[hpsdr] Penelope changes for Excalibur?

[Bruce Beford]

n3evl wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> I'm almost finished construction of Excalibur and will be ready to try 
> it out soon.
>
> I seem to recall some discussion relating to disabling the on-board 
> 10MHz osc on Penelope.
>
> Have I remembered this correctly and is this considered an essential 
> hardware change?
>
> Is it only Penelope that is affected (I don't remember seeing any 
> similar discussion for Mercury)?
>
> I plan (initially) on using Excalibur's on board TXCO but will likely 
> add an external ref later.
>
> Can any of the Penelope/Excalibur resident experts clarify the need 
> for this possible change and provide details (picture would be nice) 
> of exactly what/where/how to achieve this.
>
> Thanks, and Happy New Year to all on the HPSDR list.
>
> Pete, N3EVL


Hi Pete. I was the one who posted some info about disabling the 10 MHz clock
on Penelope. As Graham stated, Mercury had a jumper included on the board to
cut power it's own 10MHz clock. This was presumably designed in so one could
disable the onboard clock if one was using Penelope as the 10 MHz source.
This jumper was not included in the design of Penelope. 

Once I added Excalibur to my system, I added a jumper to my Penelope for
this purpose. It is a simple mod, and in my opinion does not represent
"cutting up" my board. I have posted two pictures showing the mod here:

http://home.myfairpoint.net/vze1t5sc/Penelope10MHz_detail.jpg

http://home.myfairpoint.net/vze1t5sc/Penelope10MHz_detail2.jpg

You can see the new 2-pin jumper just to the right of the 10MHz clock chip
(U2). It is located just below the 3V3 silkscreen at the top of the board.
It is a standard, 0.1 inch spacing 2 pin right angle header. First the trace
is cut to the upper right pin (3.3v line) of U2. Then the header is
installed across the gap. This allows the oscillator to be re-powered, if
ever desired.

As Phil said, it is not mandatory to do this. You could just leave the clock
running. However doing so leaves the another RF source running in the
system, with the potential for mixing products producing birdies in the
receiver. As this is High Performance SDR, I decided to do this simple mod.

As a side note, I use an external reference with Excalibur. With nothing
connected to the antenna on Mercury ,I can tune the receiver to 10 MHz and
see a nice peak from the external reference clock. If I tune to 9.999.4 MHz
USB, I can produce a nice 600 Hz note in the audio out. I have used this
note to calibrate the soundcard clock on my computer, using Spectrum Lab.
This is done to obtain the most accurate results on frequency measuring
tests. In the zoomed spectrum of the audio, I can clearly see a second peak
caused by the onboard TXCO on Excalibur. I may eventually modify my
Excalibur board to disable the onboard TXCO whenever an external clock is
detected. At some point, as an experiment I may reenable the clock on
Penelope, and see if I can discern a signal from that as well.

Again, diasbling the clock on Peenelope is not needed for most applications.
But for some, and for those that have not been dreamed up yet, it may be
beneficial.

73,
Bruce Beford, N1RX




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