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A look inside of an Agilent E4400B signal generator

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nctnico:
I'd try and find where the lock signal is generated and trace back from there. I think most of the HF stuff is there to generate a sine wave between 250MHz and 4GHz. The output frequency is divided in several ranges and this may affect the PLL as well so the first thing to check is whether the error occurs on all ranges or only at a certain frequency.

feedback.loop:
By the way, look at the info: the unit has 32K hours on it, which is about 3.6 years, about 1.5K power cycles and about 3.8M (!) attenuator cycles.

Regarding the unlock problem. I put the synthesizer back together, and couldn't reproduce the problem since. I run the unit for several hours from time to time. No errors so far.

Pete2:
I am looking at a faulty 4400B on Ebay with the same error you had: 508 Synthesizer Unlocked. Sadly, it doesn't help that your unit repaired itself. Not much experience in repairing RF stuff so maybe fixing a possibly faulty Synth module is too much for me. Although I don't know if there's a supply rail missing as well. Working modules on Ebay cost more than broken units!

R-1125F:
Have a look at this thread on the HPAK group.  There is a problem with some frac-n chips.
https://groups.io/g/HP-Agilent-Keysight-equipment/topic/synthesizer_module_in_esg/28079593

Pete2:

--- Quote from: R-1125F on October 16, 2023, 09:04:57 pm ---Have a look at this thread on the HPAK group.  There is a problem with some frac-n chips.
https://groups.io/g/HP-Agilent-Keysight-equipment/topic/synthesizer_module_in_esg/28079593

--- End quote ---

Thanks for this. Seems like getting the unit is a big risk and even if the same chip is faulty the replacement would cost 150 eur on top of the unit that is about $450 + VAT for my country (24 %) including shipping. I asked the seller if there is the unlevel error as well which MAY indicate a missing supply rail but does not gurarantee an easy fix. Explains why noone has still not bought the unit.

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