Audio bandwidth between the product detector and VOL-H

Rob French (KC4UPR)

I have a uBITX v5 that I have been playing around with in many ways, for many months.  I'm revisiting some earlier DSP work I had done, and had a thought.

I want to try a 3rd "DSP" IF of around 16 kHz or so.  The idea with that is that:
(a) I could quash some low frequency noise and birdies,
(b) I could do some passband tuning (including width) in the DSP.

So the 2nd IF would be unchanged, but then I would lower the frequency of the BFO by 16 kHz, with the result that the desired signal at the output of the product detector would be in a ~3 kHz band from 16-19 kHz (-ish).  (I'm sure I'm missing something here...)

My question is, will the circuit from the output of the product detector, to VOL-H, support audio frequencies up to ~20 kHz with minimal loss?  I don't see any significant filtering, but at the same time, I can't imagine the circuit was originally designed intentionally to pass more than ~3 kHz, so...

The DSP would be inserted between VOL-H and the actual volume control.


Evan Hand

I am not the expert on the frequency response of the audio sections of the uBITX, however the preamp should be able to handle the higher frequencies.  The issue I am sure that you will have is the transmit side.  The same BFO and product detector are used to create the transmitted signal that then goes through the SSB filter.  You will either have to use the DSP to transform the mic audio to the correct frequency so that the product detector mixer than adds the correct carrier or change the BFO in the Nano program when transmitting.  You could do some audio compression algorithms for that, it will require a second DAC or relays to feed the current mic preamp.

My question back is why not just do the DSP at the current audio frequencies?  There are audio DSP filters that can be bought to lower the bandwidth to a couple of hundred hertz for CW, so it has been done.  There are also add on boards with higher processing capabilities that can be reprogramed to what you might want to do in the DSP.  One that comes to mind is the JackAl board, though this is no longer available from QRPGuys.  There is also the Teensy upgrade to replace the Nano.  This would give you extra program and I/O space if you want to play there.

My thoughts.  WARNING, they could be wrong.

Rob French (KC4UPR)

Hey Evan,  thanks for the input.

I've actually previously done some DSP filtering with the Teensy, I had it communicating with the Raduino Nano via serial, to coordinates modes.  Unfortunately, I accidentally killed my Teensy by plugging things into my level shifter wrong once (I had to unplug the Teensy from the Raduino to update the Nano, otherwise the serial would interfere).

This time around, I will try using I2C to communicate between the Teensy and the Raduino (yes, I could use JackAl or something similar... JackAl is the only one that also uses the Teensy Audio Adapter, which is part of my setup). 

Anyway, yeah, I could simply do the DSP at normal audio frequencies like I did previously.  This was just some thinking for experimentation.  My uBITX is never done...