The bridge is the real point where the VNA measures the reflected signal from the load connected to Port 1. It is inside the VNA. Calibration is a mathematical process used to move the apparent measuring point to wherever the OSL standards are connected during calibration.
What happens when the piece of coax connecting the VNA to the calibration point is not 50ohms? The 50ohm load gets transformed to a different value depending on the electrical length of the coax at the measurement frequency and the coax's impedance. The implicit assumption is that the VNA can measure this transformed load with sufficient accuracy that the error in subsequent measurements can be ignored.
But can it? Does this implicit assumption fail as the reflected signal level falls, ie the connected load approaches 50R? And a what point does it fail? Here is how I made my own mind up. I used the best of the 4 VNA's I have for this, a NanoVNA V2 plus4. The one in the metal box.
Step 1 : Perform an OSL calibration directly at the SMA connector on the VNA. The calibration load is a string of SMA attenuators with the 50R termination at the end of them.
Step 2 : Plot S11 against frequency. Also understood to be the input return loss by most people. Save that to one of the memories.
Step 3 : Remove the 50R load but not the string of attenuators. You are now measuring an input return loss twice that of the connected loss in the string of attenuators. In my case, with 45dB of attenuation in the string, that should be a return loss of 90dB. Save to another memory
Step 4 : Repeat Step 3 but remove the last attenuator. I now had 35dB of attenuation which should show as a 70db return loss.
Step 5 : Remove the next attenuator. I now had 25dB of attenuator which should show as a 50Db return loss.
Step 5 : Remove the next attenuator. I now had 20dB of attenuator which should show as a 40Db return loss.
So what conclusions can we draw about this NanoVNA at this point?
Firstly, it's not very good at measuring loads which have a return loss of under 60dB. Instead of flat lines we have odd looking curves.
Secondly, when the return loss is 40Db or better I'd be circumspect in the actual displayed value.
Nevertheless, a useful instrument since a return loss measurement down to 40Db is useful.
Now, lets attach a piece of coax and reassemble the attenuator string for 25dB of attenuation. If the piece of coax had a characteristic impedance of 50ohm this should show as a 50Db return loss. Clearly, from the measured result, the nice blue cable is not 50ohm.
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| Should be the same line as the 50dB curve in the chart above. |
At the device the first graph shows a relatively flat line. With the piece of coax the graph suggests the coax is not 50ohm.
Now, repeating the OSL calibration at the end of the coax, which in practice is what I would do, the 25Db attenuator string still returns an error prone measurement though a 15dB attenuator appears satisfactory up to 800MHz.
Conclusion:
For the price the NanoVNA is still the most useful piece of test gear I own and I recommend you have one. However, be aware of the limitations. When measuring S11, or input return loss, the value returned when better than 30dB is at best indicative. Careful calibration cannot overcome this.
You can test, in relative terms, how good the cable you are using is by performing the calibration directly at the device then moving the reference load to the end of the cable and measuring S11, or input return loss. I've tested scores of cables and none of them are satisfactory. But it allowed me to weed out some truly terrible cables.
Finally, there are a number of posts I have read that go through some complicated mathematics, typically using python, to derive some improved number or esoteric measurement. Must make the author feel clever but they probably don't appreciate the flaws of the measurement underpinning whatever they are trying to do. The pitfalls of garbage in means garbage out. And no amount of ridiculing others by referring to their work as "Hammy Sammy" make what they do superior.
