[hpsdr] clocking janus

[Bruce Walker]

Last night, I finally got around to hooking up a GPSDO (Thunderbolt) to
Atlas.  My immediate goal is to get precision sampling (frequency) from
Janus.  I'm not a HW engineer, and my interface from the Thunderbolt is
pretty naive: terminated with 50ohms, then coupled through a capacitor to a
voltage divider between 3.3V and GND into C16.  I don't have a scope, so if
anyone has a suggestion of proper components for an interface which gets the
levels right from a ~10dBm/50ohm Tbolt input, please share.

Using PowerSDR PennyMerge, if I have Penny in the system and select Atlas
for 10MHz, it seems to work correctly. Sampling frequency is spot on as far
as I could easily measure, though there are occasional short drop-outs on
the audio which lead me to believe the clock is marginal.  With Penny not
installed, it does not work: Janus seems to revert to it's own oscillator
rather than lock to the 10MHz, off by ~100ppm.  Not clear to me why this
would be, unless just having Penny further down the bus is providing better
termination for the signal.  I have not played with slot location any
further.

If anyone has gone through this exercise or has suggestions to "do it
right", please advise.  I suppose I could put something like a Clock Block
in front of it to drive it with a stiffer square wave, but looking for a
minimal robust solution.

--bruce w1bw
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