Best Method for AGC?

Hi, I am doing AGC for a radio receiver. It's been tough. I spent a whole day on it and now I have a question.

What is the best way to perform the AGC? Should it go more positive with signal or negative? I'm thinking more positive is best because of the compression in the AGC biased amplifier but if you can manage to stay out of the cutoff region, negative bias would be more effective. I am dealing with a wide range of signal strengths. Anyone have some good tips?

Crystal ball is cloudy!...ouija board is out for recalibration!...spirit cat won't talk to me!.......how about a schematic?

Ken

KMoffett said:

Crystal ball is cloudy!...ouija board is out for recalibration!...spirit cat won't talk to me!.......how about a schematic?

Ken

Click to expand...

Thanks for reply. I don't even have it on paper yet, much less the computer. I'm thinking compression is best. When I think about it, that would take a plus and a minus signal fed to two seperate amplifiers 180 degrees out of phase like two CE amps in a string. Ever hear of this being done? Well, back to the drawing board.

AGC typically controls the gain of one or more of the IF stage amps to maintain a constant signal level and the IF output. Is your question about how to vary the gain in a particular gain stage? It's not very clear what you are asking.

Here's a link to a simple AGC amp with an 82dB AGC range which might be something you could use **broken link removed**.

crutschow said:

AGC typically controls the gain of one or more of the IF stage amps to maintain a constant signal level and the IF output. Is your question about how to vary the gain in a particular gain stage? It's not very clear what you are asking.

Here's a link to a simple AGC amp with an 82dB AGC range which might be something you could use **broken link removed**.

Click to expand...

Oh thanks man. I have been getting bad sights. Maybe this one is better.

Basically the question is...what is the preferred method? I am extremely concerned with linearity all the way through the receiver. That is because I will be using it for SSB reception. If you know anything about SSB then you understand what I talking about.

So what is the preferred method for maximum linearity? I am rectifying some audio just befor the volume control and feeding it back to one of the 1st IF stages. If the amplitude of an incoming received signal increases then I can get either a positive or a negative DC output by various mean, most likely a CE (Common Emitter) amplifier which will phase shift it's input 180 degrees. Follow me?

Now if I use a positive output and take that and apply it to the bias of a CE amplifier in a chain of IF amplifiers, then what will happen is the signal will compress as the bias approaches saturation. If the DC signal out goes more negative with an increase in in coming signal amplitude then the amplification will decrease but could go into cut-off and would have to be carefully biased to prevent this. With the very wide range amplitudes in short-wave reception it would be very difficult if possible.

Looking for best technique.

PIN diode attenuators are fairly linear and have good IP3 and low NF. Only issue is temp drift compensation is needed.
Also if you put the Audio voltage into a PIC you can make an AGC lookup table and linearize with software.

Actually AGC design is one of the most important part for good overall receiver performance. This is most seen in HF communications receivers that have to deal with large signal level variations. The well designed AGC system should be able to allow the receiver to work over it's whole dynamic range. The AGC also has to be able to deal with the different modulation modes that it is designed to handle (CW, SSB, AM, etc) as these require different attack and decay time constants. Also each mode have different IF bandwidths requirements and bandwidth filters are usually the most expensive stages in a well designed communications receiver. I've seen some advance AGC designs that would work well over a 100+ DB range.

I would suggest checking out some of the more advanced HAM radio sites for examples and suggestions. The good news it that there are very good devices available for RF, mixer and IF amplifiers these days.

However the current state of the art design in high performance receivers are so called 'software controller receivers', where the AGC, demodulation and bandwidth control are all done using DSP techniques.

Lefty

Mikebits said:

PIN diode attenuators are fairly linear and have good IP3 and low NF. Only issue is temp drift compensation is needed.
Also if you put the Audio voltage into a PIC you can make an AGC lookup table and linearize with software.

Click to expand...

Oh, thank you Mikebits. I will look into that. I believe have a couple handy. Not really sure how to apply that...what configuration.

Leftyretro said:

Actually AGC design is one of the most important part for good overall receiver performance. This is most seen in HF communications receivers that have to deal with large signal level variations. The well designed AGC system should be able to allow the receiver to work over it's whole dynamic range. The AGC also has to be able to deal with the different modulation modes that it is designed to handle (CW, SSB, AM, etc) as these require different attack and decay time constants. Also each mode have different IF bandwidths requirements and bandwidth filters are usually the most expensive stages in a well designed communications receiver. I've seen some advance AGC designs that would work well over a 100+ DB range.

I would suggest checking out some of the more advanced HAM radio sites for examples and suggestions. The good news it that there are very good devices available for RF, mixer and IF amplifiers these days.

However the current state of the art design in high performance receivers are so called 'software controller receivers', where the AGC, demodulation and bandwidth control are all done using DSP techniques.

Lefty

Click to expand...

Don't I know it. Yeah I have the complete radio done except for finalizing the AGC. Yes, I am glad you pointed that out that the dynamic range in HF (Short-wave) is the most stringent of all types of receivers.

One thing I do not want to do is add more digital. I have an incomplete PLL that uses a PIC and a Motorola MC145170 PLL chip for it. It is single loop currently, so only single band. I also have a VFO circuit built to test the 3 to 30MHz. It actually produces the offset freq. or first IF for it to receive 3 to 25 MHz.

I'm not real real concerned with the attack time as I am receiving overall stability of the signal strength. Of course like I have previously stated I am concerned with linearity. Even compression can degrade a good quality SSB signal.