Re: 40m - too much gain of whole RX chain

J68HZ
 

I have a real simple AGC circuit that works perfectly with this kit.  Im on vacation this week but will publish it when i return to my office next week. It uses a transistor and diode charge pump to control the gate of a fet for variable gain in the audio chain.  By changing the charge capacitor or the parallel resistor you can make it variable anywhere in the fast to slow range. 


Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton - J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com


email:  bill@...

 


On Nov 9, 2017, at 5:08 AM, ondrej.briatka@... wrote:

I built 40m version QCX. It Works fine, I was surprised how this simple radio clear recieve.
But as radio has no AGC, I need use a volume knob a lot. I kept it most of the time almost on zero. The total gain is just too much for 40m (or 80m), I believe optimal for higher bands.

As a 'secure' solution I modified R37 from 1 K to 10K that decrease gain by 20dB. I kept R38 and C23 as it defines cutoff frequency (1326 Hz) for low pass filter.
It works fine, maybe sidetone is quiter. But I am still thinking how to lower gain on first stage IC5.
I cannot touch 100 ohm resistors R5-R9 (input impedance of amplifier) as they are tighten with quad. sampling detector. What about R7-C4, R10-C7 in feedback?
2k2+47n, gain -13dB
3k3+30n, gain -9.7dB
cutoff freq. remains the same

What do you think? Could it affect IP3 / sensitivity of whole in negative way?

73
Ondrej OM4DW

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