At nanoVNA-H – Port 2 attenuator for improved Return Loss I explained the reasons for essentially permanent attachment of a 10dB attenuator to Port 2 (Ch 1 in nanoVNA speak).
Above, the 10dB attenuator is semi permanently attached to Port 2 principally to improve the Return Loss (or impedance match) of Port 2, a parameter that becomes quite important when testing some types of networks than depend on proper termination (eg many filters). I should remind readers that the improvement in Port 2 Return Loss comes at a cost, the dynamic range of Port 2 is reduced by 10dB.
Adding an attenuator to Port 1
Several correspondents have suggested the there is also benefit in permanently attaching such an attenuator to Port 1. These suggestions were traced to the assertions of one online expert.
The nanoVNA-H v3.3 samples the signal supplied to the measurement bridge, albeit prior to a voltage divider and we might assume that enters into calculation of the value of s11. I have not examined the source code to see if it includes the effect of that voltage divider and its variable load, but I suspect not as the voltage divider action is dispensed with in v3.4 and they both use the same firmware. I suspect normal VNA correction algorithms are relied upon to deal with non ideal hardware design.
While adding an external attenuator to Port 1 does nothing to address shortcomings in the implementation of the measurement bridge, it does degrade the dynamic range for s11 and s21 measurement.
I think it wrong to think that a network such as a filter will not measure properly if the source is not Thevenin impedance matched. The VNA is using a measurement bridge and its calibration / correction processes to come to a steady state value that depends entirely on the (complex) ratio V/I at the measurement plane, and that ratio is determined by things downstream from the measurement plane and NOT by things upstream of the measurement plane. This might not be accepted by disciples of Walt Maxwell's re-re-reflection proposition.
So, whilst a network such as a filter might not exhibit a correct response in an ordinary sweep generator – filter – detector configurations (such as a spectrum analyser with tracking generator) unless both generator and detector are matched to the design impedances of the filter, in the case of a VNA measurement it is important that the filter is terminated in a ‘matched' load, the measurement / calculation of s11 and s21 is corrected by the calibration and measurement processes of the VNA.
Analysis of the measurement source – nanoVNA v3.3
Above is an extract from the schematic for v3.3. A first approximation for the source impedance seen by the measurement bridge is the Si5351A output impedance (50Ω) in parallel with with R16+R17 (497Ω) for a combined 45.43Ω, in series with R13 (150Ω) for 195.43Ω, in parallel with R14 (56Ω) for combined 43.53Ω.
Measuring the output power @ 10MHz and applying a second termination results in a drop of 3.3dB on the power meter indication. We would expect 3.52dB for a 50Ω source and 3.32dB for a 45.53Ω source, validating the approximate circuit analysis.
Analysis of the measurement source – nanoVNA v3.4
Above is an extract from the schematic for v3.4. A first approximation for the source impedance seen by the measurement bridge is the Si5351A output impedance (50Ω) in series with R13 (0) in parallel with with R16+R17 (497Ω) for a combined 45.43Ω, in parallel with R14 (56Ω) for combined 25.08Ω.
The ‘improvements' to v3.4 move further from an idealised 50Ω Zs for the measurement bridge…. but the instrument works with the same firmware as v3.3 hardware.
I don't have a v3.4 to test, but for a double termination test, we would expect 3.52dB for a 50Ω source and 2.5dB for a 25.08Ω source. Happy to quote a valid experiment here if data is offered.
Return Loss Bridge discussion
I wrote at Return Loss Bridge – some important details of the dependence of an ordinary Return Loss Bridge and its calibration / measurement process on near ideal Thevenin source impedance equaling the calibration or reference impedance. In that case, failure to supply a correct Thevenin source cannot be solved by putting an attenuator on the ‘unknown' port.