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

[Graham / KE9H]

Bruce:

Very nice work, with useful data.
If I re-design Excalibur for a future production run, I will add some 
capability
to optionally disable the 10 MHz on board oscillator.

Thanks,
--- Graham / KE9H

==

Bruce Beford wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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
>
>
>
> _______________________________________________
> HPSDR Discussion List
> To post msg: hpsdr at openhpsdr.org
> Subscription help: http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
> HPSDR web page: http://openhpsdr.org
> Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/
>
>   


 1262549144.0