[time-nuts] Power Supply Noise Affects Thunderbolt 1 PPS

[Eric Lemmon]

I recently acquired a Symmetricom 8040C Rubidium Standard, hoping to use one
of my Thunderbolt GPSDOs to further refine its 10 MHz output.  To my
surprise, connecting the 1 PPS output to the 8040C did nothing- the "1 PPS
SYNC" indicator stayed dark.  I hooked up my other Thunderbolt and got the
same result.

The power supply I was using to run the Thunderbolt was a Hviele T-30B
switching supply made in China, that came with one of the Thunderbolts.  It
appears to be well-made, similar in quality to those made by Mean Well, and
it puts out +5 VDC at 3 A, +12 VDC at 1 A, and -12 VDC at 0.5 A.  Either
Thunderbolt works perfectly on this power supply, but I only used the 10 MHz
outputs.  When I looked at the 1 PPS outputs with a 'scope, I could see that
the 1 PPS was riding on a 5 or 10 Hz noise.  Since two working Thunderbolts
had the same noise problem, I suspected that the power supply had a spurious
output.

I hooked up a linear triple-output bench supply to run the Thunderbolt, and
now the 8040C locked up perfectly on the 1 PPS signal.  Since I don't want
to tie up one of my bench power supplies to run the Thunderbolt, I plan to
try adding some filters to the outputs of the T-30B switcher.  I'm thinking
that some low-ESR tantalum capacitors might do the trick.  Rather than
re-invent the wheel, I wonder if others found this problem, and what their
solution was.  I also wonder if this T-30B power supply requires a minimum
load on any of its outputs, for it to operate properly.  Hmmm.

73, Eric Lemmon WB6FLY