Timenuts-
I was able to borrow a Holzworth HA7062C phase noise analyzer recently and used
it to measure phase noise for several oscillators.
I have a few questions:
1) I noticed that ground loops can be a significant problem - generating noise
peaks at multiples of 60Hz and/or multiples of 120Hz. If this were merely
additive noise, then no noise peaks near the 5MHz or 10MHz carrier should
result in the analyzer, so non-linearities must be involved. Where are those
non-linearities arising and how can they be eliminated?
2) Using several 50 ohm baluns and disconneting power supplies from ground
eliminated some of the 60Hz-related noise in my tests but not all. The attached
is a phase noise plot of a 5065A, run into a Wenzel low-phase-noise doubler run
off a battery, run through a balun into the analyzer. Some 60Hz-related peaks
are still evident - should I assume they are produced internally by the 5065A?
In this case I could imagine that some additive 60Hz noise on the output of the
5065A gets nonlinearly transformed by the doubler. Unfortunately, the HA7062C
lower frequency limit is 8MHz, so I had to use the doubler (or something
similar).
3) See the 105B plot, which is drawn directly from the 10MHz OCXO (I modified
the 105B, adding a TAPR buffer of the 10MHz OCXO to feed an external
port),which has bad 60xN Hz peaks despite my using a balun to eliminate ground
loops on the output. So no doubler in this case - where are these 60Hz-related
peaks coming from?
4) Both 5065A and 105B oscillators show peaks at 10KHz (multiples), 100KHz,
1MHz, etc. Is this nonlinear feedback from the dividers in the OCXO devices? I
could remove the dividers to see if that has an effect...
Another question - I can't afford to buy the Holzworth analyzer for my personal
use, but I would like to continue my phase noise experiments. Any advice on
acquiring or building an adequately sensitive instrument that doesn't break the
bank?
BTW, I recently visited the Holzworth factory. Their designs and manufacturing
processes are impressive.
Thanks in advance-
Jim
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