dynamic amplitude equalization AGC - Page 1
Forum section: electronics / projects

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 09, 2014, 11:09:03 am
content: I have a transmission line with amplitude irregularities down to approximately -40db. I would like to correct this using dynamic amplitude equalization (AGC) to main a constant gain of 0db between its input and output. through out the transmission line chain. However I have used the following circuits:http://electronicdesign.com/analog/effective-agc-amplifier-can-be-built-nominal-costAnd the following attached circuitI'm not getting the expected results. with the first circuit. I struggling a bit with the circuit values and on the attached circuit I am not sure how much control voltage, values of resistor I should use.thank you for all your help

author: dannyf
author_rank: Super Contributor
timestamp: June 09, 2014, 03:49:29 pm
content: First of all, you have to explain what -dynamic agc- is.secondly, a gain of 0db can be achieved via a simple wire, or a follower. No need for a complex contraption as proposed.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2014, 04:22:17 pm
content: What are you hanging off the first circuit to measure it's output?Where does the second circuit come from? I don't immediately understand it's operation ... is that second jfet some kind of temperature compensation? The first circuit seems to be intended for audio, it has a large time constant in it's gain adaptation (at least for gain increase, it clips it down fast ... which given the potentially large amplification you definitely want). The second regardless of how it works is not an AGC at all but a VCA (you could use the same BJT trick from the first to turn it into an AGC).

author: retrolefty
author_rank: Super Contributor
timestamp: June 09, 2014, 04:28:49 pm
content: Quote from: R_G_B_ on June 09, 2014, 11:09:03 amDo you really mean you wish constant gain or constant output level? You can't have both.I think you mean to say is you wish to maintain a constant output level for a variable level input signal? That is a classic definition of applying AGC to an amplifier (the G in AGC is varied). The limits will be of course the maximum gain available from the controlled amplifier stage(s) and the linearity and minimum and maximum sensitivity values (total dynamic range)of the AGC detection circuit.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 09, 2014, 04:40:07 pm
content: Do you really mean you wish constant gain or constant output level? You can't have both.I think you mean to say is you wish to maintain a constant output level for a variable level input signal? That is a classic definition of applying AGC to an amplifier (the G in AGC is varied). The limits will be of course the maximum gain available from the controlled amplifier stage(s) and the linearity and minimum and maximum sensitivity values (total dynamic range)of the AGC detection circuit.I want to overcome the channel amplitude distortion with a constent gain using automatic gain control to mitigate these effects. Looking at it another way consider a passive filter with a constent gain of 0dB up to the 3db point. This is the objective can anyone help with this?As for dynamic amplitude equalization I thought this was another terminology.Thanks

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2014, 05:39:29 pm
content: Quote from: R_G_B_ on June 09, 2014, 04:40:07 pmI think you need to learn how quotation works ... also this sentence doesn't parse at all. You should just describe your entire problem instead of giving small almost incomprehensible pieces. What way is your signal transmitted? What is your signal? What is causing such large gain changes? On what time scales?PS. I'll just repeat ... that second circuit is not AGC at all.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 09, 2014, 06:15:36 pm
content: I would like to to achieve the frequency response shown in the attachment plot b using AGC.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2014, 06:55:08 pm
content: Well it's not going to happen because you want equalization and that has bugger all to do with AGC, just forget about those circuits you selected they won't help you.Maybe you didn't understand me the first time though ...describe your entire problem instead of giving us the pieces you think are relevant, you are wasting everyone's time.If this is for audio and speakers just use an equalizer and tune it till it sounds to your liking, chasing "objectively" perfect sound is a fool's errand.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 09, 2014, 07:27:16 pm
content: The phase in constent I just need to sort out the amplitude irregularities. Get a flat response.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2014, 07:38:01 pm
content: I'm done here.

author: dannyf
author_rank: Super Contributor
timestamp: June 09, 2014, 11:36:18 pm
content: QuoteStart with something simple, and you have a chance to do fancy stuff later.Start with something fancy, you get nowhere.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 10, 2014, 01:31:54 pm
content: Quote from: dannyf on June 09, 2014, 11:36:18 pmI don't know the point of your sarcasm. Maybe you miss understood my objective.

author: madshaman
author_rank: Frequent Contributor
timestamp: June 10, 2014, 02:23:05 pm
content: Do you have a transmission line whose response isn't flat (over your operating range) that you want to flatten?If so, I think what some people are saying is that you don't need AGC for that, you just need equalisation.I'm curious though, you say you have non-flat amplitude response but a flat phase response?I find that fishy.My apologies if I didn't understand what you're asking.

author: R_G_B_
author_rank: Frequent Contributor
timestamp: June 10, 2014, 03:23:29 pm
content: Yes correct madshaman, I want a flat response over the frequency range of interest and you are right regarding equalization which I have managed to achieve with respect to the phase but the amplitude is not so great . I also thought this was not possible maybe Lt spice is not correct but I also ran the simulation on Tina spice and the result show its possible. There seems to be a none causal relationship between amplitude and phase. So this is why I was trying to implement the AGC to correct the amplitude. But this ended up distorting both the phase and amplitude.