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HP87512 /VNA

HP87512 /VNA

HP87512 /VNA

(OP)
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 'bride' from HP have nothing to do with a Wheastone bridge (from what Icould 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 ????
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

RE: HP87512 /VNA

When you say you opened it up, what exactly did you see?  I would assume there was some sort of ferrite based directional coupler inside.  What did the parts look like.

RE: HP87512 /VNA

(OP)
Hello Biff44,

No in fact there is no ferrite based transformer or something at all. In fact it's made of resistors, but not in a way to build a Wheastone brigde. In fact the Output signal is divided in two signal of different level, one for the Reference port, the other for the A port... If necessary, I'll be able to give you the exact value of those resistors and the schematic on Monday.

If it can help ...

Didier

RE: HP87512 /VNA

Yeah, get the resistor values and schematic.

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