Glad to hear you made some progress. Sounds like the replacement YTO was
too sensitive (degraded phase margin and increase in loop BW due to increase
in overall loop gain), correct? I wonder if the best way to fix that would
be a shunt resistance across the tuning coil. There's a lot to be said for
not altering the loop filter itself, if possible. It has to work over a
rather-wide range of conditions as the analyzer sweeps across the band.
-- john, KE5FX
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-----Original Message-----
From: hp_agilent_equipment@...
[mailto:hp_agilent_equipment@...]On Behalf Of Chris Bartram
Sent: Monday, November 20, 2006 6:09 AM
To: hp_agilent_equipment@...
Subject: [hp_agilent_equipment] HP8568B Phase Noise
I was able to spend a couple of hours looking at my venerable 8568B this
weekend.
After running through the adjustments, and sorting-out some minor
alignment
issues with the 50MHz VTO and 249/259 synthesisers, the it became
clear that
the major problem was the tuning sensitivity of the YIG
oscillator. This is a
replacement part (sourced from a redundant 10MHz - 2.5GHz plug-in for an
HP8620, BTW...) This was causing the phase-noise to peak by a few
dB at about
150kHz. I was able to reduce the peak, and move the corner
frequency back to
just below 100kHz by slightly changing the constants of the YTO
loop filter.
I hope that's of use to somebody else.
Incidentally John, KE5FX's phase-noise measurement software was
very useful
indeed in chasing this problem.
Vy 73
