I am thinking that I should simply make a very simple notch filter
that suppresses the spurious line by 10 or 20 dB. I can then do
an A/B comparison of the measured ADEV with and without the filter.
If there is no change, I'm done. Otherwise, I add filtering in
steps until the ADEV reaches an asymptote.
Rick
On 8/6/2018 6:39 PM, John Miles wrote:
Again, this is analogous to the 5071A having 1 MHz
spurs of about -90 dBc, yet the ADEV is not much
different than an open loop 10811. How high would
the 1 MHz spurs have to be to affect ADEV?
With ADEV, you have to consider the measurement bandwidth before you can ask
questions like this, much less answer them. :) 1 MHz spurs are far outside
the restricted measurement bandwidth that would be desirable when measuring
a high-performance source like a 5071A. I suppose there might be some
influence if you use a DC-to-daylight sampling process such as a counter,
but the instrument noise floor will probably obscure any interferers in the
-90 dBc range.
With a TimePod or similar device, a general rule of thumb is that a spur a
few Hz away from a 10 MHz carrier at -120 dBc will cause ripple in the 1E-11
to 1E-12 range. This coincides with typical levels of coupling between
nearby RG-58 cables -- see page 38 of
http://www.miles.io/TimePod_5330A_user_manual.pdf for instance. That
particular situation wouldn't have shown up on a traditional counter-based
measurement, but others might.
For instance, in your 1 MHz example, I doubt you'd see any effect at exactly
1 MHz, but if your spurs are at 1.000001 MHz and your counter is capable of
1-ns single-shot resolution, I can imagine that there will be some
corruption. The exact numbers will depend on so many factors that it's
almost easier to set up a test and observe the effects firsthand.
-- john, KE5FX
Miles Design LLC
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