[TowerTalk] NanoVNA port impedances Re: Test Fixture for Common Mode Chokes

[Brian Beezley]

Jim Lux wrote:

"At HF frequencies, I’d be surprised of the Z is far off.  (But I can 
measure it… I have two NanoVNAs - I’ll do that later today)"

Jim, I'd be interested in what you measure. Port 1 for N6LF's NanoVNA-H4 
rev. 4.3 was about 57 ohms at HF, about 14% high. Another user reported 
a similar value. Port 2 was about 50.6 ohms, not far off. In addition to 
the resistive deviations, the equivalent parallel capacitance was 1.9 pF 
for port 1 and 1.4 pF for port 2. The capacitive reactance might affect 
some measurements. Then there is the LF roll-off due to coupling 
capacitors in both ports (not present in some earlier revisions). N6LF 
used an HP 908A 50 ohm load as the cal standard. The circuit has changed 
slightly with each hardware revision. I believe it's past 4.3 now. I 
don't know whether these results apply to the current offering.

I don't know the details of the Enhanced Response feature that partially 
compensates for port impedances when making a two-port series 
measurement. What I recall is that there wasn't enough code space for a 
full compensation! If your VNA has this feature, there's probably little 
reason to worry about port impedance.

Brian