Regarding AGC!! - RF Cafe Forums

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sky1
Post subject: Regarding AGC!! Posted: Tue Jul 26, 2005 6:19 am

Lieutenant

Joined: Tue Jul 26, 2005 5:55 am
Posts: 2
Hi Guys,

I am building a 822Mhz Transceiver, and with a receiver IF = 90Mhz.

I use a pin variable attenuator provided by agilent after my first IF stage.

The problem is the attenuation is logrithmatic and the control voltage is
linear, this leave me a non-linear output gain even it is within some
acceptable range.

Therefore, I wondering if there are any method can solve this problem other than using a expensive log amplifier?

Thank you for helpping me out

Ken

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IR
Post subject: Posted: Tue Jul 26, 2005 6:33 am

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Ken,

If I understood correctly your problem, then one possible solution would be to convert the linear voltage into its logarithmic value. This can be done by means of a logarithmic amplifier - a simple circuit made of operational amplifier with diode in the feedback path:

http://cnx.rice.edu/content/m0059/latest/

A more advanced version:

http://www.peg.si/electro/logamp.html

Good luck!

_________________
Best regards,

- IR

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sky1
Post subject: Posted: Tue Jul 26, 2005 7:29 am

Lieutenant

Joined: Tue Jul 26, 2005 5:55 am
Posts: 2
Hi IR,

Thank you so much, these circuits are indeed quite nice.

All the best.

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