Using a 1N4004 or similar as a varicap or pin diode for AGC control

Allison KB1GMX finds it  odd that every one seems to be bent on levelling the audio volume in the audio circuit.

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 depending on the band. If you feel that is not enough AGC range then add the same mod at R33 and 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 diode (x2 if using both diodes). That gives a manual gain control. To make it automatic use a circuit 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.

The 1n400x series with minor reservations makes a fine substitute for a PIN diode, the preferred but more costly device for this function. Beside being widely available and cheap  makes it useful.  It also makes a good 20pf varicap and a 1A rectifier to 1000V (1n4007).

This was tested on the first bitx20 that Allison built over a decade ago to test AGC.  It has been used on several older Tentec radios and more than few of her own design. That said its far from a new idea or design as its documented in EMRFD and an older book (Solid State Design, ARRL press, now out of print).

Jerry KE7ER climbs into the conversation saying:

“I have no idea what the capacitance of a slightly forward biased 1n400x is,
figure 6 here suggests it’s north of 30pf:    Therefore, it might be marginal at 45mhz, and can vary wildly with diode type and brand.

Consensus seems to be that a 1n4007 is preferred over other 1n400x flavors for use as a PIN.   Some experimentation may be required using diodes from different manufacturers:

If you are paying $5 postage to ship in some 1n4007’s, you might consider
also getting some BAP64-02’s at $0.43 each single unit pricing, Mouser 771-BAP64-02-T/R.  These are fully specified for use as an RF PIN diode.