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# Regarding AGC!! - 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.

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

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.

Posted  11/12/2012
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