Back in the 90s, I designed a receiver that used a pair of 1350s in the I.F.
strip. I could not find a suitable AGC circuit, so I designed my own.
A 160 millisecond time constant is low enough for your low-pass filter. This
will prevent bass guitar frequencies from modulating the AGC bus. Next, make
sure that you use filtering ahead of your first I.F. amplifier....if you
don't, you will not only have broadband noise from your I.F. strip, but
strong out of bandpass signals will capture you AGC system.
The manifestation will occur by having strong stations as far as 100kHz away
causing desense of your receiver.
I actually perfected this complicated design........then, I discovered the
Philips TDA1572. This chip offers a much better solution, since it has
on-board AGC, low distortion demodulator, etc. The AOR7030 uses this chip as
the 2nd I.F./AGC/demodulator. If you have ever listened to one of these
receivers, you will have an idea of the capability of this chip.
The only difficult thing about the 1572 is that it is almost impossible to
get a fundamental crystal to oscillate on the proper frequency when using
the on-board oscillator. The oscillator is great for a conventional LC
oscillator, since this chip was really designed to be used as an AM radio on
a chip. Another nice thing is the buffered 50 Ohm I.F. output, which you can
use to drive a sync detector, product detector, etc.
E-mail me directly, and I will be glad to send you design information for
both of these chips. I've been meaning to get my website up again, and
repost these designs, but that won't be for awhile. Have fun..........that
is the best part of homebrewing!

Pete

"N9NEO" wrote in message
...
Greetings to my friends at RRS.

I am working on a new pc board. I actually have gone out for boards
and got them working, but I've had to rethink one of the circuits.

In a nutshell the board is a result of the work published by Dallas
Lankford on his investigations into audio elliptic filters. Mr
Lankford uses these filters to reduce the distortion resulting from
fading - fading distortion. Terry had brought up the paper here a
while back and it's taken me a long while to do anything with it.

The board has a MC1350 Gain Controlled IF Amplifier followed by a
precision detector, tone control, audio amplifier, elliptic filter,
and a final amplifier to drive a speaker. There is also a dedicated
amplifier to provide some type of AGC action. I'm not quite sure of
the requirements necessary in order to provide a decent AGC. I'm just
trying to smooth out the audio fading some. My plan was to just put a
low pass filter with a cut-off frequency of a couple of hertz to
modulate the MC1350 amplifier. I intend to drive this thing with my
Red Sun RP2100 radio.

My friend pointed out that the noise would be amplified as well so
that I would need to devise a circuit that would not respond to
anything under a few uV. My answer is to have a switch and switch in
the agc when I got the station half-ass tuned in - HATI. Circuit has a
lot of hooks and empty board space so I can jam a little bit of stuff
in there to accomplish a basic agc. I've run through the rest of the
board and all of the other circuits are good to go.

So the point here is that I'm in a little bit over my head. I would
like to have some discussion with you brainiacs about AGC and maybe
some links to circuits or comments on radios that have had good agc
action. If you think I'm ****ing in the wind with an audio controlled
AGC circuit then I would like to hear that as well.

regards,
Bob
N9NEO
(just say 'NEO')

On Mar 16, 2:01*am, "Pete KE9OA" wrote:
Actually, you do need AGC when using an MC1350. Each stage provides about
60dB of gain at 455kHz. If you don't have some sort of gain control on the
stage, you will drive it into clipping. Been there, done that. My first
attempt at a sync detector was a quasi-sync detector, similar the the
implementation Drake uses with their "Synchro Phase" detector, and similar
to the design used for AM recovery in the Racal 6790.
It used a 1350 as a limiting amplifier that was driven into clipping by the
radio's I.F. strip. This chip squared up the signal for the LO input of a
Philips SA-637 digital FM radio chip. The point of this is that I
intentionally let the MC1350 run at full gain, so that I could run it into
clipping, thus, squaring up the signal.
Now, about that time constant...................true, that 6Hz is the -3dB
corner frequency, but this is fine for attack/decay characteristics. In the
real world, there is no problem with modulation of the AGC bus. Ten times
this time constant would be way to long of a decay time.........we are
talking about 1.66 seconds here. 500 milliseconds would be better for a
release time for AM signals.
A hang AGC is fine for SSB signals, but for AM full carrier signals, it
would be very inconvenient. As far as cascaded AGC, there can be oscillation
problems of the control system if you are not careful when designing it. You
could grab the AGC line from the radio that you would be using, and
scale/translate the levels with a couple of op-amps. The MC1350 requires a
5V knee point at Pin5 that your AGC voltage is superpositioned over. As I
stated before, the AGC system for a 1350 is fairly complex; that is the
reason that I suggested a TDA1572. The 1572 is a much better chip for this
purpose, since it has its own AGC/demodulator system on board. In addition
to that, a few external components can be used for independant attack/decay
parameters. If you don't need SSB demodulation, a TDA1072 has the same
functionallity, minus the buffered 50 Ohm I.F. output.
The 160 millisecond time constant was mentioned in Ulrich Rohde's book,
entitled "Communications Receivers, Principles and Design". I didn't pull
that figure out of a hat.
You do have some good suggestions, Tom.

Pete

"You don't need another AGC if the Redsun AGC is controlling its IF
output, do you? Cascaded AGC's might be a problem. Maybe the Redsun
AGC is poor and you will want to replace it with an outboard one. AGC
can cause audio distortion if it is fast enough to chase the
modulation. Pete's mention of 160mS AGC must be referring to the
attack time constant or speed. A release time constant of about 10
times that is advisable to keep modulation tracking very low. If it
was symmetrical 160ms attack/release, then it would do a very good job
tracking 6Hz modulation (a rarity) but would still have a 10%
sensitivity to 60Hz and 1% at 600Hz. Then there is the concept of
'hang' AGC, where the release is delayed, maintaining constant gain
through the delay unless a greater attack is enountered that requires
a gain reduction and a new 'hang' delay. This is particularly
effective at avoiding modulation tracking if the hang time is equal or
greater than the attack time and allows a faster release without
jeopardising modulation tracking distortion.

I don't know whether there is any big advantage between audio derived
and IF derived AGC as long as there is modulation. You mention talk
radio so I assume that is DSBAM with carrier. The IF derived AGC will
output an AGC voltage affected by carrier strength and to some extent
by modulation so it will maintain an appropriate gain even during long
pauses in modulation. An audio derived AGC would increase IF gain
during silence and thus might permit peak distortion or clipping of
the next audio, especially if it begins with a plosive. Were this
suppressed carrier AM, then both would increase gain during pauses.

You would probably get more informed opinion than I can offer if you
were to post to rec.radio.amateur.homebrew.

Good luck with your project!"

Tom

I agree with Telamon about the two release speeds for speech
programming: the long release for fairly stable signals and the
shorter for rapid fading. The longer one gives lower distortion due to
modulation tracking. Under really stable conditions (local
groundwave), lowest distortion would be obtained by turning AGC off
and setting the RF Gain manually.

Pete, given that the Redsun already has AGC, why couldn't the 1350 be
manually set to an appropriate gain to match the controlled level
coming out of the tuner?

I have done a fair bit of investigation and modification of AGC
circuits in the Radio Shack DX-394 and the Yaesu FT-817 and been
surprised by the experience. Did a lot of research on published specs
or test reports for high-end radios and other articles. There seemed
to be a rough consensus about release speeds corresponding roughly to
what Telamon cites for speech/music; for interrupted CW and for data
modes, below 25ms seemed to be the desired speed. What was more
confusing or contentious is the attack - some radios citing sub 1ms!
Moreover, good audio compressor/limiters also have very fast attack to
prevent peak clipping and broadcast Peak Program Meters integrate over
10ms, effectively ignoring peaks very much shorter in duration as
these are psychoacoustically inaudible. I modified the AGC for the
DX-394 with sub 10ms attack for all release speeds and then was
dismayed with the effect of impulse noise - long holes in audible
modulation or even continuous suppression if the repetition rate was
high enough. Unless there is an IF noise blanker ahead of the AGC
detector, an attack that is proportional to the release is necessary,
something on the order of 5-10%.

With the FT-817, I found that the effective AGC speed is inversely
related to the RG Gain control setting and is very much faster than
the RC time constant would suggest. Pete, you mention a 'knee' with
the 1350 around 5V - I think the FT-817 has a knee around 1V and
follows a square law characteristic such that most gain variation
occurs over maybe a 100mV slice out of the 3V range over which the AGC
and RF Gain control swing. Hence, I have been unable to eliminate what
I think is quite objectionable distortion due to modulation tracking,
even with a release RC time constant on the order of 2.5s and an
attack RC time constant pushed up from a few ms to over 100ms.
Modulation wiggling the AGC by a millivolt distorts the IF envelope
delivered to the AM detector. Listening to DSBAM in ECSS mode (SSB)
significantly reduces distortion because the filter skirt suppresses
bass energy.

Tom

I agree...........longer release times are fine, as long as they don't get
too long for AM DSB mode. For SSB reception with a product detector, a long
hang time is fine.
As for the MC1350 device, I wouldn't push more than 30uV into that device if
it doesn't have AGC. You could ride the gain manually, but what is the point
of installing a device that has 50dB additional gain if you are going to set
it down to 10dB of gain so that it doesn't run into clipping. I
know.......both Ten Tec and at least one other company were using that chip
as an I.F. amplifier, but better results can be had with the Philips
TDA1072, TDA1572, JRC NJM2552, Analog Devices AD600, and AD607.
The 1350 has quite a bit of gain, and it makes a great limiting amplifier.
Even though some manufacturers have implemented this device with an
unbalanced output circuit, it really works better if you feed the V+ through
a center tapped I.F. transformer. The Toko RMC502-182NO is perfect for this
application. I ended up modifying my old Ten Tec receiver in this manner,
and it picked up quite a bit more gain. This chip has a 15 to 18dB noise
figure, which means that you need to have quite a bit of gain in the front
end to override the noise in the I.F. strip.
A perfect example of what happens to this device can be illustrated with
some of the measurement receivers, such as the Sierra 128A or some of the
Siemens measurement receivers. These units don't have AGC, and they will
overload on relatively weak signals. When properly implemented, the MC1350
is a great device........it's just that there are better devices available
today. You can even use an FM demodulator chip for listening to AM signals.
There was an article in RF Design magazine back in the mod 90s that used an
antilog amp to linearize the RSSI output of the chip, and demodulate AM
signals in this manner.
I was actually going to take that route, but then I discovered some of the
other chips that were suited for this purpose. Somewhere, I have an article
that uses some sort of CMOS switch to demodulate AM signals. This
demodulator has around 70 or 80dB of dynamic range, so AGC isn't needed with
this type of demodulator. I have to dig it up and scan it one of these days.
Telamon is right on the money about those decay times...........the 160
millisecond time constant is ok, but longer times do tend to give a smoother
sound to the demodulated audio.
I hope this helps.

Pete

"Tom" wrote in message
...
On Mar 16, 2:01 am, "Pete KE9OA" wrote:
Actually, you do need AGC when using an MC1350. Each stage provides about
60dB of gain at 455kHz. If you don't have some sort of gain control on the
stage, you will drive it into clipping. Been there, done that. My first
attempt at a sync detector was a quasi-sync detector, similar the the
implementation Drake uses with their "Synchro Phase" detector, and similar
to the design used for AM recovery in the Racal 6790.
It used a 1350 as a limiting amplifier that was driven into clipping by
the
radio's I.F. strip. This chip squared up the signal for the LO input of a
Philips SA-637 digital FM radio chip. The point of this is that I
intentionally let the MC1350 run at full gain, so that I could run it into
clipping, thus, squaring up the signal.
Now, about that time constant...................true, that 6Hz is the -3dB
corner frequency, but this is fine for attack/decay characteristics. In
the
real world, there is no problem with modulation of the AGC bus. Ten times
this time constant would be way to long of a decay time.........we are
talking about 1.66 seconds here. 500 milliseconds would be better for a
release time for AM signals.
A hang AGC is fine for SSB signals, but for AM full carrier signals, it
would be very inconvenient. As far as cascaded AGC, there can be
oscillation
problems of the control system if you are not careful when designing it.
You
could grab the AGC line from the radio that you would be using, and
scale/translate the levels with a couple of op-amps. The MC1350 requires a
5V knee point at Pin5 that your AGC voltage is superpositioned over. As I
stated before, the AGC system for a 1350 is fairly complex; that is the
reason that I suggested a TDA1572. The 1572 is a much better chip for this
purpose, since it has its own AGC/demodulator system on board. In addition
to that, a few external components can be used for independant
attack/decay
parameters. If you don't need SSB demodulation, a TDA1072 has the same
functionallity, minus the buffered 50 Ohm I.F. output.
The 160 millisecond time constant was mentioned in Ulrich Rohde's book,
entitled "Communications Receivers, Principles and Design". I didn't pull
that figure out of a hat.
You do have some good suggestions, Tom.

Pete

"You don't need another AGC if the Redsun AGC is controlling its IF
output, do you? Cascaded AGC's might be a problem. Maybe the Redsun
AGC is poor and you will want to replace it with an outboard one. AGC
can cause audio distortion if it is fast enough to chase the
modulation. Pete's mention of 160mS AGC must be referring to the
attack time constant or speed. A release time constant of about 10
times that is advisable to keep modulation tracking very low. If it
was symmetrical 160ms attack/release, then it would do a very good job
tracking 6Hz modulation (a rarity) but would still have a 10%
sensitivity to 60Hz and 1% at 600Hz. Then there is the concept of
'hang' AGC, where the release is delayed, maintaining constant gain
through the delay unless a greater attack is enountered that requires
a gain reduction and a new 'hang' delay. This is particularly
effective at avoiding modulation tracking if the hang time is equal or
greater than the attack time and allows a faster release without
jeopardising modulation tracking distortion.

I don't know whether there is any big advantage between audio derived
and IF derived AGC as long as there is modulation. You mention talk
radio so I assume that is DSBAM with carrier. The IF derived AGC will
output an AGC voltage affected by carrier strength and to some extent
by modulation so it will maintain an appropriate gain even during long
pauses in modulation. An audio derived AGC would increase IF gain
during silence and thus might permit peak distortion or clipping of
the next audio, especially if it begins with a plosive. Were this
suppressed carrier AM, then both would increase gain during pauses.

You would probably get more informed opinion than I can offer if you
were to post to rec.radio.amateur.homebrew.

Good luck with your project!"

Tom

I agree with Telamon about the two release speeds for speech
programming: the long release for fairly stable signals and the
shorter for rapid fading. The longer one gives lower distortion due to
modulation tracking. Under really stable conditions (local
groundwave), lowest distortion would be obtained by turning AGC off
and setting the RF Gain manually.

Pete, given that the Redsun already has AGC, why couldn't the 1350 be
manually set to an appropriate gain to match the controlled level
coming out of the tuner?

I have done a fair bit of investigation and modification of AGC
circuits in the Radio Shack DX-394 and the Yaesu FT-817 and been
surprised by the experience. Did a lot of research on published specs
or test reports for high-end radios and other articles. There seemed
to be a rough consensus about release speeds corresponding roughly to
what Telamon cites for speech/music; for interrupted CW and for data
modes, below 25ms seemed to be the desired speed. What was more
confusing or contentious is the attack - some radios citing sub 1ms!
Moreover, good audio compressor/limiters also have very fast attack to
prevent peak clipping and broadcast Peak Program Meters integrate over
10ms, effectively ignoring peaks very much shorter in duration as
these are psychoacoustically inaudible. I modified the AGC for the
DX-394 with sub 10ms attack for all release speeds and then was
dismayed with the effect of impulse noise - long holes in audible
modulation or even continuous suppression if the repetition rate was
high enough. Unless there is an IF noise blanker ahead of the AGC
detector, an attack that is proportional to the release is necessary,
something on the order of 5-10%.

With the FT-817, I found that the effective AGC speed is inversely
related to the RG Gain control setting and is very much faster than
the RC time constant would suggest. Pete, you mention a 'knee' with
the 1350 around 5V - I think the FT-817 has a knee around 1V and
follows a square law characteristic such that most gain variation
occurs over maybe a 100mV slice out of the 3V range over which the AGC
and RF Gain control swing. Hence, I have been unable to eliminate what
I think is quite objectionable distortion due to modulation tracking,
even with a release RC time constant on the order of 2.5s and an
attack RC time constant pushed up from a few ms to over 100ms.
Modulation wiggling the AGC by a millivolt distorts the IF envelope
delivered to the AM detector. Listening to DSBAM in ECSS mode (SSB)
significantly reduces distortion because the filter skirt suppresses
bass energy.

Tom

SSB has always Sucked.Can't understand anything they say on SSB.Besides,
who cares anything about how to fry eggs in a cook stove pan anyway.I
called those Alanama Dumbasses then, I still call them Alabama Dumbasses
now.(but not really though)

www.devilfinder.com Alabama State Song
cuhulin

Pete KE9OA wrote:
That sounds good........................since you are doing your own PC
board layouts, I can also send you a product detector schematic that uses an
NE602, and a sync detector schematic that uses an Analog Devices AD607.

I'd love to see those myself.

--
Iran tells us what the mainstream media won't:
"A new opinion poll suggests that over 54 percent of Americans do not trust
mainstream media and consider news websites more reliable."
http://www.presstv.ir/detail.aspx?id=46837&sectionid=3510203

Britney Spears has a new 2008 SLK (something like that) white Mercedes
Benz car.Can you buy a new fancy Mercedes Benz car every year?
www.devilfinder.com Kentwood Louisiana
cuhulin

In article ,
wrote:

SSB has always Sucked.Can't understand anything they say on SSB.Besides,
who cares anything about how to fry eggs in a cook stove pan anyway.I
called those Alanama Dumbasses then, I still call them Alabama Dumbasses
now.(but not really though)

OK nice try. Any time you have a feed back loop it is negative so you
have control. Positive feedback would cause the amplifier to the maximum
output at all times or in other words would be out of control like Ms.
Spears.

--
Telamon
Ventura, California

Local Jackson tv news.Sahhing pants thingy.$25.00 to $100.00 fine and up
to eight hours communiity service.

I would make their familys pay Ten Thousand dollars for the Bullet.
cuhulin

Sometimes, Britney Spears goes to Nyla's (spelling) Burger Basket in
Osyka,Mississippi for some real good burgers.Much better than anything
the rest of the World puts out.
www.devilfinder.com Nyla's Burger Basket Osyka Mississippi

Dont feel sorry for me, California boy.
cuhulin