio amplifier
output. That should sound *interesting*
I know. He should have used the dual of the circuit. Series L rather
than parallel C. Even then, the source impedance isn't known.
A simple buffer amplifier would make the impedance known. If I was
betting on what it is, I'd bet under and Ohm for the impedance of even
a modest amplifier.
I never
bothered to see if the component values are correct.
I stopped at the first part I saw.
The whole
implementation is stupid enough that I didn't believe checking the
design was worth the effort.
I have little faith in Mr. Lankford's theories, but at least it keeps
him off the streets. Granted, it sounds like he did use some cheap
synchronous demod radios since he complained about the growl. As I
said, sync is hard to do well in practice. Drake wouldn't have made
the B rev if the sync in the older model didn't suck. The AR7030
probably had the best off the shelf synchro for a consumer radio, but
even the designer complained about it.
See the bottom of this page:http://www.aoruk.com/archive/pdf/sync.pdf
Some other interesting reading about the design:http://www.aoruk.com/archive/comments.htmhttp://www.aoruk.com/archive...
I never own a Sherwood sync demod, so I can't comment on the quality.
In fact, I never met anyone that owned one. However, Sherwood does
test a lot of radios and is considered well in the industry.
It begs the question why he didn't use an active filter.

I forget who said it but: When stupidity will serve as an explanation,
you need look no further.
At those
frequencies, the quality would be better. In any event, the DSP boxes
can bandlimit just fine. Clearly, this paper is Lankford >at his worse.

At communications quality sound, a micro controller can make the
filter if you want a low cost DSP.

If you have signal with all these problems we aren't talking HiFi.

You could also build a radio with a very narrow IF strip and clean the
signal up a lot before you have to demodulate it.

Unless I'm being paid, I just buy COTS. There are exceptions, such as
if the product is extremely expsensive or doesn't even exist.

In the case of Mr. Lankford, I could just select a tighter filter in
my radio. Of course, that doesn't do much for fading.
 
M

MooseFET

Jan 1, 1970
0
Both of you seem to be talking as the noise arriving before the
modulator.  It is not.  It is between the tx antenna and the rx
antenna.

No we (or at least I am) talking about noise that gets added in the
channel. The problem is that you have to do math on the stuff and
relate it all to the carrier. Since how the noise got created doesn't
really matter, it is best to call it all "side bands". We are talking
about the signal in the IF strip heading towards either a 1N914 or a
sync demodulator. By time you get there, the bandwidth is quite
narrow.

 It is not just Gaussian white noise, it include many impulse
sources, single frequency transients, harmonic series from many
sources, and even other tx on the same (and adjacent) frequency
sources.

I am lumping it all together and doing an FT on it. I am also taking
a very long time frame so that all the different clicks and pops get
included so that we can assume every frequency and phase will get in
there some time. On this basis, I say it is just "random".

We can take any random function and break it into two functions that
add together to make it. For my explanation I suggested that it be
broken into one part that gets through the sync demodulator and the
other part that doesn't. This requires that it be recast into things
related to the carrier of the station being listened to. The whole
sub thread assumes that we have a PLL with a very narrow band width so
that it has a VCO with no noise in it.
 There are frequency domain models for each of these sources.
There is even a tractable model for the typical aggregate spectrum for
medium wave and short wave for 500 kHz to 30 MHz and slices thereof.

Remember that the OP said normal AM radio. We are dealing with a
normal AM radio design up to the point of demodulating the signal to
make the audio. We have no noise near the harmonics to worry about
etc.
 
No we (or at least I am) talking about noise that gets added in the
channel. The problem is that you have to do math on the stuff and
relate it all to the carrier.

No,it doesn't. It is a conversion to make some of the math easier.
Since how the noise got created doesn't
really matter, it is best to call it all "side bands". We are talking
XXXX->easier ^When we
about the signal in the IF strip heading towards either a 1N914 or a
sync demodulator. By time you get there, the bandwidth is quite
narrow.
Depends on the particular definition of narrow.
I am lumping it all together and doing an FT on it. I am also taking
a very long time frame so that all the different clicks and pops get
included so that we can assume every frequency and phase will get in
there some time. On this basis, I say it is just "random".

The math for two sine modulation is not much harder than that for
single sine modulation. Then by induction you may stipulate that
modulating functions of the form f(t)=Summation(from n=1 to 20000;
a[n](t) * sin(nwT=t) + b[n](t) * cos(nwt)) will analyze just the same
as for two sines.
We can take any random function and break it into two functions that
add together to make it. For my explanation I suggested that it be
broken into one part that gets through the sync demodulator and the
other part that doesn't. This requires that it be recast into things
related to the carrier of the station being listened to. The whole
sub thread assumes that we have a PLL with a very narrow band width so
that it has a VCO with no noise in it.


Remember that the OP said normal AM radio. We are dealing with a
normal AM radio design up to the point of demodulating the signal to
make the audio. We have no noise near the harmonics to worry about
etc.

Fine. Thus we are typically discussing single superhetrodynes with
455 kHz or 250 kHz IF strip and AGC. LO may be variable LC or synth.

For the add on a standard PLL at 4 * fc of IF for 455 kHz and doing
I&Q synchronous has some interesting advantages. We can easily have
each sideband demodulated separately and select either one or the sum.


Mit der Dummheit kaempfen Goetter selbst vergebens.
--Schiller (from "Die Jungfrau von Orleans")
 
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