Topics

Who has AGC... Fun Facts

MadRadioModder
 

Well I was wrong about Zenith having the first AGC.  It was RCA that sued Zenith over a similar AGC design (now ancient history).  I had to read my old papers again.

 

This is fun fact from the Antique Radio forum circa 1970:  “RCA's Radiola 64 (below, 1929) is credited for being the first radio with AGC. Since the tubes in it were triode 27s, they were difficult to control with a simple rectified carrier, as later circuits had. This set, as well as others in those early days, had to have a DC amplifier to provide the voltage swing to bias the RF and IF stages from normal bias to near cutoff. This circuitry was tricky, but it worked. RCA also had a tuning meter on these sets, supposedly another first. In a couple of years, variable-mu tubes (35/51) came along and made AGC a lot easier. And it got better after that.  Now in general, superheterodynes had AGC, but there were some exceptions. In about 1931, Western Electric made their 10A AM high fidelity tuner. It's monstrous, but it was a TRF with AGC and a tuning meter”.  The Antique Radio guys would eat us alive for not knowing the distinction between AGC and AVC.

 

 


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Jerry Gaffke
 

I can believe there is a historical basis for drawing a distinction between AGC and AVC.
And that there are still a few folks that might maintain this difference.
But in 2019, it seems almost everybody just lumps it all together as AGC.

Jerry, KE7ER


On Fri, Apr 19, 2019 at 10:12 PM, MadRadioModder wrote:

The Antique Radio guys would eat us alive for not knowing the distinction between AGC and AVC.

 

Jerry Gaffke
 

A curious tidbit about 1920's era radio:
They used to use a rheostat (a big honking pot) on the filament current to vary the gain.

A bit off topic, as I don't recall anybody closing the loop
by driving a servo motor to control that rheostat based on received signal strength. 

Jerry, KE7ER

MadRadioModder
 

Schematic of the Radiola 64

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Jerry Gaffke via Groups.Io
Sent: Saturday, April 20, 2019 12:54 AM
To: BITX20@groups.io
Subject: Re: [BITX20] Who has AGC... Fun Facts

 

A curious tidbit about 1920's era radio:
They used to use a rheostat (a big honking pot) on the filament current to vary the gain.

A bit off topic, as I don't recall anybody closing the loop
by driving a servo motor to control that rheostat based on received signal strength. 

Jerry, KE7ER


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Joe Puma
 

I’m taking this discussion in and learning. Jerry you always find a way to educate the group when topics fly by. As I’ve been learning I understood that there is automatic volume control and automatic gain (RF) control. I learned that RF gain control is better to use. For me I just want to be able to hear weak stations and clamp down on the real close ones.  But as to your AGC AVC discussion and why we don’t properly label the two. Show my a radio that has AVC on the knob. I’m a relatively new ham but all I’ve ever seen is AGC. Maybe some of the older tube boat radios used AVC, I don’t know but it’s easy to label it all AGC because I’ve never seen a AVC lable on a radio knob. 

Joe
KD2NFC 



On Apr 20, 2019, at 1:46 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

I can believe there is a historical basis for drawing a distinction between AGC and AVC.
And that there are still a few folks that might maintain this difference.
But in 2019, it seems almost everybody just lumps it all together as AGC.

Jerry, KE7ER

On Fri, Apr 19, 2019 at 10:12 PM, MadRadioModder wrote:

The Antique Radio guys would eat us alive for not knowing the distinction between AGC and AVC.

 

Jerry Gaffke
 

Here's some antiqueradio guys talking about AVC, it is all about controlling RF and IF gain:
This is now commonly called AGC.
    https://www.antiqueradios.com/forums/viewtopic.php?f=6&t=263244
    http://www.johnsvintageradio.com/notes03.html
    https://www.radioremembered.org/detector.htm
    https://www.radiolaguy.com/info/AVC.htm

That last link mentions rheostats on filaments to control gain, though this was not commonly done.
See the final post here:  https://www.vintage-radio.net/forum/showthread.php?t=60467
 
I doubt there are many radios with AVC/AGC that ever did other than adjust the gain of RF and/or IF stages.
There's no real advantage for AGC to mess with the audio stage gain, and it has the disadvantage 
that this does not solve signal overload in earlier stages.  The uBitx and Bitx40 AGC schemes in the forum
often adjust the audio gain, but only because it's the easiest thing to slap in there as an add-on.

Can somebody point to a passage somewhere that claims AVC and AGC are different animals?
Not that it matters much, but I don't want future discussions of AGC schemes on the uBitx/Bitx40
to spend days arguing about the rather silly distinction of whether the controlled gain stage
happens to be operating at 1khz or 12mhz.   

Jerry, KE7ER


On Sat, Apr 20, 2019 at 08:32 AM, Joe Puma wrote:
I’m taking this discussion in and learning. Jerry you always find a way to educate the group when topics fly by. As I’ve been learning I understood that there is automatic volume control and automatic gain (RF) control. I learned that RF gain control is better to use. For me I just want to be able to hear weak stations and clamp down on the real close ones.  But as to your AGC AVC discussion and why we don’t properly label the two. Show my a radio that has AVC on the knob. I’m a relatively new ham but all I’ve ever seen is AGC. Maybe some of the older tube boat radios used AVC, I don’t know but it’s easy to label it all AGC because I’ve never seen a AVC lable on a radio knob. 
 

Scott McDonald
 

The AGC/AVC thing probably can’t be resolved at this point, but it would be unambiguous to me if folks just called their AVC/AGC IF-derived or audio derived.

I’m not sure that would be a silly distinction as in one case you’re looking at an RF waveform that should be more representative of what was transmitted than one that’s at the tail end of an audio amp that’s done who knows what to it.

And because I’m a dinosaur I want to believe a decent IF-derived system is better, though I have to admit the ubitx audio-derived stuff is pretty good. :)

Scott Ka9p

Make something good happen!

On Apr 20, 2019, at 11:51 AM, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

Here's some antiqueradio guys talking about AVC, it is all about controlling RF and IF gain:
This is now commonly called AGC.
    https://www.antiqueradios.com/forums/viewtopic.php?f=6&t=263244
    http://www.johnsvintageradio.com/notes03.html
    https://www.radioremembered.org/detector.htm
    https://www.radiolaguy.com/info/AVC.htm

That last link mentions rheostats on filaments to control gain, though this was not commonly done.
See the final post here:  https://www.vintage-radio.net/forum/showthread.php?t=60467
 
I doubt there are many radios with AVC/AGC that ever did other than adjust the gain of RF and/or IF stages.
There's no real advantage for AGC to mess with the audio stage gain, and it has the disadvantage 
that this does not solve signal overload in earlier stages.  The uBitx and Bitx40 AGC schemes in the forum
often adjust the audio gain, but only because it's the easiest thing to slap in there as an add-on.

Can somebody point to a passage somewhere that claims AVC and AGC are different animals?
Not that it matters much, but I don't want future discussions of AGC schemes on the uBitx/Bitx40
to spend days arguing about the rather silly distinction of whether the controlled gain stage
happens to be operating at 1khz or 12mhz.   

Jerry, KE7ER


On Sat, Apr 20, 2019 at 08:32 AM, Joe Puma wrote:
I’m taking this discussion in and learning. Jerry you always find a way to educate the group when topics fly by. As I’ve been learning I understood that there is automatic volume control and automatic gain (RF) control. I learned that RF gain control is better to use. For me I just want to be able to hear weak stations and clamp down on the real close ones.  But as to your AGC AVC discussion and why we don’t properly label the two. Show my a radio that has AVC on the knob. I’m a relatively new ham but all I’ve ever seen is AGC. Maybe some of the older tube boat radios used AVC, I don’t know but it’s easy to label it all AGC because I’ve never seen a AVC lable on a radio knob. 
 

MadRadioModder
 

Your suggestion is good as I have just found one of my notable IEEE colleagues calling an AFC (automatic frequency control) circuit… an AGC circuit in a phase-locked loop.  Otherwise, at this point a volume control pot could otherwise rightly be called AGC too if you include the human.

 

 

From: BITX20@groups.io [mailto:BITX20@groups.io] On Behalf Of Scott McDonald via Groups.Io
Sent: Saturday, April 20, 2019 1:07 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Who has AGC... Fun Facts

 

The AGC/AVC thing probably can’t be resolved at this point, but it would be unambiguous to me if folks just called their AVC/AGC IF-derived or audio derived.

 

I’m not sure that would be a silly distinction as in one case you’re looking at an RF waveform that should be more representative of what was transmitted than one that’s at the tail end of an audio amp that’s done who knows what to it.

 

And because I’m a dinosaur I want to believe a decent IF-derived system is better, though I have to admit the ubitx audio-derived stuff is pretty good. :)

 

Scott Ka9p

Make something good happen!



 


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Jerry Gaffke
 

You don't have to be a dinosaur.
IF derived is better because it comes on faster, it would take milliseconds to sense low frequency audio,
and in that time the RF/IF stages will go into overload and distort.
So we have audio derived AGC and RF derived AGC (that is where the signal level is detected),
and while most radios then adjust the RF and/or IF gain accordingly, we could also adjust the AF gain.
We might need 3 or 4 different acronyms. ;-)



On Sat, Apr 20, 2019 at 11:06 AM, Scott McDonald wrote:
And because I’m a dinosaur I want to believe a decent IF-derived system is better, though I have to admit the ubitx audio-derived stuff is pretty good. :)
 

Adrian Chadd
 

A decent IF system would be great if we did AM/FM, right? :) But it's OK how it is!



-a