I wrote the other day about seeing a transient that I thought
was bouncing off the primary center.
Now after dismantling and measuring a few things I find
that transient was reflecting from the LPF which follows the
transformer. The transient width was about the same as the
FET switching time and it came back to the drain of the off
transistor at a voltage many times higher than the supply.
Pinging the transformer itself I can see no sign of any
transmission line behaviour up to 50MHz, the turns are just
too tightly coupled. In fact the coax cable with outer as
primary and inner as secondary has a very tight coupling.
351uH per winding and 1uH leakage inductance. I wont be
using coax again though, the inability to tap the secondary is
a severe limitation.
So I learn that it's no good just looking at the bandpass
and bandstop of the LPF, one might have to consider transient
reflections because they end up back at the drain. Almost all
the departure from idealness of the drain waveform was down
to the LPF and not the transformer. I still haven't accounted
for the excessive heat of the windings when the primary center
is AC grounded.
The divided down 477.69kHz multiplier output was compared
with a GPSDO reference and the RMS jitter was less than 2
degrees over timescales up to 60 minutes. Here is a plot of
the 2-sample RMS jitter over a run of eight hours
http://abelian.org/vlf/tmp/160302a.gif
The plot means for example, that the average difference
between two samples of the phase taken 512 seconds apart is
1.04 degrees. The plotted phase is scaled back up to the
tx frequency. The jitter at the soundcard output of EbSynth
is one ninth of the plotted value.
This pretty much convinces me that the spreading seen by Markus
was unlikely to caused by my tx. Some doubt remains because
I'm using the same GPSDO (Trimble) to reference both EbSynth
and the phase comparison. Also the PA and antenna were not
in the loop.
The phase comparison is done using vtsid from vlfrx-tools,
sampling the phase 100 times per second.
--
Paul Nicholson
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