ubitx.net recommends installing an AGC circuit
Why include an AGC circuit in your µBITx?
The µBITx comes without an Automatic Gain Control (AGC) circuit. This means that very loud stations over-power the audio amplifier. This is particularly noticeable when you have been listening to a weaker station. With headphones on, the inability of the uBITx to reduce gain in the presence of a strong signal can be deafening. On a speaker it can disrupt the household. All modern commercial transceivers include an AGC circuit, and the µBITx needs one too!
Difficulties with AGC circuits
The original design from March 2017 included an AGC, built around the TDA2822’s second amplifier (it is a stereo chip). However, it was removed from the design in the production model, presumably because of circuit weaknesses. You can try this mod if you want to as it can be found here.
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) has proven to be difficult. Probably nobody has tried out more AGC mods on this rig than John VK2ETA. See the thread here for his experiences with a range of AGC mods.
There are numerous AGC circuits that constructors have experimented with. These are documented here on ubitx.net. The problem is in recommending a preferred solution that most constructors would agree was satisfactory. The circuit below seems to be the current preferred solution. It uses a pre-built microphone preamp module, but is adapted for useMAX9814 AGC circuit
John VK2ETA used the Adafruit MAX9814 board. Other variations on eBay may require less hacking than the Adafruit design. John had to solder a wire on to 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 his perspective. He plans to use the AGC voltage as an s-meter input. This setting does produce a “gap” in AGC effect at the bottom end, but this does not appear to be critical.
In both cases he noted small “clicks” when the AGC kicked in on strong sudden signals.
The maximum gain of the MAX9814 as indicated in the schematic for the part is 50dB. Screened cables are recommended for the audio circuit. John 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.
ubitx.net tentatively recommenDS THIS AGC MOD