On 7/14/2012 2:15 PM, George Dubovsky wrote:
> All,
>
> I guess it's time for me to put up my "common mode choke" test technique
> for all to shoot at. I have a HP 4815A Vector Z meter, a N2PK VNA, and a
> HP 8753C VNA. I have all the probe adapters for the Z meter, and I have
> been bitten by the small amount of stray C that you just can't eliminate
> (but you should be able to calculate out). I have finally decided that
> my standard test is based on the following:
>
> I made up a board out of .062 FR-4 that is approximately 6" across (and
> about 4" wide). I cut three 50 Ohm microstrip lines full width across
> the 6" dimension. I soldered an N connector on each end (6 connectors
SINCE YOU ASKED, there are a number of unresolved issues here from
a strict metrological viewpoint:
1. Your fixture is not an enclosed box. A defined Faraday cage is
recommended. The results might be affected by lead dress or whether
the test bench is conductive or not. Any fixture imposes an arbitrary
and finite spacing between the DUT and the ground plane. In general,
the measured impedance will depend on this. There is no "correct"
answer.
2. The N connectors should be male and female and you should use
a 2 port type N calibration kit.
3. You used the common short cut of a through cal instead of a
full 2 port cal. This means that you are not calibrating out source
and load mismatch. A common short cut to fix this is to put pads
around the DUT. This can work fairly well.
The designer of the network analyzer can recognize that the through cal
shortcut is popular and accomodate the user by going to a lot of
trouble to provide well matched ports. Or not. It turns out that
getting well matched ports translates into massive padding. This
has the disadvantage of loss of dynamic range. Then there is the
question of allowing the user to turn the internal padding on and
off. What if he turns it off and then proceeds to do a through
cal anyway? There is no simple answer to this discussion.
If the environment seen by the DUT in the test fixture is not
the same as it sees in the application, then the measurement
is just an approximation.
It is entirely possible that you will get results accurate enough
for ham radio since you are working at HF. But the technique
would not be deemed completely rigorous by metrologists.
Rick N6RK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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