Residual phase error

[windytan]

There's a residual phase error of around 6° in the liquid-dsp symbol synchronizer. Probably not a big deal, but not perfect either; it could potentially cause more bit flips than necessary in noisy conditions.

The relevant function is Subcarrier::demodulateMoreBits(). This phase error seems to come from the PLL NCO that follows the difference frequency between 57 kHz and the actual subcarrier (nco_exact_).

• Perhaps nco_exact_.stepPLL() should be called at a different point?
• Calling nco_exact_.step() anywhere makes the constellation rotate wildly. But according to Liquid docs, you should call step() on an NCO or it will not advance. So what's happening here?
• What about loop filter bandwidth?
• Or perhaps the PLL can't lock onto such a low frequency? It's on the order of ±10 Hz.
• Links to liquid docs: nco, symsync

Here's the constellation of PSK symbols returned by the liquid-dsp functionsymsync_crcf_execute():

[windytan]

Fixed in v0.7.4