Re: AGC circuit to try?

Tim Gorman


These amplifier circuits are very carefully designed to provide 50ohm
inputs and outputs so that proper terminations are provided to the
mixers in order to maximize rejection of unwanted carriers. The output
of the second amplifier is to provide the proper termination to the
crystal filter.

If you replace the emitter resistors with a diode then you have
effectively changed the input and output impedances of the amplifiers.
This would be especially bad for the termination into the crystal

Google "w7zoi" and "termination insensitive amplifiers" for a

What you suggest would probably work but without testing it is
difficult to determine the actual result on the overall operation of
the receiver.

tim ab0wr

On Sat, 07 Apr 2018 12:15:15 -0700
"ajparent1" <kb1gmx@...> wrote:

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).   


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