[time-nuts] time-nuts] OP-Amps for 10MHz distribution...?

John Miles jmiles at pop.net
Fri Mar 2 01:02:30 UTC 2012


Apologies if this is a dupe -- I'm not sure it ever made it to the list:

> The attached plot indicates the phase noise performance obtainable with
> a wideband FET (OPA653) input opamp.
> With a 10MHz +9dBm input, the phase noise floor is around -163dBc/Hz at
> 1kHz offset and around -154dBc/Hz at 10Hz offset.
> A quieter test source would be useful particularly for offsets below 10Hz.

What was the test setup?  Ideally the 10 MHz test source won't influence the
measurement if you use a splitter to drive both the input and reference
jacks, with the DUT in the input arm.   (I'm assuming that's what you're
doing, or you wouldn't have been able to see 1 Hz PN down to -140 dBc/Hz.)

In reality a noisy test source can still degrade a 2-port measurement, but
this will usually be seen at wideband offsets where the broadband noise
decorrelates due to differences in the input and reference paths other than
those caused by the DUT's additive noise.  I usually use a 10 MHz LPF for
2-port measurements even when the test source is a good crystal oscillator,
just to be on the safe side.

If you let it run overnight, does the close-in measurement floor drop any
further?   It looks like there's still a lot of instrument influence below
100 Hz.  Higher signal levels will help quite a bit, as well.

-- john





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