On 5/17/22 11:17 PM, Phil via groups.io wrote:
It's looking to me like my NanoVNA-H has an 87 ohm input impedance when I try to do a through measurement. Shouldn't it be 50?
Here's how I came to that conclusion:
I connected a 4.99k ohm resistor from CH0 to CH1,
Then I configured the NanoVNA as follows:
Display Format LOGMAG
Channel CH1 THROUGH
Then I did a calibration.
The display shows -35.2 dB.
The attenuation would be 10^(35.2/20) = 0.0173
Which means the NanoVNA's input impedance is 88.33(1-0.0173) = 87.5 ohms.
As a confirmation, I replaced the resistor with a 0.01 uF capacitor, and it was 3 dB down at 187 kHz, which is what I would expect for an input impedance of 87.5 ohms.
Am I doing something wrong here?
You should cal using the cal standards (i.e. a zero ohm thru), then make the measurement.
If you're doing the cal with the UUT in the loop, who knows what you'll get.
If you want to measure the Ch0 Z, you'd need to look at the actual cal coefficients (not the displayed values) calculated with a standard cal - I don't know that those are accessible. Or, reset the cal to none, measure some standards, and from that you can calculate the port's apparent impedance.
Or, hook up another NanoVNA, and directly measure it.
