[hpsdr] Spectrum pics of 10 MHz clocks (longish)

[Bruce Beford]

Hello to the list.

I have just uploaded two HPSDR-related pics to my web account. They are
spectrum captures of the output of Mercury with no antenna connected.
Mercury is tuned to 9,999.400 MHz, USB. This results in the 10 MHz clock
that gets into the receiver to be "heard" at a nominal 600 Hz frequency.

I have an Excalibur board fitted, and selected as the 10 MHz source in
PowerSDR. Excalibur is fed by a Trimble Thunderbolt GPSDO as the external
standard. The clock select jumper on Excalibur is in the "external"
position. The mechanical fine adjust pot on Excalibur's on-board TXCO has
been adjusted so that it is within 1 Hz of the external reference. The audio
output from Mercury is fed into the soundcard on the PC, and Spectrum Lab
software is used to analyze the audio spectrum. The HPSDR hardware has been
powered up for about 2 hours to stabilize.

As previously discussed, I have modified my Penelope board to add a jumper
that disables the 10 MHz clock, much like the one that is included on
Mercury. At the beginning of this test run, both of those jumpers were
removed. i.e.- no power to the onboard 10 MHz oscillators on Mercury and
Penelope.

Here is the first "wide" scan:

http://home.myfairpoint.net/vze1t5sc/HPSDR10MHz_a.jpg

I ran PowerSDR and Spectrum Lab for a few minutes to collect a "baseline"
trace, showing the signal resulting from the clock from Excalibur to the
Atlas Bus. After a few minutes, I enabled the clock on Penelope by
installing the jumper.

At the left of the spectrum, you can clearly see where I applied power to
Penelope's clock. It drifted down slightly during warm-up, then settled in
about 68 Hz below the reference signal from the GPSDO. As an additional
experiment, I then enabled the 10 MHz oscillator on Mercury. You wouldn't
normally run like that, but I decided to try it. You can see the clock
on-board Mercury warm up, and settle in with a somewhat stronger signal in
the receiver, about 32 Hz below the GPSDO.

This experiment clearly shows that the clock on Penelope does get into
Mercury's receiver, even if it is not selected as the 10 MHz source in the
software. This is why I have chosen to modify my Penelope to allow turning
off this clock. In a typical receiver application, it would probably not
make any difference at all. However, in a demanding application such as a
spectrum analyzer (Cyclops) with or without a tracking generator, it could
be a bother. Other weak signal applications may also be impacted.

In this second photo, I have zoomed in on the Excalibur signal:

http://home.myfairpoint.net/vze1t5sc/HPSDR10MHz_zoom.jpg

Here you can see the GPSDO external reference as the strongest trace. (It is
also nice and straight of course, because Mercury is locked to this.) To the
left of that you can see the signal from Excalibur's on-board TXCO. It is
running about 180-200 milli-Hertz below the GPSDO. Just below that, you can
see another trace that I believe is a mixing product of these two clocks.
The presence of the TXCO signal in the receiver, even when it is not
selected, is the reason that I may modify my Excalibur at some later date. I
would like to have the on-board TXCO to only be powered when there is no
external reference detected.

I hope that you find these pictures interesting and informative. I know I
enjoyed doing the experiments.

73,
Bruce Beford, N1RX