Dave Haupt wrote:
> A vector voltmeter is what I used to make my first
> complex Z measurements at home. I was doing a
> consulting job, so I could purchase a pair of
> mini-circuits connectorized directional couplers and
> deduct the cost. But, one could built a resistive
> return loss bridge, using only one twisted-pair balun
> to deal with the "ungrounded leg" issue.
>
> Send the signal through the two dir. couplers, with
> one configured to create a "forward" sample, and the
> other configured to provide a "reverse" sample.
> "Forward" goes to Vector Voltmeter channel A,
> "reverse" goes to channel B. With an open circuit,
> adjust the phase offset knob on the VVM to show zero
> degrees. If you need an amplitude correction, you can
> put an attenuator in the line from whichever coupler
> sampled port you need to get an amplitude of 1. Now
> connect the unknown. The amplitude and phase reading
> of the VVM represent the magnitude and phase of the
> reflection coefficient. Plot this point on a Smith
> Chart and read the complex impedance from there.
>
> I still have the instrument, but along with the rest
> of my lab, it's packed away in some unknown storage
> place, awaiting the day that I finally select a house.
>
> 73,
>
> Dave W8NF
Although your method assumes the couplers are perfect, which is not so.
They will not have infinite directivity. A lot of the sophistication of
modern vector network analysers is in the removing these less
significant effects. I expect a review of some Agilent/HP application
notes would allow you to improve on it, using open, short and 50 Ohm loads.
Ian, G3SEK mentioned a system based on a card that fits in a PC that is
very good too.
--
David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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