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HP8753D fails Operator's Check on port 2

Question asked by mankan on Mar 20, 2018
Latest reply on Mar 25, 2018 by mankan
Hi all,

 

new owner of a HP8753D with 1D5 and 006 options and new to this forum as well. Currently running firmware 5.34 but have ordered 6.14 EEPROMs. Touchstone file and extra marker together with numerous fixes seems to make it worth the trouble and cost.

 

I got the instrument really cheap from work since it "sometimes looses calibration and displays things like SWR 0". This is nothing I have encountered previously when doing some hobby HF/VHF measurements (ham radio) at work.

 

Anyway, the firmware upgrade instructions recommends a pre-upgrade check, i.e. run test 21 (S11 on port 1) and test 22 (S11 on port 2) also known as Operator's Check in the service manual.

 

First I started out with a short N male/SMA male cable with a female SMA connector which I shorted with solder. With this both test 21 and 22 failed.
So I changed to just using an N male/SMA female adapter and a solder shorted male SMA connector. Now test 21 passes but test 22 still fails.

 

The failing part of the test is before port and attenuator switching occurs and it seems like it is only the measurement at 30kHz that is outside the +/- 4dBm limits, it measures -4,3dBm. Already at 31.3kHz the S22 value is -4dBm. For S11 test the values are more like -3dBm@30kHz and -2.3dBm@31.3kHz.

 

I have double checked this by doing manual S11 and S22 measurements in the kHz range and the -4dBm value is still at 31.3kHz, i.e. no interpolation is going on.
I have also verified the same measurements by borrowing a SMA calibration kit and N/SMA adapter from work that is used for a 26.5GHz R&S VNA.

 

The advice I got from a HW guy at work is to remove the APC/N adapter and clean it. Tried that on port 2, almost no dirt to be cleaned away. Same results as above afterwards.

 

I've done some basic tests following the service manual (internal tests, test 17, 18 & 19, ABUS/analog aux curves all pass) but since the copy I have, has no pictures and graphs in it so I have combined it with the 8753E service manual. Anyone have a link to a complete 8753D service manual?
The A & B input traces test, shows 1-2dBm lower value than 8753E graph. Is the problem in the receiver end or should I run RF output power correction tests?
What do you guys think could be wrong? Any advice on further tests? Or should I just ignore this and go ahead with the firmware upgrade?

 

Best regards

Marcus Gustafsson. SA5PMG

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