I think that Vref is derived from the internal voltage regulator that
powers the oscillator. Is it possible that any noise will appear on
both and therefore, they'll cancel each other out? That could even
reduce power supply sensitivity.
You could test that by measuring ADEV with an EFC pot powered by Vref
vs. powered by an external quiet supply.
Ed
On 4/21/2013 2:47 PM, Poul-Henning Kamp wrote:
Ok, this surprised me a LOT!
I used my HP3458A to measure the stability of the Vref output from
an IsoTemp OCXO-107 and the results were anything but stellar:
The OCXO's Vref has 18 microVolt RMS noise
For comparison my HP6626A power-supply has 2 microVolt RMS noise.
Obviously, the Vref only has to be good enough to not be a relevant
source of noise when used on the EFC input.
If the OCXO has a EFC sensitivity of 10PPM/Volt, then a microVolt
more or less means a 1e-11 change in frequency.
The Vref is also quite sensitive to the OCXO supply voltage, but
since it was powered by the HP6626A, that's not where the noise
comes from.
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