[hpsdr] Frequency control

[Jim Sanford]

All:

I've been dealing with some interesting issues, subsequently resolved, 
but have questions.

My HPSDR setup is locked to a GPS-stabilized 10 MHz reference. 
(Excalibur, Mercury, Penelope, Metis, PennyWhistle)

Yet, I received reports of being 150 Hz or so low and slowly drifting on 
10 meters.  I was able to confirm with a GPS-locked frequency counter 
(EIP 545A)

AFter a lot of measuring and digging, I resolved the frequency issue -- 
the 10 MHz reference was not getting to the radio.  Fixed that and I'm 
now on frequency, verified on-air and with the frequency counter.

Here's the question:  I don't know how it worked at all without the 
reference.

I have Excalibur jumpers set to use the external reference. Unless 
there's a LOT of leakage, there should have been NO 10 MHz on the Atlas 
bus.  Mercury jumpers are set to get 10 MHz from the Atlas bus.  I 
couldn't find any similar jumper on the Penelope board.  In SETUP, the 
Excalibur Present flag is set.  Mercury is set to be the source for the 
122.88 MHz signal.

Looking at the schematic, I see that Penelope has an on-board 10 MHz 
reference which goes to the FPGA.  It appears that the PLL for the 
122.88 MHz is within the FPGA, with a discrete loop filter outside of 
the FPGA.   I see that the Atlas bus 10 MHz signal also goes to the FPGA.

SO, it would APPEAR that the FPGA senses the presence of the 10 MHz 
signal from the Atlas bus, and if there, uses it, if not, it uses the 
onboard 10 MHz and uses it for the PLL reference, AND puts it on the 
Atlas bus for Mercury. *_/Is this correct?/_* I can see nothing about 
this behavior in any of the documentation.  Absent that behavior (which 
looks like a nice fallback, if intentional), I would expect the radio to 
not work at all, without the external reference.  A friend with a 
similar setup (Hercules amp) has noticed the same behavior, so I don't 
think there is something else wrong.

I would really like to understand what is going on, and would be most 
grateful if anyone could explain what I am seeing.

Thanks & 73,

Jim

wb4gcs at amsat.org



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