[TowerTalk] Measuring Common Mode Choke with Anritsu VNA

Avery Davis wb4rtp at mindspring.com
Tue Jan 2 23:34:54 EST 2018

On 1/2/2018 10:00 AM, Jim Brown <jim at audiosystemsgroup.com>:
> Somehow I failed to clearly make the point that measurement errors
> associated with measuring high Z chokes with S11 is NOT a relatively
> small error, like 10%. It can be a VERY LARGE error, like 50-70% in
> resonant frequency. Which FAILS to allow assessment of suitability for
> use in a given frequency range.


I agree the error can be very large, but whether that error is
significant depends on the application.  The Keysight charts clearly
show that using a professional-grade VNA, the S11 measurement can be
accurate up to 2,000 Ohms impedance, and that can be enough for many
common-mode choke application.  If I only need 1,000 Ohms impedance for
my application I don't care if the actual impedance is 1,000, 10,000 or
100,000 Ohms.  At the very least, the S11 Smith chart test is a good
quick sanity check that a choke is basically working.  If I need to
prove a choke has a lot more than 1,000 Ohms impedance, then I agree
that some other technique is needed, and the Keysight charts discuss
those other methods. 

Again, I thank you for bringing this to my attention, prompting me to
search for impedance measuring techniques and leading me to these
Keysight charts. 


Avery, WB4RTP

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