Re: AGC circuit to try?

ajparent1/KB1GMX
 

It is incomplete..

You have the audio detection to get an AGC control voltage but you don't show what your controlling.

What I find odd in general every one seems to be bent on leveling the audio volume in the audio circuit.
Why?  I understand protecting ones ears and all but what an audio compressor?  

The Bitx or uBitx has enough gain and handy places that RF gain control based on audio detection works
very well.  The easy way is replace R13 (ubitx) with a diode such as 1n400x (x=1 to 7) and controlling the
current through the diode to make it behave as a variable resistance at RF.  

The current would be about 4-6ma at max gain and decrease to zero (0) at minimum gain.  For that design
the AGC range is about 26 to 32db depening on band.  If you feel that is not enough agc range then add
the same mod at R35 then with both the agc range is near 60+ DB, generally enough.  AGC in this form
is less prone to overload distortion as you are lowering gain.  The control could be a pot between 8V
(or RX-V) and ground and a series 1K resistor to the he diode (x2 of your using both diodes).  That is a
manual gain control.   To make it automatic use a circuit like just published to detect the voltage at the
top of the audio gain pot and feed that voltage to the gain control diodes.  The circuit should be
arranged to put 4-8V out at NO Audio and decrease to zero volts with increasing audio.

Side note the 1n400x series with minor reservations makes a fine substitute for a PIN diode, the
prefered but more costly device for this function.  Beside being widely available and cheap
( I buy them in quantities of 100 for pennies each) makes it useful.  It also makes a good
20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 I built over a decade ago to test AGC and is used on several  older Tentec
radios and more than few of my own design.   That said its far from a new idea or design as its documented
in EMRFD and the older SSD (Solid State Design AARL press, out of print).   


Allison

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