Topband: Mother of all ferrite common-mode coaxial chokes

[Jim Brown]

On 7/13/2012 6:46 PM, Richard (Rick) Karlquist wrote:
> Another thing you can do to measure chokes accurately is
> to simply use a Vector Impedance Meter instead of a network
> analyzer.  Like the HP 4815A.  In essence, it uses the VI
> method, as the name implies.  It doesn't care if the
> impedance is nowhere near 50 ohms.  For really high
> impedances, it can be helpful to mathematically subtract
> the stray capacitance of the probe.  I've gotten excellent
> results with my 4815 for years.

That's exactly what's needed, and is the RF I-V method that both Wes and 
I cited. And yes, the capacitance of the test fixture can be a major 
source of error -- the equivalent circuit of a typical choke is parallel 
RLC, where R is typically on the order of 5K and C is on the order of 
2-4 pF..

73, Jim