IF agc ciircuit design using voltage feed-back ampifiers

[huvarda]

variable agc using op amp

Hi,

I need information about designing IF amplifier with agc (Automatic gain control) circuit. Actually, I am interested in the second part right now. How can I design an AGC circuit using voltage feedback amplifiers (for example LMH6622 ? What should be the parameters I am interested in for the begining?

I want this circuit detect the output signal of the IF amplifier and using this signal, control the gain of IF amplifer. That's it.

thanks

 

[flatulent]

gapershi

There are several things to keep in mind with AGC circuits.

1. The method of reducing the gain should not produce distortion.

2. The gain reduction of the stages should be done with the last one reduced first since it will have the largest signal. Do a noise and distortion analysis for all of your candidate reduction schemes to verify your choice.

3. The gain reduction should be delayed. That is, for small signals below a certain value the gain should not be reduced, only signals above the threshold should activat the AGC.

4. The time constants are important. You should have a shorter constant for reducing the gain and a slow one for raising it. This will reduce the distortion of the modulation on the received signal produced by the gain variations.

5. The stability of the feedback loop is important. Keep in mind the nonlinearity of some detectors (output vs input) and some gain elements (control voltage vs gain). You will have to calculate the loop gain for all signal levels to make sure of stability. The old Drake R7 receiver had this problem. You had to reduce the AGC loop gain below the factory setting to make it stop oscillating the AGC at certain signal levels.

 

[huvarda]

agc design using integrator

Thanks,

Where can I get article or application note about this topic. I am not very familiar with the topic.
Thanks again...

 

[flatulent]

rohde if amplifier ham

One source is amateur radio handbooks put out by the US ARRL and the UK RSGB.

There is a book on receiver design by Rohde that is good. Any of the old pre-1950 radio engineering handbooks are useful, especially the ones by Terman. Try a visit to a library for these.

 

[huvarda]

lmh6622 multisim

I will do it, but before I start I need block diagram of an AGC circuit. What circuits doed it consist?

 

[flatulent]

four articles

Here are all 4 AGC articles from Ham Radio Magazine which was the most technical ham magazine of all times until the editor passed away.

 

[molloy]

well, i dont know if this can be useful (i designed an AGC control in a upper band of frequency) and anyway this is an article you can find on the net (i suppose on the site of HP) about the attenuator you could use (a pi-attenuator with pin diode).
Pay attention in the timing of the voltage that have to control he attenuator because the circuit can easily oscillate. It should be delayed of some ms (lets say even 20ms, but this depends also on the time you want the circuit works)
bye

 

[huvarda]

AGC circuit

Thanks for your concern guys, the articles are really helpful.

I saw a circuit diagram of a receiver. What may be the function of an Opamp circuits at the output of a IF amplifier after envelope detector in AGC circuit in the receiver? AGC circuit is fed back to the VGA (Variable Gain Amplifier) by a SPDT (analog switch) after opamp circuitry. If you drop a few comment, it will be very helpful as well.

 

[flatulent]

The switch is to manually remove the AGC action. This is a common function in higher priced receivers. This is done in situations where there is no fading on the signal and there are static crashes which would trigger the AGC to reduce the gain.

The op amp is to increase the AGC loop gain so that the output level stays more constant with a wide range of input signal levels.

 

[Borber]

Hi huvarda,
If you want to take a look at the schematics of complete receiver with AGC in IF you may wisit:
**broken link removed**

 

[flatulent]

good site

This link is to a first rate site. The radio is well done and the descriptions will help you understand the radio.

 

[WMAN]

Hi huvarda,
If you want to take a look at the schematics of complete receiver with AGC in IF you may wisit:
**broken link removed**

This web site link didn't work!

 

[huvarda]

agc

Hello guys,

you are really helpful, but still I wonder something; if you check the first mail, i wrote a specific part number, lmh6622 which is a voltage feedback amplifer. There are three lmh6622 circuits, first one is difference amplifier, second is modified integrator and third is integrator. This circuitry is between the detector and spdt (analog switch) in feedback circuit. Output of the spdt is connected to VGA (variable gain amplifier). I simulated this circuit in Multisim and I was expecting that there should be different voltages at the output for different input voltages. But it is not . It is supposed to change, right, in order to increase or decease the gain. I am totally confused. OK one purpose is to increase the agc loop gain. But what can be wrong. It is very different from the other agc circuits that you showed me.

 

[flatulent]

post diagram

Post the diagram of your circuit.

 

[janet]

One source is amateur radio handbooks put out by the US ARRL and the UK RSGB.

There is a book on receiver design by Rohde that is good. Any of the old pre-1950 radio engineering handbooks are useful, especially the ones by Terman. Try a visit to a library for these.
can you give tell me the name of these reference you suggested?

Added after 1 minutes:

One source is amateur radio handbooks put out by the US ARRL and the UK RSGB.

There is a book on receiver design by Rohde that is good. Any of the old pre-1950 radio engineering handbooks are useful, especially the ones by Terman. Try a visit to a library for these.

can you tell me the name of these reference?

 

[guopeng]

Thanks, i need more papers

 

[A_D_G]

@Flatulent: Can you please explained to me more about this statement: "The time constants are important. You should have a shorter constant for reducing the gain and a slow one for raising it. This will reduce the distortion of the modulation on the received signal produced by the gain variations".
I need to design an analog AGC for WLAN application but the demodulated signal constellation degrades significantly

 

[zhaozenghui]

thanks!!!!!

 

[zhaozenghui]

thank you very much!!