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8568B - bad frequency stability and jumping baseline noise level...

Question asked by Uky on Aug 23, 2012
Latest reply on Oct 5, 2012 by Uky
Forum,

I have a 8568B analyzer with the following problem:

I assume that whatever the problem is,
it is in the lower 19" RF unit.

There seems to be a problem with LO locking. Both wide
and narrow band, although it is more notable when sweeping narrow band.

When in narrow band (Span ~hundreds of kHz) and the instabilities occurr, the baseline
noise level jumps + 20dB or so. When sweeping wide band, a test RF signal shifts in frequency,
but not to much.

The problem is not persistent. It comes and goes in seconds
or minutes. I am thinking "leaky capacitor in a PLL somewhere"

I have checked for bad solder joints
in the power supply regulation circuitry as well as correct voltages
and also for unscrewed filter capacitors. When sweeping narrow band,
there are no red LED warnings.

Before I start digging, I wanted to know if there are anyone
who have had a similar problem and can help me narrow my search.

I know that this is an old instrument, but it would be pity to scrap it.  

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