Author Topic: HP 8753C Calibration/Use Issue Phase Measurement  (Read 380 times)

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Offline calmissile

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HP 8753C Calibration/Use Issue Phase Measurement
« on: November 01, 2018, 10:45:38 pm »
We are using a HP 8753C and 85046A S-Parameter Test Set to make some antenna/feedline measurements.

The acceptance test has the VNA connected through about 25 feet of 50 Ohm coax and then connected to a 4 ft coax pigtail that connects to a slot antenna.

The center frequency we are measuring is 113 Mhz.  We are doing a Full 2 Port calibration, and connecting the SOL to the end of the test coax and the antenna pigtail coax is disconnected.  The test specs indicate that we should have about 0 deg phase shift at midband (113 Mhz).  Our measurements are about 2-4 deg out of tolerance.

In order to study the problem, we have disconnected the test coax and performed a Full 2 Port Calibration at the instrument itself.  The 85046A has APC7 connectors and we do not have APC7 standards.  We use a pair of identical APC7 to N adapters and a calibrated set of Maury N type standards.  The calibration kit called up is the 50 ohm N kit.  We have not changed or customized any calibration tables in the analyzer.

After the 2 port calibration we set the display to Phase format and reinstalled the open and short that were used in the calibration process.  The Open standard gives a phase reading of -1.68 deg.  The Short standard gives a phase reading of +178.54 deg.  This is part of what I do not understand..... The why?

We performed the identical tests/calibration on two identical 8753C/85046A and get nearly identical results.  In reading the theory of operation it is my understanding that the analyzer reads the phase shift from the Short and Open and makes an average of the two to arrive at the 0 degree reference plane at the calibration standard (end of the coax).

The delay specs for the Maury Open and Short standards are 37.026ps and 42.063ps which does not equate to anywhere near 1.5 deg at 113 Mhz.

Has anyone encountered this kind of issue in measuring phase of antennas and coax before?

Online TheSteve

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Re: HP 8753C Calibration/Use Issue Phase Measurement
« Reply #1 on: November 01, 2018, 11:11:33 pm »
Why have you not programmed the full spec's for your cal kit into the 8753c?

Offline hendorog

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Re: HP 8753C Calibration/Use Issue Phase Measurement
« Reply #2 on: November 01, 2018, 11:45:56 pm »
There is a heap of info here about setting up the cal kit on an 8753:

Personally I use this application with a GPIB adapter:

Basically you enter the provided cal kit 'coefficents' into the analyzer for each standard and calibrate the analyzer using those standards.
Then when you re-measure the same standards you should see the same coefficents in the measurements.

So if your open standard has a 37.026ps delay, then you should get a result similar to that when you measure the open standard after calibration.
If not then the only possibility is that you haven't entered the coefficents correctly!

i.e. Even if your open standard is knackered, it will still measure ~37.026ps, as you told the analyzer that is what it is.

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