Topics

Confused about how AGC works?

digger AB3XU
 

Hello All,

(sorry if this topic has been hashed to death elsewhere, if so I couldn't find it).

As the subject says I'm a bit confused about how AGC, and specifically the ND6T version, works. There must be something I'm missing because it doesn't make sense. Here's a link to the basic circuit and how it works.


To begin with I can follow along with the theory and it does makes sense. Very simply, the signal from Vol-Hi via Q1 is used to control the conduction of Q2 and how much Q3 shunts the IF signal. Ideally RF levels below S9 are left relatively untouched, however if greater than that, the action of Q2 conducting less and Q3 acting as a shunt  results in reducing clamping (reducing) the IF level.

So here's where I get lost.  As the IF level gets attenuated by the action of the AGC doesn't that in turn:
  1. attenuate the level you will see at Vol-HI
  2. which will then decreases the effect of the AGC
  3. leading to an increased IF signal level
  4. and increasing Vol-HI
  5. which then increases the AGC's effect
  6. reducing the IF signal level
  7. resulting in a feedback loop of the IF level going up and down...
Am I describing my confusion/question in a way that makes sense?

73, digger AB3XU

Curt
 

check this out - this is a similar version of the same circuit

http://www.nd6t.com/bitx/AGC

the main idea is sampling the audio, feeding the transistor to boost this signal.  the 2 diodes develop a rectified signal based upon the audio, and this is used to bias a MOSFET used as a variable resistor. 

Curt

Jerry Gaffke
 

A poorly designed AGC circuit could oscillate exactly as you describe.
Whether or not it will oscillate depends on the delays through each element
and the gain of each element.  Both can vary depending on the frequency
at which they are calculated. 
    https://en.wikipedia.org/wiki/Barkhausen_stability_criterion

Jerry, KE7ER



On Thu, Apr 18, 2019 at 06:00 PM, digger AB3XU wrote:
To begin with I can follow along with the theory and it does makes sense. Very simply, the signal from Vol-Hi via Q1 is used to control the conduction of Q2 and how much Q3 shunts the IF signal. Ideally RF levels below S9 are left relatively untouched, however if greater than that, the action of Q2 conducting less and Q3 acting as a shunt  results in reducing clamping (reducing) the IF level.

So here's where I get lost.  As the IF level gets attenuated by the action of the AGC doesn't that in turn:
  1. attenuate the level you will see at Vol-HI
  2. which will then decreases the effect of the AGC
  3. leading to an increased IF signal level
  4. and increasing Vol-HI
  5. which then increases the AGC's effect
  6. reducing the IF signal level
  7. resulting in a feedback loop of the IF level going up and down...
Am I describing my confusion/question in a way that makes sense?

73, digger AB3XU

Don, ND6T
 

Hi Digger,

Often the simplest of things are revealed as complex. As Jerry says, the delays are important. The values used allows the attenuation to smoothly increase and track the signal level variations such that the response is fast enough to produce a pleasant result but not so fast that it will oscillate. Your confusion may be that you perceive that the end result is a tightly controlled output. It is not. Stronger signals result in slightly stronger audio levels. A S9+40 dB signal will be a bit louder than a S9 signal, just not horribly so. That increase is designed to be slight and gradual. This simple circuit is almost (but not quite) linear in that respect. Fortuitously, the shunt resistance takes over as the series resistance nears maximum conduction and it is a nice, gentle, transition. Close but not perfect. Good enough.

To get a better AGC, especially one that tames the initial burst, you would need a much more complex circuit. The better ones include an additional detector and multiple additional gain control points throughout the RF and IF amplifiers. If we had a carrier to use as a signal strength reference then it would be easier. Without the carrier we have to rely on the modulated signal. Tricky.

I use a compromise circuit. Even then, it seems that it is so complex that the majority of builders need kits or completed boards. This circuit is the best that I can do (so far) in keeping it simple yet effective.

Did this help?
73, Don

Jerry Gaffke
 

Here's my shot at a quick summary regarding AGC circuits for the Bitx40 and uBitx
Opinions may vary.

When tuning across the band with a stock radio, you might run across a weak station
and turn the volume up to hear it.  Then a local QRO station responds and
you have to rip the headphones off to protect your ears, or be very quick in reaching
for that volume control.  An AGC circuit (automatic gain control)  turns the volume down
for you when a strong station shows up like that.

This is an early version of Don's (ND6T) AGC circuit:
    https://groups.io/g/BITX20/message/68037
Still viable, works well enough, simple and easy to build from scratch.
Really strong local signals can still be annoyingly loud, though much less so.

Don refined that FET shunt attenuator design by adding a FET series attenuator
to get additional dynamic range, keeps even the strongest stations down in volume.
    http://www.nd6t.com/uBITX/AGC.htm
It is a bit more complicated, but performs better.
Was kitted up by Kees (no longer available),
and is now available from  https://shop.kit-projects.com/

Either of the above AGC circuits will work on the Bitx40 or the uBitx.


Don's design attenuates the RF signal as it comes in from the antenna,
using a control signal derived from the audio. 

Most of the other AGC designs presented in this forum (there have been a
half dozen or so) attenuate the audio after the audio pre-amp, simple and
works well enough to protect the ears, and you avoid the tricky business of 
attenuating the RF path.  Here's a very early example of this approach:
    http://bitxhacks.blogspot.com/2016/11/agc-for-bitx.html

Attenuating the RF path is preferred, since the stock receiver dynamic range
is limited primarily by that audio pre-amp, and also to some extent by the IF amps
and mixers.  Don's design reduces the signal right where it comes in from the
antenna jack, so nothing in the radio will get overloaded and cause distortion.  

So an AGC design with attenuation in the audio stage works to protect the ears,
but strong signals may be distorted enough to be unintelligible.  With RF 
attenuation, strong signals remain undistorted.

All successful AGC circuit designs presented in the forum thus far detect the
incoming signal level just before the volume control.  Signal detection must occur
after the 12mhz crystal filter, otherwise we would also detect strong nearby stations
that the crystal filter rejects.  Detection of audio is not quite ideal since it takes time for
the signal to arrive there, so there will be a brief pop when a strong signal shows up
before it is properly attenuated.  But detection of 12mhz RF between the crystal filter and the
demodulator is difficult, because the unshielded 12mhz BFO on CLK0 will sneak into
our AGC signal detector and look like a very strong signal.

Here's an example of a well regarded AGC scheme for a high performance receiver:
    http://www.ka7exm.net/hycas/hycas_200712_qst.pdf
It's about as complicated as an entire uBitx receiver.

Jerry, KE7ER



On Thu, Apr 18, 2019 at 06:43 PM, Curt wrote:
check this out - this is a similar version of the same circuit

http://www.nd6t.com/bitx/AGC

the main idea is sampling the audio, feeding the transistor to boost this signal.  the 2 diodes develop a rectified signal based upon the audio, and this is used to bias a MOSFET used as a variable resistor. 

MadRadioModder
 

As discussed before…

 

“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”

 

That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.

 

MRM

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, April 19, 2019 11:14 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Confused about how AGC works?

 

Here's my shot at a quick summary regarding AGC circuits for the Bitx40 and uBitx
Opinions may vary.

When tuning across the band with a stock radio, you might run across a weak station
and turn the volume up to hear it.  Then a local QRO station responds and
you have to rip the headphones off to protect your ears, or be very quick in reaching
for that volume control.  An AGC circuit (automatic gain control)  turns the volume down
for you when a strong station shows up like that.

This is an early version of Don's (ND6T) AGC circuit:
    https://groups.io/g/BITX20/message/68037
Still viable, works well enough, simple and easy to build from scratch.
Really strong local signals can still be annoyingly loud, though much less so.

Don refined that FET shunt attenuator design by adding a FET series attenuator
to get additional dynamic range, keeps even the strongest stations down in volume.
    http://www.nd6t.com/uBITX/AGC.htm
It is a bit more complicated, but performs better.
Was kitted up by Kees (no longer available),
and is now available from  https://shop.kit-projects.com/

Either of the above AGC circuits will work on the Bitx40 or the uBitx.


Don's design attenuates the RF signal as it comes in from the antenna,
using a control signal derived from the audio. 

Most of the other AGC designs presented in this forum (there have been a
half dozen or so) attenuate the audio after the audio pre-amp, simple and
works well enough to protect the ears, and you avoid the tricky business of 
attenuating the RF path.  Here's a very early example of this approach:
    http://bitxhacks.blogspot.com/2016/11/agc-for-bitx.html

Attenuating the RF path is preferred, since the stock receiver dynamic range
is limited primarily by that audio pre-amp, and also to some extent by the IF amps
and mixers.  Don's design reduces the signal right where it comes in from the
antenna jack, so nothing in the radio will get overloaded and cause distortion.  

So an AGC design with attenuation in the audio stage works to protect the ears,
but strong signals may be distorted enough to be unintelligible.  With RF 
attenuation, strong signals remain undistorted.

All successful AGC circuit designs presented in the forum thus far detect the
incoming signal level just before the volume control.  Signal detection must occur
after the 12mhz crystal filter, otherwise we would also detect strong nearby stations
that the crystal filter rejects.  Detection of audio is not quite ideal since it takes time for
the signal to arrive there, so there will be a brief pop when a strong signal shows up
before it is properly attenuated.  But detection of 12mhz RF between the crystal filter and the
demodulator is difficult, because the unshielded 12mhz BFO on CLK0 will sneak into
our AGC signal detector and look like a very strong signal.

Here's an example of a well regarded AGC scheme for a high performance receiver:
    http://www.ka7exm.net/hycas/hycas_200712_qst.pdf
It's about as complicated as an entire uBitx receiver.

Jerry, KE7ER



On Thu, Apr 18, 2019 at 06:43 PM, Curt wrote:

check this out - this is a similar version of the same circuit

http://www.nd6t.com/bitx/AGC

the main idea is sampling the audio, feeding the transistor to boost this signal.  the 2 diodes develop a rectified signal based upon the audio, and this is used to bias a MOSFET used as a variable resistor. 


Virus-free. www.avg.com

--

…_. _._

Jack Purdum
 

Yep...seems to be a lot of confusion on this point...


Jack, W8TEE


On Friday, April 19, 2019, 2:21:43 PM EDT, MadRadioModder <madradiomodder@...> wrote:


As discussed before…

 

“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”

 

That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.

 

MRM

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, April 19, 2019 11:14 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Confused about how AGC works?

 

Here's my shot at a quick summary regarding AGC circuits for the Bitx40 and uBitx
Opinions may vary.

When tuning across the band with a stock radio, you might run across a weak station
and turn the volume up to hear it.  Then a local QRO station responds and
you have to rip the headphones off to protect your ears, or be very quick in reaching
for that volume control.  An AGC circuit (automatic gain control)  turns the volume down
for you when a strong station shows up like that.

This is an early version of Don's (ND6T) AGC circuit:
    https://groups.io/g/BITX20/message/68037
Still viable, works well enough, simple and easy to build from scratch.
Really strong local signals can still be annoyingly loud, though much less so.

Don refined that FET shunt attenuator design by adding a FET series attenuator
to get additional dynamic range, keeps even the strongest stations down in volume.
    http://www.nd6t.com/uBITX/AGC.htm
It is a bit more complicated, but performs better.
Was kitted up by Kees (no longer available),
and is now available from  https://shop.kit-projects.com/

Either of the above AGC circuits will work on the Bitx40 or the uBitx.


Don's design attenuates the RF signal as it comes in from the antenna,
using a control signal derived from the audio. 

Most of the other AGC designs presented in this forum (there have been a
half dozen or so) attenuate the audio after the audio pre-amp, simple and
works well enough to protect the ears, and you avoid the tricky business of 
attenuating the RF path.  Here's a very early example of this approach:
    http://bitxhacks.blogspot.com/2016/11/agc-for-bitx.html

Attenuating the RF path is preferred, since the stock receiver dynamic range
is limited primarily by that audio pre-amp, and also to some extent by the IF amps
and mixers.  Don's design reduces the signal right where it comes in from the
antenna jack, so nothing in the radio will get overloaded and cause distortion.  

So an AGC design with attenuation in the audio stage works to protect the ears,
but strong signals may be distorted enough to be unintelligible.  With RF 
attenuation, strong signals remain undistorted.

All successful AGC circuit designs presented in the forum thus far detect the
incoming signal level just before the volume control.  Signal detection must occur
after the 12mhz crystal filter, otherwise we would also detect strong nearby stations
that the crystal filter rejects.  Detection of audio is not quite ideal since it takes time for
the signal to arrive there, so there will be a brief pop when a strong signal shows up
before it is properly attenuated.  But detection of 12mhz RF between the crystal filter and the
demodulator is difficult, because the unshielded 12mhz BFO on CLK0 will sneak into
our AGC signal detector and look like a very strong signal.

Here's an example of a well regarded AGC scheme for a high performance receiver:
    http://www.ka7exm.net/hycas/hycas_200712_qst.pdf
It's about as complicated as an entire uBitx receiver.

Jerry, KE7ER



On Thu, Apr 18, 2019 at 06:43 PM, Curt wrote:

check this out - this is a similar version of the same circuit

http://www.nd6t.com/bitx/AGC

the main idea is sampling the audio, feeding the transistor to boost this signal.  the 2 diodes develop a rectified signal based upon the audio, and this is used to bias a MOSFET used as a variable resistor. 


Virus-free. www.avg.com

--

…_. _._

Jerry Gaffke
 

From 
    https://en.wikipedia.org/wiki/Automatic_gain_control
"Automatic gain control (AGC), also called automatic volume control (AVC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, ..."
A google search for "AGC vs AVC" shows they are considered synonomous in radio design (but not for power grids), and that AGC is the more modern term.

In this forum, all such circuits for receivers have been getting referred to as AGC.
For some reason, the term AVC has only been used in this forum for automatic level control while transmitting
(Except for these posts from MRM.)
Works well for me, as few of us care that much about AVC while transmitting.
I suggest we keep it that way. 


Anyways, regarding the various AGC schemes:

There is this AGC kit from Sunil, it detects audio, and uses that to somehow attenuate the signal in one of the IF amps.
    https://amateurradiokits.in/product/agc-kit-for-bitx-and-ubitx/
Unfortunately, nobody seems to know how to hook it up:
    https://groups.io/g/BITX20/topic/4809028#65586
I'm sure it's easy enough to resolve with a quick email to Sunil (he monitors this forum).
And is definitely a contender, especially if you are already ordering one of the excellent Bitx40/uBitx enclosures from Sunil.

There have been several other AGC kits made available.
Let us know if any are still active, other than Sunil's and the one from   https://shop.kit-projects.com/

An AGC scheme that attenuates audio is a perfectly fine solution.
As stated in my previous post, one that attenuates RF (or IF energy) instead of audio
can give better dynamic range, but is in some ways more complicated and error prone.
An audio only AGC scheme could be mounted on the back of the volume pot, with the only
additional wire required being one to supply power.  No worries about coax and such. 
If you never hear stations so strong that they are still distorted after you turn down the volume knob
on your stock uBitx, you don't need the extra dynamic range offered by an RF (or IF) attenuated AGC.

And if you occasionally do, that could be taken care of with a manual RF gain control,
which could be just a 1k pot in the received RF from the antenna.

Jerry, KE7ER



On Fri, Apr 19, 2019 at 11:21 AM, MadRadioModder wrote:

As discussed before…

 

“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”

 

That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.

 

MRM

Joe Puma
 

Jerry I have had great success in hooking up Sunil’s AGC. I could be the poster child. I even determined a better location to grab audio instead of Vol H which is on the relay side of the R70 resistor. 

I do admit I had a hard time figuring things out in the beginning because of my ‘I’m still learning skillset’ but also because the CD that came with my radio kit could not be read and I only got half of the images and info off of the disk.  Then Sunil sent me some links that helped. Tuning the AGC meter was tricky but just follow the instructions again. The AGC is pretty simple. Just hook up the attenuation where Sunil stated in his instructions and you should be good to go, tune the 2kohm variable. Actually you know what.  I put a 10k because I thought it wasn’t attenuating enough I even changed one of the resistors before the AGC pot to 15kohm more then what was there. I can get specifics if anyone needs I’m going off memory.  

I ended up using coax for my attenuation hookup as that area is sensitive on the board and I was getting RF in from that wire as well as my IF tap for SDR. 

 Here is a pic before the coax 

image1.png


Joe
KD2NFC 





On Apr 19, 2019, at 2:57 PM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

From 
    https://en.wikipedia.org/wiki/Automatic_gain_control
"Automatic gain control (AGC), also called automatic volume control (AVC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, ..."
A google search for "AGC vs AVC" shows they are considered synonomous in radio design (but not for power grids), and that AGC is the more modern term.

In this forum, all such circuits for receivers have been getting referred to as AGC.
For some reason, the term AVC has only been used in this forum for automatic level control while transmitting
(Except for these posts from MRM.)
Works well for me, as few of us care that much about AVC while transmitting.
I suggest we keep it that way. 


Anyways, regarding the various AGC schemes:

There is this AGC kit from Sunil, it detects audio, and uses that to somehow attenuate the signal in one of the IF amps.
    https://amateurradiokits.in/product/agc-kit-for-bitx-and-ubitx/
Unfortunately, nobody seems to know how to hook it up:
    https://groups.io/g/BITX20/topic/4809028#65586
I'm sure it's easy enough to resolve with a quick email to Sunil (he monitors this forum).
And is definitely a contender, especially if you are already ordering one of the excellent Bitx40/uBitx enclosures from Sunil.

There have been several other AGC kits made available.
Let us know if any are still active, other than Sunil's and the one from   https://shop.kit-projects.com/

An AGC scheme that attenuates audio is a perfectly fine solution.
As stated in my previous post, one that attenuates RF (or IF energy) instead of audio
can give better dynamic range, but is in some ways more complicated and error prone.
An audio only AGC scheme could be mounted on the back of the volume pot, with the only
additional wire required being one to supply power.  No worries about coax and such. 
If you never hear stations so strong that they are still distorted after you turn down the volume knob
on your stock uBitx, you don't need the extra dynamic range offered by an RF (or IF) attenuated AGC.

And if you occasionally do, that could be taken care of with a manual RF gain control,
which could be just a 1k pot in the received RF from the antenna.

Jerry, KE7ER



On Fri, Apr 19, 2019 at 11:21 AM, MadRadioModder wrote:

As discussed before…

 

“An AGC circuit (automatic gain control)  turns the volume down for you when a strong station shows up like that.”

 

That’s known as AVC or “Automatic Volume Control”… not necessarily AGC.

 

MRM

digger AB3XU
 

Thanks Don and Jerry for your explanations. In my OP I wasn't thinking about that what I described were oscillations, and as you wrote the parts are chosen to stabilize the circuit and prevent that. Does the delay in part prevent oscillations by introducing a phase shift?

Jerry, the initial link you posted Barkhausen stability criterion was a bit dense for me, but your summary  was useful. Even so it lead me to read some on feedback loops which was helpful.

Don, you are certainly right, I could understand the simple theory of your circuit but got lost on the complexity of how it actually works.

73, digger

Jerry Gaffke
 

Joe,

Thanks for responding.

Can you point us to Sunil's instructions that state how to hook up his AGC board?
I clicked on "Download Manual" on his website, got an archive with a dozen files,
but could not find any description of how to wire the thing up.
Is this information available from his website?
Or is it only available from a CD that comes with the kit?

Looks like it could be a nice AGC implementation, does attenuation in one of the
IF stages to allow greater dynamic range than could be had if attenuating after the audio pre-amp.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 01:43 PM, Joe Puma wrote:
Jerry I have had great success in hooking up Sunil’s AGC. I could be the poster child. I even determined a better location to grab audio instead of Vol H which is on the relay side of the R70 resistor. 
 
I do admit I had a hard time figuring things out in the beginning because of my ‘I’m still learning skillset’ but also because the CD that came with my radio kit could not be read and I only got half of the images and info off of the disk.  Then Sunil sent me some links that helped. Tuning the AGC meter was tricky but just follow the instructions again. The AGC is pretty simple. Just hook up the attenuation where Sunil stated in his instructions and you should be good to go, tune the 2kohm variable. Actually you know what.  I put a 10k because I thought it wasn’t attenuating enough I even changed one of the resistors before the AGC pot to 15kohm more then what was there. I can get specifics if anyone needs I’m going off memory.  
 
I ended up using coax for my attenuation hookup as that area is sensitive on the board and I was getting RF in from that wire as well as my IF tap for SDR. 
 

Joe Puma
 

I just dumped all the photos I had that I was able to get from him. I put them here: https://groups.io/g/BITX20/album?id=89704&p=Created,,,20,2,0,0

I put other non image files here: https://groups.io/g/BITX20/files/Amatureradiokits%20Files

Hope that helps.

Joe
KD2NFC


Jerry Gaffke
 

Barkhausen stability criterion is dense.
If you find an explanation that is sufficient to do design work without being dense, do let us know! 
There are thousands of 3'rd year undergraduate electrical engineering students who want to see it.

> Does the delay in part prevent oscillations by introducing a phase shift? 

Specifying a delay at a given frequency is equivalent to specifying a phase shift.
For example, a 1khz audio signal has a period of 1 millisecond.
If we put it through a delay line that delays it by 0.5 milliseconds, 
then the output of the delay line is phase shifted 180 degrees with respect to the input.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 02:36 PM, digger AB3XU wrote:
Thanks Don and Jerry for your explanations. In my OP I wasn't thinking about that what I described were oscillations, and as you wrote the parts are chosen to stabilize the circuit and prevent that. Does the delay in part prevent oscillations by introducing a phase shift?

Jerry, the initial link you posted Barkhausen stability criterion was a bit dense for me, but your summary  was useful. Even so it lead me to read some on feedback loops which was helpful.

Don, you are certainly right, I could understand the simple theory of your circuit but got lost on the complexity of how it actually works.

73, digger

digger AB3XU
 

Makes sense. Thanks Jerry

Jerry Gaffke
 

Digger previously wrote:
    Does the delay in part prevent oscillations by introducing a phase shift? 
 
I guess didn't quite answer your question

Consider an amp with positive feedback and no delays anywhere.
It would instantly ramp up to infinity.

If you want it to oscillate, you need to have a delay in that feedback loop
that gives a full 360 degrees of phase shift at the frequency of interest.
So the next cycle at that particular frequency gets reinforced by feedback from the previous cycle.
And other frequencies don't get this positive reinforcement at exactly the right time, so they die out.

If you want the oscillation to be stable, the gain around the entire loop
must be exactly 1.0  (so no gain and no loss).
If it is less than 1.0, the oscillation will die out.
If it is greater than 1.0, the amplitude of the oscillation will keep getting bigger till it swallows the universe.
However, any practical amplifier will have reduced gain as the oscillation gets too big,
so we will soon reach a steady state when the oscillation is so big that the amplifier gain is reduced to 1.0.
In this way the amplifier is considered non-linear, since it treats big signals differently than small signals.

The diagram on that wikipedia page shows a pure amplifier with a voltage gain of A (and no delays).
There is also a feedback network with a transfer function of Beta (a Greek B).
The transfer function mathematically describes all the phase shifts (delays) and attenuations at all possible frequencies,
all wrapped up in a nasty equation.  This is the part that keeps EE students drinking coffee the night before an exam.
But that transfer function is linear, and could be described by a bunch of resistors and capacitors and inductors
all wired up into a big rats-nest.  For example, here's a fairly simple model of the 12mhz quartz crystal
used in the feedback loop of the BFO oscillator on the Bitx40:  
    https://www.maximintegrated.com/en/app-notes/index.mvp/id/2127

Jerry, KE7ER




On Fri, Apr 19, 2019 at 03:45 PM, Jerry Gaffke wrote:
> Does the delay in part prevent oscillations by introducing a phase shift? 

digger AB3XU
 

Thanks again Jerry. I used to work with electronics decades ago and have forgotten almost all of it. Not in design but with repair and maintenance of telemetry and then audio equipment. The uBITX has been a great way to get reintroduced to all of it.

73, digger AB3XU

Tom, wb6b
 

HI,

An additional, somewhat oversimplified, way to look at the AGC control loop is this:
 
If you consider the transistor (or other type of amplifier in the control part of the loop) provides 180 degrees of phase shift then an addition delay of 180 degrees is all that is needed for the control loop to go from being a control loop to an oscillator. This can be done with as little as two equal RC time constant type delays chained together. 
 
Assuming there are delays in the AGC control loop due to other components not being infinitely fast, or other circuit compromises, you can make sure one of the RC time constant delays is much larger than the rest. That will swamp out the effects of the other delays. Many AGC circuit go even further by causing the AGC to cut in fast but restore the gain at a slower rate. 
 
If there was no delay in the control loop the AGC might be so good as to adjust the gain microsecond by microsecond and result in an output signal (audio) of zero volts AC, across every cycle of the audio. 
 
Finally if you are controlling a system with a lot of inertia (such as the hot water temperature to your shower head, air conditioning, motors or the melting temperature of the heated head of your 3D printer) another type of control loop, called the PID loop, is used.
 
In the PID loop the rate of change of the temperature/speed/whatever is fed into the control voltage to help the loop anticipate when to shut off (or turn on) just ahead of the desired value, so the temperature/…/…  will arrive at but not overshoot the desired value (set point).
 
The PID loop would likely not be as beneficial in an AGC circuit when your neighbor down the street keys up his kilowatt with his beam pointed your way, at there is nothing gradual to anticipate. So simpler control loops are the better choice for AGC.

Tom, wb6b

Jerry Gaffke
 

All the AGC circuits proposed in this forum (and there have been many) have a fast attack and then a relatively slow decay.
That way when the QRO guy across town keys up, it saves our ears as much pain as possible, and keeps the gain throttled
back in case he decides to utter yet another syllable.  The fast attack and slow decay thing is non-linear, and thus harder
to model with equations.  

The PID algorithm is worth knowing about, an ideal approach to many feedback loops for us punters:
    https://en.wikipedia.org/wiki/PID_controller
No nasty math, easy to understand. 
You get three knobs, and you twiddle them till the system starts to behave itself.
A good example of where you might want to use PID would be on a heater for an oven-controlled-crystal-oscillator, or OCXO.
A processor such as the Nano would be entirely adequate for implementing the algorithm.
The goal would be to add enough heat when the oven is cold to quickly bring it up to temperature,
but not leave the heater on so long that the oven temperature overshoots and gets too hot.
Note that there will be a time lag between when the heater shuts off and when the
sensor near the quartz crystal sees the temperature stop rising.
This system could easily oscillate if the knobs are set wrong,
with the temperature alternating between too high and too low. 


> The PID loop would likely not be as beneficial in an AGC circuit when your neighbor down the street
> keys up his kilowatt with his beam pointed your way, at there is nothing gradual to anticipate.


In that case, you want the "D" knob turned way up, for a very fast AGC attack.
However, since the PID loop is a linear system, the AGC decay will be equally fast, and that is not what you want.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 07:30 PM, Tom, wb6b wrote:
Many AGC circuit go even further by causing the AGC to cut in fast but restore the gain at a slower rate. 

MadRadioModder
 

I tend to use the definition of AGC from the Radio Amateurs Handbook and other IEEE sources that pertain to the Radio-Electronics industry specifically rather than Wiki… which is typically diminished science for the masses from random writers who may or may not actually understand the topic.  The Wiki article you refer to discusses AGC applied to Audio systems, Power systems (generators), Process Control systems (like PID controllers), and even biological systems. While a tiny bit of this definition is applicable to radio systems (feedback with variable gain in some form), it’s by no means specific or useful.  Historically (that being from the late 1930’s and early 40’s when the first patents for AGC were granted), AGC was defined as a closed-loop system that provides attenuation to one or more RF-IF gain stages of a receiver.  The goal was to keep the output level of the receiver constant and prevent gain stages from saturating at high RF front-end input while increasing gain for weak (low voltage) signals yet irrespective if the input.  The underlying algorithms to do this generally consist of signal measurements in the IR-RF chain and post detector, filter and integrator for response shape, to be fed back to a variable attenuator of some type.  Our ears are logarithmic in response (a second outcome of Fletcher-Munson) and since we can (and do often) define signal strength as logarithmic voltage, the feedback algorithm can be linear or semi-linear (logarithmic).  This is called Input-VGA or IVGA (attenuator-amplifier) as opposed to Output-VGA (which is fixed gain amplifier followed by a passive attenuator).   I believe Zenith held one of the first patents on AGC, but it was contested early and proliferated by others while in litigation… and it was evident on a myriad of AM receivers starting in the 50’s and forward.  This is what I know with 45+ years of designing AGC circuits for commercial and military receivers.

 

I don’t know much about AVC other than it was used in the audio industry and proliferated there for recording and delivery (dBX had a compression form of it).

 

I have no idea what “Transmit AVC is”…

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Friday, April 19, 2019 1:58 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Confused about how AGC works?

 

From 
    https://en.wikipedia.org/wiki/Automatic_gain_control
"Automatic gain control (AGC), also called automatic volume control (AVC), is a closed-loop feedback regulating circuit in an amplifier or chain of amplifiers, ..."
A google search for "AGC vs AVC" shows they are considered synonomous in radio design (but not for power grids), and that AGC is the more modern term.

In this forum, all such circuits for receivers have been getting referred to as AGC.
For some reason, the term AVC has only been used in this forum for automatic level control while transmitting
(Except for these posts from MRM.)
Works well for me, as few of us care that much about AVC while transmitting.
I suggest we keep it that way. 



 


Virus-free. www.avg.com

--

…_. _._

Jerry Gaffke
 

Do a google search for "AGC  AVC"
Let me know if you find any hits that draw a distinction.
I only see entries that state they are synonyms.

My 2015 ARRL Handbook has no index entry for AVC,
Perhaps yours is an older edition?

AGC systems, oscillators, and PID algorithms are all closely related as feedback systems.
Learning about one can help understand the others.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 08:16 PM, MadRadioModder wrote:

I tend to use the definition of AGC from the Radio Amateurs Handbook and other IEEE sources that pertain to the Radio-Electronics industry specifically rather than Wiki… which is typically diminished science for the masses from random writers who may or may not actually understand the topic.  The Wiki article you refer to discusses AGC applied to Audio systems, Power systems (generators), Process Control systems (like PID controllers), and even biological systems. While a tiny bit of this definition is applicable to radio systems (feedback with variable gain in some form), it’s by no means specific or useful.  Historically (that being from the late 1930’s and early 40’s when the first patents for AGC were granted), AGC was defined as a closed-loop system that provides attenuation to one or more RF-IF gain stages of a receiver.  The goal was to keep the output level of the receiver constant and prevent gain stages from saturating at high RF front-end input while increasing gain for weak (low voltage) signals yet irrespective if the input.  The underlying algorithms to do this generally consist of signal measurements in the IR-RF chain and post detector, filter and integrator for response shape, to be fed back to a variable attenuator of some type.  Our ears are logarithmic in response (a second outcome of Fletcher-Munson) and since we can (and do often) define signal strength as logarithmic voltage, the feedback algorithm can be linear or semi-linear (logarithmic).  This is called Input-VGA or IVGA (attenuator-amplifier) as opposed to Output-VGA (which is fixed gain amplifier followed by a passive attenuator).   I believe Zenith held one of the first patents on AGC, but it was contested early and proliferated by others while in litigation… and it was evident on a myriad of AM receivers starting in the 50’s and forward.  This is what I know with 45+ years of designing AGC circuits for commercial and military receivers.

 

I don’t know much about AVC other than it was used in the audio industry and proliferated there for recording and delivery (dBX had a compression form of it).

 

I have no idea what “Transmit AVC is”…