I purchased an early vintage HP3325B synthesizer generator, from about 1988.

Unit is working all right, as I were checking most of the verification items. I have locked the HP3325B to my HP5370B OCXO, and measured the stability of the synthesized frequency with the same 5370B.

I recognized, that there's a frequency instability of about 0.003 Hz, which is about constant for all frequencies.

At 10Mhz output, that would give a stability of 3E-10, that's really great, but at lower frequencies, e.g. at 1Hz, that would be 3E-3 only.

This frequency stability of the synthesizer circuit is not directly specified in the manual. I assume, due to the high PLL division ratio at low output frequencies, relative to the nominal 30MHz VCO frequency, that this is a normal behaviour.

So my question to the time-nuts list, is my instrument working properly, or should the lower frequencies be as stable (in relative terms) as the higher ones?

Is there any adjustment, or maybe later circuit revision, that would improve the feedback loop to better stability?

I only have the 1990 service manual (03325-90003) available, so I have no information about later changes.

Thanks - Frank

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