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,...)