HP 8720A VNA Calibration Issue

[neuschs]

Hi all,

I purchased a HP 8720A for a good price because while testing it on site the RIFA caps blew.
Shortly after repairing that - still waiting for a good measured calibration kit - I wanted to do some simple measurements. I used Rosenberger open, short and a mini circuits load. Everything works. I am happy. some reference attenuators look pretty good in respect to magnitude. (All polar/smith/phase plots had issues as expected due to wrong calibration standard models.)

But from now and then - and only on S11 not on S22 calibration - the VNA decides to have 40dB of error after applying the calibration measurement corrections.

Concrete step-by-step:
0. Reflected Magnitude (uncal) looks as expected with a lot of magnitude errors
1. Enter Calibration Menu
2. Choose S11 Port Cal
3. Measure Open, Short, Load
4. Press Calibration Done
5. 8720A "Calculating calibration coefficients ..."
6. Reflected Power is a straight line (with an open or short connected) as expected reading 0db

Some times I redo the steps above and at No 6. I start reading - 40dB and worse.

Do you think that could come from a degrading memory chip which some times work and sometimes not ?
In case someone has any ideas on what to try or how to narrow down the issue, please respond !

Kind regards
Sebastian

[TheSteve]

Cal kit making poor contact during calibration?

[neuschs]

Hmm... I don't have connector gauges, but I will try to make microscope shots of the connectors and have a look.

Edit: But the realtime measurement view looks fine while doing the calibration, so full reflection for short/open and high return loss for load calibration.

[neuschs]

I exchanged some adapters in between and actually yes one of them seems to be just faulty. Even thought I can not measure any difference between this one or another one.