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HP87512/VNA - RF Cafe Forums

 The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Below are all of the old forum threads, including all the responses to the original posts. -- Amateur Radio -- Antennas-- Circuits & Components-- Systems-- Test & Measurement

DIDIER40
Post subject: HP87512/VNA
Unread postPosted: Thu Nov 11, 2004 4:32 pm

I have a question for those who have 'worked' closely with the equations involved in the VNA, and particularly for the calibration procedure.
At work we have a HP8751A VNA (5Hz-50MHz) working with a
reflexion/Transmission Test Set HP87512. As I'm always loking for 'the way it work', I opened it, thinking it was a directional bridge made with 50ohms resistors (Wheastone bridge) with the design adapted to work from 5Hz to 500MHz.
But it was a great surprise for me to see that the 'bridge' from HP,made of resistors, have nothing to do with a Wheastone bridge (from what I could understand !!). With the schematic from HP, I can't manage to find a 'directivity' behavior of the 'bridge' made of resistor. As the HP8751A work with a S parameter HP87511A or B, I know that the 'bridge' is not a Voltage/Current method of measuring the reflexion parameter. And if I look on the 'mathematics' for the calibration of a VNA (TRL or SOL methods,...), there is always a correction factor for the unperfect directivity of the directional coupler.
So is it possible to obtain the correcting factor even
with a nondirective bridge, or a SOL method is able to accept a nondirectional bridge ???? It seems that the signal from the Output connector is 'split' with diferent levels to the Reference Port and the A port...
I'm in a real confusion ...!!!

If you have any idea about the way the HP87512 work with the HP8751A, I would be pleased to read you on the subject (or if you know a link with the description of the way it work,...)

Didier

Posted  11/12/2012