Saturday 6 June 2020

Update on Digital SSB

I had been keen to implement a better drive AF amplifier between the Teensy output (designed to drive 10k) and the balanced mixer inputs (120R).

I have three choices, a small module using a TDA2822 stereo audio amplifier, an emitter follower or a couple of LM386 amplifiers. I wired up the TDA2822 first, then powered it up - making a grand mistake using a 12V supply, when the TDA2822 is rated at 5V. It blew up.

So now I have used a couple of LM386s which give x20 gain and use a 12V supply. The output of the Teensy Audio Adapter is max 1.2Vp-p, this is the digital limit, no wy togo higher, with an overhead margin a working outputis around 0.2Vp-p. This would give an output of 4Vp-p from the LM386. Too high. So I have included a couple of 10k trim pot volume controls. This has the added advantage of being able to balance the AF L & R channels and minimise the unwanted sideband. This is now the circuit



New circuit

And the board and system look like this


Board


System



LSB output

The rebuild has reduced the carrier break though and it is now 40dB below peak signal. There is still the issue of USB break though as I have seen before. Here is the breakthru by frequency, equals the USB dB  down level,

70 taps USB down in dB

But this is due to weakness in the FIR Hilbert filters in the Teensy, Above is the filters using 70 coefficients or taps. With 100 taps and a slightly narrower BW, the USB breakthru is improved


100Taps, narrower BW, USB down in dB

And this is the Bandwidth using white noise as a source, the SDR is set to 3KHz

SDR at thing 3kHz with white noise input

BOOST
Now I will try something else. The Audio Adapter chip can pre-process audio before passing to the I2S converter and on to the Teensy. I have enabled this and also a Auto Volume Control. The result is a more "punchy" modulation and it is more difficult to over modulate.

I still have the USB breakthru. I have tried a larger Hilbert filter (ideal would be at least 200 taps), but the Teensy cannot handle above 100 taps in two filters. 

The sketch TEENSY_TX_100_BOOST is here.






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