I wrote:

> I can reproduce this using EbSynth and a weak near-field
> 9th harmonic signal.  2 to 3dB loss every time on a similar
> strength signal to last night's test.

> There is also an error floor showing up, about 6 to 8% BER
> with a very strong signal ...

I tracked this down to a problem in my receive timing system,
the one that mixes the signal and PPS into a BT878.

With that fixed, I can no longer reproduce the anomaly here.

EbSynth checks out ok - measuring the output phase against a
GPSDO reference shows a phase jitter around 0.14 degrees RMS
at 53077.222222 Hz and the peak phase deviation on a run of
a few hours was only 0.34 degrees.  The phase plot of a BPSK
transmission looks real good.

So maybe there's a problem with the multiplier, which uses
junk box parts and an unregulated supply.  It wouldn't be too
hard for the thing to introduce an intermittent phase error,
eg by a fluctuating threshold of a Schmitt trigger or an
unstable capacitor.

--
Paul Nicholson
--