Repair HP83751A RF Generator 2-20GHz  (Synthesized Sweeper) - Page 1
Forum section: electronics / repair

author: Papoulis_RF
author_rank: Contributor
timestamp: December 19, 2021, 06:42:43 pm
content: Hi everyone,Today I write for the first time a repair subject.In my lab, I have a "gorgeous" HP83751A which is an RF synthesized sweeper 0-20 GHz. I experience some problems using it in the very beginning frequency of its frequency range: 2 GHz to 3.33 GHz which is a little bit annoying.The fact is when I generate any signals into this band, I get the "Unlock" and "Unlevel" warning blinking at the screen with no signal at the output of the generator. For the rest of the band, nothing to report, the internal oscillator doesn't show any locking problem and it's able to generate whatever I ask him to do. I also made a full self-test and I get the following error "LowBandIF". "A8YoLoop FAILED".To have a better representation of the situation I made a measurement at the RF output of the generator with my spectrum analyzer using the max hold function (see the pictures). From that, we can clearly see the problem locatedbetween 2GHz and 3.5GHz.The service manual indicates checking the sampler IF Input by placing and measuring a signal at a specific port. But at the specified port, I get no signal.. so following the manual, I checked the output signal of the sampler (between A8 and A16) but I what I measured don't match the one indicated in the picture (10MHz -15dBm), i get something higher around hundred of megahertz.I'm wondering if there is some RF ayatollah kind of man who had ever experienced this problem and would give me some advice and hints to find the solution... I'm also looking for someone who can explain how this architecture works in more detail.Have a great day

author: paul@yahrprobert.com
author_rank: Regular Contributor
timestamp: December 20, 2021, 03:49:31 am
content: If I recall those generators contain a few yig oscillators of different frequency ranges. Maybe one of the oscillators is bad?

author: Papoulis_RF
author_rank: Contributor
timestamp: December 20, 2021, 09:16:29 am
content: Hi Paul,You right, on this unit there is a dual YIG oscillator generating signal between 2 GHz and 20 GHz, its composed of two YIG oscillator in one mechanical housing.I found a great old article from the 93" explainning the architecture of HP 83750 series (page 38 to 51)https://worldradiohistory.com/Archive-Company-Publications/HP-Journal/90s/HPJ-1993-04.pdf

author: Papoulis_RF
author_rank: Contributor
timestamp: January 08, 2022, 06:18:09 pm
content: Hi,I post an update of my investigation. I did some measurements in order to first try to isolate functional blocks from the non-working ones.First I checked power rails. All the rails are present on all the boards. Then I look the Fractional-N (A7). It is internaly composed of a VCO, frac divider, and a PLL. Its output (LO signal) is connected to the sampler who acts like a mixer. The frequency output of A7 is in the range specified by the schematic in terms of frequency and amplitude i.e 250MHz to 500MHz ~15dBm. So nothing unusual here.For the dual YIG oscillator (A18 DYO), I still can't verify the generated frequency under 3.4GHz simply because when I set something between 2GHz and ~3.4GHz the all system became unlocked and unleveled. However for all the others frequenciesI managed to measure correct and stable values of freqs and amplitudes for the all the: ~3.4GHz to 20GHz band. The 2 GHz to~3.4GHz still remains unusable. All this stuff make me think that the YIG oscillator is OK.The part where I am stucked in my investigations is the diagnose of the A8 YO Loop function. The global full-test output an A8 YO Loop FAILED. When I look into the detailed sub-tests performed for this A8 function a get :- YO_LO_FM : PASSED- FMCoil : PASSED- FMCal : PASSED- FMSummer : PASSED- FMInput : PASSED- YOIntegrator : PASSED- YOPhaseDete : PASSED- HighBandIF : PASSED- LowBandIF :FAILEDWhen I take a look to the schematic of the A8 YO Loop function, I can easily locate where the tests have been performed on.I was exepting something to fail in the lower right corner, more precisely on the YO LO FM path which is in charge of driving the Low band YIG oscillator (where the problem occur). But when I'm look the performed tests, both YO_LO_FM and FM Coil are PASSED..It's a bit confusing because the system output a fail coming from A8 labeled as "LowBandIF". However, when I look to the the detailed tests performed in A8, all seems to be OK except one : LowBandIF which I consider not to physicaly belonging to A8.Is anyone have its own interpretation or some experience in debugging PLL, Yig, Loop or synthesizer to share ?I put a photo of the A8 board it's a 3 layers board, all the chips are populated on the top.Have a good day