On 6/13/22 1:31 PM, Brian Beezley wrote:
I've been analyzing Touchstone files Rudy Severns, N6LF, has recorded with his NanoVNA-H4. The magnitude of S11 is greater than 1 for many files. For example, after calibration right at the VNA connector, |S11| was 1.0007 maximum for the open cal part itself. For the short the maximum was 1.0005. For both, |S11| > 1 for all 401 points from 0.1 to 50 MHz.
The images show calculated permittivity and conductivity for a ground probe with the rods in air. The image with most of the points missing used uncorrected data. The other image is for normalized data where |S11| = 1 maximum.
Although normalization solves the problem for my software, I'm curious why |S11| is ever > 1.
Brian
Measurement uncertainty? You're essentially measuring (V refl)/(V incident) with a noisy sensor.?? 1 part per 1000 (1.001) is 60dB.. the SNR of the measurement is in that ballpark.
Calibration peculiarities - you determine the cal coefficients with noisy measurements, so the combination of cal coefficient high, and measurement high,.
The ADC measuring the output of the mixer has an ideal SNR of ~90 dB.? It's a 16 bit adc, so there's some quantization uncertainty. Then there's the arithmetic aspect. The basic software multiplies the ADC numbers by sin and cos, then sums to get I/Q.?? That's done with 16 bit signed integers, 32 bit products, but then truncated before integrating.
There's also the single precision floating point calculation of the various calibration coefficients using single precision float (32 bit, with 24 bit mantissa/significand).
