Steve Hunt wrote:
>jimlux wrote:
>> Steve,
>> what about a high reactive impedance.
>>
>> Mostly, the problems I've had with this kind of thing have to do with
>> fixturing and/or calibrating the fixture out. I was trying to measure
>> the Z of traps, and never did get a good fixture scheme worked out.
>> It's not like you can just take a couple banana plug clipleads.. I
>> finally decided to remain ignorant of the electrical properties of the
>> traps in a 6-BTV.
>Jim,
>
>Instruments like the AIM attempt to "calibrate out" the fixture. You
>show the instrument a short-circuit, an open-circuit, and a known load,
>right at the measurement plane. It can then remove the effects of the
>fixture for unknown impedances connected at the measurement plane. I
>maintain a set of calibration files for my various test fixtures.
>
I agree with Steve; you *can* take a couple of clip leads... but then
you have to pre-calibrate the whole setup quite carefully against three
impedance standards.
Analyzers that have no facilities to do an "Open, Short, Load"
calibration are completely, categorically out of this game! They will
not give accurate measurement results on common mode chokes.
To make measurements on physically large objects like traps and many
kinds of CM chokes, the "Open circuit" standard will have to be clip
leads carefully arranged in the physical locations where they will
eventually need to be connected (and preferably taped down so they
cannot move). For the "Short circuit", I use a length of 1/2in copper
tape between the two alligator clips; and for the "Load", a 49.9 ohm
chip resistor embedded into an identical length of copper tape.
All three of these "standards" obviously have parasitic reactances of
their own, but the calibration math takes that into account. The
subsequent measurements display the properties of the Device Under Test,
in excess of those of the test and calibration setup.
Beyond that, we enter the realms of philosophy. If you are measuring the
traps in a 6-BTV, should the reference standard be a textbook
short-circuit, an equal length of plain tubing, or what? And in each
case, why?
The one thing on which we can all agree is that these measurement/
calibration problems get worse with physically larger (longer) Devices
Under Test.
>As I said earlier I tried measuring a 10k resistor ; I also just tried
>measuring a 10k resistor in parallel with a 3.3pF capacitor. Over the
>range 1-50MHz the results seemed plenty accurate enough for making CM
>impedance measurements on chokes.
>
Calibrating with a 50 ohm standard and then attempting to measure 10k,
is quite a severe test - for the operator as well as the VNA. As Steve
says, a few percent error at the 10k level isn't serious for
measurements on CM chokes... but I do like to see some indication that
the operator is conscious of these problems (operators who were trained
on the more familiar "SMA, plug and play" systems often are not).
The effects of shunt capacitance on the resonant frequencies of chokes
can be much more severe. As Jim Brown and I have both pointed out, the
self capacitance of a typical "inductive" CM choke is only a few pF, so
any lack of calibration and control in the measurement system can shift
the resonance by several MHz at HF.
But ask yourself this: if the resonance a CM choke is
frequency-sensitive, even under laboratory conditions, what are my
chances of the resonance "hitting the band" when the choke is installed
on the antenna?
All of this argues very strongly for the use of BROADBAND chokes that do
not have such frequency-sensitive characteristics... and that always
means ferrite (purely air-wound chokes are *acutely*
frequency-sensitive). It also argues for chokes that have a
PREDOMINANTLY RESISTIVE impedance over as wide a bandwidth as possible,
and also a VERY HIGH impedance when measured in the lab. With those
reserves of performance in hand, we have a much better chance of good
performance out there in the wild.
>I'm still puzzled why Jim (K9YC) was questioning the accuracy of the
>Balun Designs impedance data. The chart shown here:
>http://www.balundesigns.com/catalog/1-1%205kw%20Choking%20Impedance.jpg
>doesn't seem to contain impedances that would cause significant errors
>on the AIM.
But only if the AIM has been calibrated correctly! I don't feel
comfortable with CM choke measurements until the operator has shown that
he is aware of these issues, has taken the appropriate precautions and
has proved out his test setup at the 10k level.
The other question (as Jim will surely say) is how much of that "Zmag"
impedance is resistive? Possibly quite a lot, especially across the
middle bands... but again, please show me.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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