Challenges with AGC circuits
Finding an AGC mod that works, and works well (with sufficient AGC range, that does not impact on the dynamic range of the receiver, and that does not distort) is proving to be difficult. Nobody has probably tried out more AGC mods than John VK2ETA.
See his thread here for his experiences with a range of AGC mods.
MAX 9814 Circuit
John VK2ETA has now settled on using the MAX9814 circuit for his AGC.
He used the Adafruit MAX9814 board but there are a few variations on eBay with some probably requiring less hacking than the Adafruit design. John had to solder a wire on an SMD component to access the CT (time constant capacitor) pin of the MAX IC, and remove the Electret capsule.
Refer to the schematic and a few pictures of the AGC circuit. The 5VDC required for the Adafruit board is taken from the Raduino.
John took two sets of measures, one with the AGC turned down low and one with the AGC turned one third of the way up.
He used an A/B comparison with an FT-817, with the pre-amp off, receiving a carrier at 1,500Hz from local radio stations (with attenuation). The FT-817 S-Meter seem non-constant in the steps between the S-units, but nevertheless, this was John’s reference for calibrating the AGC. The AGC voltage was taken on the CT pin of the MAX9814.
The AGC voltage fluctuates quite a lot, so he used the average value shown over time.
To determine whether any saturation was coming from the AGC circuit or the uBitx upstream at high signal strengths, he would bypass the AGC and keep the volume down to prevent the audio circuit after the volume pot from saturating. If harmonics of the audio disappeared, the AGC alone was producing distortion, otherwise it appeared at least prior to the AGC, and possibly from the AGC circuit as well.
“Medium” AGC: input pot turned to about 30% of full scale.
S-Meter voltage(mV) Notes
S8 1,700 Large variation. FT-817 S-meter S8 plateau issue?
S9 2,200 Some saturation of AGC noted (starts to appear in audio FFT, not noticeable)
S9+10 2,460 Saturation of AGC audible, but not unpleasant.
S9+20 2,460 Audible saturation of both uBitx and AGC (harsh sound).
The AGC kicks in early and keeps the volume pretty constant until saturation occurs. Saturation of AGC does limits the dynamic range of receiver.
“Low” AGC: input pot turned to about 7-10% of full scale.
S-Meter voltage (mV) Notes
S0 -S4 0
S5 50-200 (100mv avg)
S9+20 2,300 Saturation of both uBitx and AGC (visible in audio FFT, but not really audible)
S9+30 2,400 Audible saturation of uBitx and mostly of AGC.
This is the most “FT-817 AGC” like, from my perspective, and is what I have settled for. I want to use the AGC voltage as an s-meter input and this setting does produce a gap at the bottom end, but this is not critical IMO.
In both cases I noted some small “clicks” when the AGC kicked in on strong sudden signals.
The maximum gain of the MAX9814 as set in the schematic attached is of 50dB and requires screened cables in the audio circuit. I originally had the input and output of the circuit fed to a two core “stereo” screened cable and I would get feedback. I had to use two single screened audio cables.