To start off, the common mode choke (CMC) I'm addressing is also referred
to as a transmission line transformer. Physically it consists of a single
bifilar set of windings on an appropriate ferrite toroid. The windings
form a short (for HF) transmission line on the toroid core with unknown
impedance. Here are methods on measuring that impedance using a vector
Network Analyzer. The NanoVNAs work well for the purpose.
All the following are reflection measurements, so no need to complete the
through and isolation calibration options on the VNA. I'm assuming the VNA
is properly calibrated over the required frequency range including any
fixtures or adaptors which might be used. It's best to attempt all of the
following at a relatively low frequency unless you are specifically
interested in the higher frequencies. A wide sweep of frequencies is not
recommended. I usually use something around 2 to 3 MHz. This avoids the
effects of introducing unknown parasitics into the measurement.
All the following is without renormalizing which can introduce unknown
additional errors to the measurements.
From what I've read and utilized, the three methods are:
METHOD 1:
1) Connect one side of the CMC choke to the s11 port or source port.
2) With the "other" side of the CMC open terminated, measure and note the
capacitance.
3) With the "other" side of the CMC short circuit terminated, measure and
note the inductance.
4) Calculate the Zo using the following formula:
Zo = SQRT [L / C]
Be sure to use basic units, Farads and Henries.
1 pF = 1E-12 F 1 ?H = 1E-6 H
METHOD 2:
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port with a known (measured) potentiometer of
nominally 100 to 150-ohms over the frequency range of interest. Use
minimal leads from the pot. to the CMC.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are
simultaneously minimized along the real, horizontal, axis on the Smith
Chart. The real axis is the only place on the Smith Chart that is purely
resistive.
4) Now remove the potentiometer and read its DC resistance on a DMM. That
is Zo.
METHOD 3 (my favorite):
1) Connect one side of the CMC to the s11 port or source port.
2) Terminate the "other" port of the CMC with a known potentiometer
measured to be non-reactive over the frequency range of interest. A value
of 100 to 150-ohms is useful since most of the CMC chokes have a Zo between
70 to 125 ohms. Keep leads to an absolute minimum.
3) While observing the Smith Chart on the VNA, adjust the potentiometer
such that both the capacitance (-j) and the inductance (+j) are minimized
simultaneously along the central (real) axis of the Smith Chart.
4) Observe the numerals at the top of the VNA. You can read all the
"interesting" parameters of your CMC from that data, including the Zo of
the choke.
No calculations or renormalizing is required for this third and last
method, minimizing potential sources of error.
Dave - W?LEV
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Dave - W?LEV
