A recent series of articles discussed the question of how accurate does a calibration LOAD need to be.
Following on from that I requested a change to allow the actual resistance of LOAD to be used
These tests are not conducted in a temperature stable laboratory, so allow some latitude in results.
The NanoVNA-H running NanoVNA-D firmware v1.2.35 was SOL calibrated, but the calibration kit had a LOAD that measured 51.273Ω at DC using a high accuracy ohmmeter.
Above is an |s11| sweep after calibration. Measurement is limited by the instrument noise floor, about -80dB @ 1MHz. This says nothing about the load as it is based on a flawed calibration, but it shows us the noise floor. For reasonable accuracy, we might say here that we can measure |s11| down to about -70dB… subject to an accurate calibration LOAD.
Now that LOAD sets the instrument’s calibration reference impedance to 51.273Ω and if I was to measure a true 50Ω DUT, we would expect |s11|=-38dB.
I do not have a termination that is exactly 50Ω, so let’s measure another termination that measured 49.805 at DC using a high accuracy ohmmeter. We would expect the VNA based on the calibration above to show|s11|=-36.8dB.
Above is a screenshot of exactly that measurement, calibrated with 51.273Ω and measuring 49.805Ω, expect |s11|=-36.8dB and we get -36.6dB. That reconciles well.
Now lets specify the “Standard LOAD R”.
Above is a screenshot of the data entry.
Now with the correction applied, let’s measure the termination that measured 49.805 at DC using a high accuracy ohmmeter. We would expect the VNA to now show|s11|=-54.2dB.
Above is a screenshot of exactly that measurement, calibrated with 51.273Ω, “Standard LOAD R” correction applied for that value, and measuring 49.805Ω, expect |s11|=-54.2dB and we get -53.1dB. That reconciles well.
So what is the effective directivity with the “Standard LOAD R” correction?
If the measurement of the LOAD was without error, then the limit for low jitter measurement would be around 10dB above the noise floor measured earlier, so say |s11|=-70dB, which corresponds to ReturnLoss=70dB.
You could use that ReturnLoss as coupler Directivty in Calculate uncertainty of ReturnLoss and VSWR given coupler directivity to calculate uncertainty of a given measurement.
Above is a sample calculation.
But, measurement of LOAD has uncertainty.
Let’s say you measure LOAD with 1% uncertainty, and you measure 49.0Ω , but it could be as low as 48.5Ω (ie -1%), we can calculate the implied inherent ReturnLoss of the 48.5Ω using Calculate VSWR and Return Loss from Zload (or Yload or S11) and Zo) .
The LOAD could be specified as 48.5Ω with ReturnLoss=45dB.
So, entering “Standard LOAD R” is better than using the default 50Ω, but due to the uncertainty of measurement of that LOAD, the instrument Directivity is around 45dB. So, you can see that the accuracy of measurement of LOAD flows into the effective instrument Directivity which feeds calculation of s11 measurement uncertainty.
The instrument I used to measure the terminations has uncertainty of 0.035Ω on a 50Ω reading, which corresponds to a ReturnLoss of 69dB.
By contrast for example, a Fluke 106 has specified uncertainty of 1.1% at 50Ω which implies ReturnLoss 45dB… you are probably better just accepting 50Ω unless you know the LOAD is really bad.
SDR-kits sells relatively inexpensive calibration kits where they supply the measured DC resistance of LOAD.