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I ran a comprehensive RF-DIAGS error test on a recently acquired HP 8920A.? One failure occurred and it reports as follows:

"Output Assembly
Output Frequency Range Closed Loop
1 MHz Lower Vdc: 0.750V? Upper Vdc: 2.50V? Measured:? 0.588V **? "

So, it appears the voltage measurement at 1 MHz is below spec range.? All other frequencies up to 1 GHz test OK and are within range except for 1 MHz as shown above.

The error also shows:

"Change RF Generator Amplitude, Output Port, or Atten Hold (if on)."

The test routine doesn't allow for on-the fly changes that I'm aware of.? I did try going to the RF GEN menu to make those changes and the result is the same.

Any clues here?? Thanks in advance.

Paul


?

looks like you have a bad output assembly but since it's on the low frequency side I would try to re-seat the output , referencne and synthesizer boards and also re-seat and? tighten the rf connections on the backplane? ?these are the coax connections that have a 5/16 nut (IIRC)? ?...you can loosen the nut and pull out the coax and re-seat.


Could be the synthesizer output is dropping at the higher end and due to the different path failing the diagnostic.



The second error message is kind of the same thing where it cannot get to the output level you specified for the port selected.

?if the DUPLEX port is selected you can get to higher outputs? like -12dbm? while the RF I/O port is like -20dbm max.

If you try to set 0dbm on the RF I/O port you will see that error for example.

This may be another? symptom of a bad output assembly as well as other issues with the RF level not being correct.

just a quick note on my post.

The DUPLEX port is capable of higher level outputs ....like 7dbm IIRC?

The RF I/O can make it up to 0dbm? IIRC in some cases .

Please refer to the documentation

The short version is that you can try and set the output to whatever you want but if the 8920 cannot make the level on the port you selected that message will come up

Good advice.? I did try re-seating the modules and re-tighten the bottom plane SMA connectors.? No go.? 1 MHz still fails.? I have ordered two spare output modules to try.? They should arrive later in the week.? The diagnostic result says the problem is localized to the Output Module but I realize that depending on switch paths, this could be misleading.?

Paul

The replacement Output Module arrived and the 8920A now passes all self tests.? Thanks to all for their assistance.

If i can figure how to crack open the Output Module, I may try to find the culprit.? Because it failed at 1 MHz and was on the lower edge of acceptance at 10 MHz, is it reasonable to first suspect a capacitive coupling problem???

Paul, W9AC

The way that the 8920 generates the low frequency is somewhat counter intuitive.

There are three paths in the Outpur Assembly

The synthesizer runs from 500-1ghz? ?so that range is just straight amplification for path 1 and the 500 -1G range
Path 2 uses a? /2 divider on the Synthesizer so you get 250 -500 Mhz range?

Path 3 which is what you are having problems with? mixes the synthesizer with 1Ghz? to get the .4 to 250 range
For a 1 Mhz? output you have 999 Mhz mixing with 1Ghz.
The higher the output frequency needed the lower the Synthesizer frequency needed.

This is the more complex of the three but at least at this point you know your 1Ghz and Synthesizer? signals are correct going into the assembly??

You can use a 9 Pin D extender which gets you everything but the RF signals? ...usually use some small coax and RS-232?Male and female pins to extend those as needed for the inputs.

You should have an external Spectrum Analyzer? ....without that I wouldn't bother.

It is better to fix the original module to maintain the calibration

Wow, that really is counter-intuitive but it now makes sense.? Thanks for the detailed explanation about the signaling paths.?

?

I’m inclined to leave well enough alone for now, then perhaps at a later time I’ll ship it off to Amtronix for a full cal with the new output module.?

?

Paul

just happened to be looking at the CLIP ....there are only two MMIC's in that that like an MSA-0486 IIRC.

Would just replace both and see what happens.

The 8920? CLIP may be in the files area ....at least it used to be