Some time ago I was reading an old article of a USA HAM and, having an old CA3089 in the drawer, I got the idea of developing a small circuit to take the reception signal from the µBITX IF and obtain a good DC signal to drive a Smeter or an ADC input of the ATmega328 on the NANO.
A couple of clarifications:
1) To drive an ADC input of the NANO via the output S.mt. it is necessary to put in parallel a 5V ZENER diode, to prevent the voltage level from exceeding the maximum allowed by the ATmega328.
2) To drive an analog Smeter it is necessary to connect it between the S-mt output. and ground with a 100kΩ trimmer in series to the positive of the indicator for calibration.
3) The voltage level of the AGC works in negative, that is, it is high for low signals and tends to zero for strong signals (as is seen in the data sheet table included), obviously this factor must be taken into account to drive a MOS attenuator of the type designed by ND6T, but it can be used directly on a PIN diode attenuator placed in series to the rx signal.
4) The IF-IN input must be taken from the CW-2 connector directly connected to the IF output of the µBITX motherboard.
5) The Vrx power supply must be connected to the RX power supply of the µBITX motherboard.
6) The CA3089 is easily available on e-bay (I have personally checked this and bought a couple), but you can also look for one of the many equivalents indicated in the Cross Reference of the diagram.
Obviously, the circuit can be used on all receivers that have the possibility of taking a medium frequency IF signal.
Greetings to all ... 73 de IW4AJR