Bruce Griffiths wrote:
Mark Spencer wrote:
Seasons greetings.
I was pleasantly surprised to find that a new to me HP 5328A had an
10811 in
it. (I was expecting the older OCXO..) After leaving it
running for a
week I adjusted the frequency and while doing so I noticed that the
out put
frequency changes by a third of a hz or so if I add or remove a 50
ohm load. I'm measuring the frequency at the timebase output
connection of the
5328A. Is this to be expected ?
Many thanks
Mark Spencer
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No, that seems to be a couple of orders of magnitude larger than the
specs for the 10811A would lead one to believe.
Maybe there's a problem with the output buffer amp in the 10811A?
Bruce
The spec for the 10811A actually states that the shift is < 5E-10 (5mHz)
for a 10% (5 ohm) change in a 50 ohm load and a for a 25% change (250
ohm) in a 1K load
A naive extrapolation would suggest a shift of around 20mHz for a 50 ohm
to 1K load change your frequency shift (333mHz) is about 15x this.
However the 10811A is notorious for injection locking if the reverse
isolation of the external circuitry is inadequate.
Bruce
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