Formulas & Data:
- Electronics
- Mathematics
- Physics
- RF & Microwaves

Parts Vendors:
- all hand-selected

- Amateur Radio
- Vintage Articles:
- Electronics World
- Popular Electronics
- QST
- Radio & TV News
- Radio Craft
- Short Wave Craft
- Wireless World

- Events Calendar
RF Cafe Homepage
Copyright 1999-2015   •−•  ••−•    −•−•  •−  ••−•  •   "RF Cafe" in Morse Code  >> Listen to It <<
- EW & Radar Handbook
- Quizzes       - Software
- App Notes
- Calculators
- Advertise on RF Cafe
- About RF Cafe™
- RF Cafe Shirts & Mugs
- RF Cafe Archives
RF Cafe Book Contest
- RF Cafe on Visit RF Cafe on Facebook Visit RF Cafe on Twitter Visit RF Cafe on LinkedIn
- Tech Humor
- Forums Archive
- Job Board
- Donate to RF Cafe
- Kirt's Cogitations
Custom Search
More than 10,000 searchable pages indexed.
Greetings: There is so much good stuff on RF Cafe that there is no way to list or link to all of it here. Please use the Search box or the Site Map to find what you want - there is a good chance I have it. Thanks!

AGC to which stages? - Comments welcome - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


Charl
Post subject: AGC to which stages? Posted: Mon Aug 21, 2006 7:58 am

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Hello all,

Looking at old schematics of tube radios, the AGC is usually wired to all stages: RF preamp, RF mixer, and IF stages (sometimes just the IF stages). Would I be able to get away with hooking it to just the RF preamp - possibly (DC) amplfied?

I presume they used to connect the AGC to all stages to get more negative feedback out of a low voltage swing: -1V on one grid only brings gain down by so many dB, but -1V on 4 grids brings it down by much more.

Thanks in advance,
Charl


Top

languer
Post subject: Posted: Mon Aug 21, 2006 3:39 pm

Captain


Joined: Sun May 14, 2006 8:53 pm
Posts: 17
Location: Earth
Do you have a particular application in mind?

AGC is mainly there to keep everything linear, so if the first thing to non-linear is the RF-amp that's where you need it the most. Usually though the first thing is the IF. And because sometimes you have strong unwanted signals within your passband, you may want to start introducing loss (i.e. AGCing) in the IF so as to not affect your NF (and thus not heavily impacting a low-power desired signal).

As I said before, it depends in your application.


Top

fred47
Post subject: AGCPosted: Mon Aug 28, 2006 11:11 pm

General


Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!

Just a quick note - it's actually very hard to design an amplifier with a gain that's variable over a wide range. That's why many circuits apply AGC to several stages - to get a wider control range.

However, there are other considerations: intermodulation and noise figure, for example. That leads designers to use "delayed AGC", where some stages receive the AGC at levels that other stages don't receive it at. There are different nonlinearities used to get this, some not very visible.

Good Luck!

Fred


Top

Charl
Post subject: Posted: Tue Aug 29, 2006 12:01 pm

Colonel


Joined: Fri May 19, 2006 5:01 am
Posts: 25
Location: Netherlands
Thanks fred, that's very helpful. Confirms my suspicions! :)




Posted  11/12/2012

Your RF Cafe
Progenitor & Webmaster

Click here to read about RF CafeKirt Blattenberger… single-handedly redefining what an engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem!
(Seize the Day!)

5th MOB: My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website