Re: ND6T AGC Troubleshooting

WA6PXC Rich <sigma@...>

Hi Jerry,
Thanks for your input. I agree, his circuit is elegant & simple .
I wanted to build an AGC and Squelch circuit on a separate board and just
remove one component & connect two wires to the BitX board (minimum mod to BitX board).
See attachment..

-----Original Message-----
From: "Jerry Gaffke via Groups.Io"
Sent: Jan 12, 2019 9:18 AM
Subject: Re: [BITX20] ND6T AGC Troubleshooting


You are correct, it is better to have AGC reduce gain at RF or in one of the IF amps.
The audio pre-amp at Q70 of the uBitx (or Q16 of the Bitx40) is the weak link, 
restricting available dynamic range if AGC attenuation occurs after that audio pre-amp.

However, take another look at the ND6T AGC, it attenuates the RF as it comes in from the antenna.
Not unlike your design, but using FET's instead of op-amps.
Simple and effective.

Would be interesting to do an A/B comparison of your implementation vs the ND6T AGC.
Lots of variables here regarding dynamic range of the attenuation, attack and release times, distortion, ...

Jerry, KE7ER

On Sat, Jan 12, 2019 at 08:46 AM, WA6PXC Rich wrote:
Hi Armin,
I developed this AGC because all of the others act on the audio signal.
When the signal is really large, the audio is already distorted so we
just control a distorted signal. Please see the Attachment.
To get an idea of how well this type of AGC will work on your radio,
remove R-12 and replace it with the FET as shown in the schematic,
with just the 10K resistor going to the gate.
Using a 10K pot, connect it across 5v and connect the wiper to the Gate resistor.
As you turn the pot, it will vary the RF gain.
Have fun...

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