HP87512/VNA - RF Cafe Forums
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
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...
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