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HP87512/VNA - RF Cafe Forums

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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

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