[time-nuts] Trimble Thunderbolt E frequency pulling.

GandalfG8 at aol.com GandalfG8 at aol.com
Sun Nov 22 12:39:28 EST 2015

Please note that this question refers to the branded "Thunderbolt  E" 
GPSDO, in a silver coloured case with blue label and using a single 24V  supply, 
and not the gold coloured "Thunderbolt" that is often found marked  "Rev E".
Whilst testing a Thunderbolt E I've observed that the  frequency of the 
10MHz output is affected by the load impedance.
Switching a counter input impedance from 50ohms to 1Mohm for  example will 
decrease the frequency around 1 part in 10^9, and this is reflected  in a 
consequent increase in the DAC voltage over the course of a few  minutes with 
the frequency recovering at the same time.
Similarly, with the frequency steady with a 1Mohm load, switching to 50ohms 
 will increase the frequency by a similar amount with the DAC voltage 
decreasing  to provide correction.
I'm familiar with the concept of frequency pulling in oscillators  but 
would have expected the Thunderbolt E output to be better  buffered than this, 
it's certainly not something I've experienced  with the gold cased 
Before I start probing any deeper I'd be interested to hear if anyone  else 
come across this with the Thunderbolt E.

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