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Regarding AGC!! - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

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


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



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:


A more advanced version:


Good luck!

Best regards,

- IR


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


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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