A test of NanoVNA s21 series through impedance measurement.
This technique is very popular, its users believing it gives superior accuracy, especially on higher impedance DUT.
Above is the test setup, a nominally 100Ω resistor is connected from Port 1 inner contact to:
- Port 2 shield for s11 reflection measurements; and
- Port 2 inner contact for s21 through measurements.
Note: my NanoVNA-H4 has been modified by addition of a short direct wire between the ground side of Port 1 and Port 2 jacks inside the case (see above).
I have intentionally used an ‘ordinary’ metal film resistor with wire leads to demonstrate with a readily available component… but you could buy some similar value 0.1% tolerance SM resistors (50pcs of 100Ω 0.1% 1210 resistors for less than $10 on Aliexpress) and solder some pigtails to them and depend on their marked value.
The instrument was SOLIT calibrated with the reference plane at the Port 1 connector. A 200mm coax jumper was used for the through calibration, it has a delay of 1.5ns and loss of 0.05dB @ 1MHz which needs to be adjusted out.
s11 reflection measurement of impedance using the NanoVNA
Above is a screenshot of s11 reflection measurement @ 1MHz gives 98.91+j0.510Ω. This reconciles well with measurement of the resistor @ DC using a high accuracy ohmmeter, 98.7Ω.
s21 through impedance using the NanoVNA
HP and their successors describe this technique in many publications, the following is from 5988-0728EN.pdf.
Note in the second formula the use of 2Zo and 100 synonymously. The formula depends on the Port 1 source being a Thevenin source with source impedance Zo and the Port 2 input impedance Zo (which they take to be 50+50Ω).
Derivation of the expression for the unknown impedance in an s21 series through measurement shows that the critical value is actually Zsource+Zload. If Zsource+Zload ≠ 100 then the formula is not correct, the measurements will have error.
Above is a screenshot of the native s21 series through measurement of DUT R and X, @1MHz Z=112+j2.06Ω.
This is significantly different to an s11 measurement of the DUT (R is 13% high), and significantly different to high accuracy DC measurement of the DUT.
What is wrong?
s21 improved impedance measurement
Let’s exercise the first calculation phase of Calculate Z from series s21 – enhanced to find Zs+Zl.
Above is a screenshot of the s21 path with the resistor in series. It is adjusted for the missing coax jumper’s delay, but 0.05dB more loss needs to be added to |s21|.
The partial calculation result using the previously measured resistor for the calibration part is that Zs+Zl=87.28-j1.099Ω, definitely not even close to 100Ω. This is 13% low and contributes most of the 13% error discussed above.
Conclusions
NanoVNA’s inbuilt s21 series through facility for measuring Z depends on an assumption that Zs+Zl=100Ω. Verify it on a known DUT before trusting it blindly.
Do and learn
Repeat the first test and second tests on your own NanoVNA, did the results reconcile?
This is essentially verification that the instrument gives correct results on a known quantity. If it does not, you cannot depend on it, and you should be reluctant to recommend the measurement technique… well unless you are an armchair expert.
References
- Agilent. Feb 2009. Impedance Measurement 5989-9887EN.
- Agilent. Jul 2001. Advanced impedance measurement capability of the RF I-V method compared to the network analysis method 5988-0728EN.