On 7/15/21 5:54 AM, Richard Clemens wrote:
Given my limited work with a NanoVNA I ask:
It appears the calibration routine (so far I have only needed for one port) is done without regard to any parameters being set.
Once it is done and saved you can reload it later and use it again.
Questions:
When would be a case that requires a new calibration vs just reloading one that has been saved?
Is it better to do a new calibration each time you fire up the nano rather than used a saved one?
What makes a calibration worth saving? ie. how should they be labeled?
Reasons to recalibrate (as opposed to use a saved cal set):
1) A different test setup, where the "reference plane" for the measurements is different.? For instance, if you have test port cables with different length, and you do the cal at the end of the cables.
2) If the frequency span of the measurements is different than the frequency span of the calibration.? The NanoVNA does some interpolation, but if you did a cal from 0-50 MHz and now you're measuring a DUT from 200-300 MHz, the measurement might be iffy. Likewise if you did a cal from 0-900 MHz, and the measurement is from 20-30 MHz.
3) You can't remember what you did for a cal before
4) The temperature or something else has changed significantly. I don't know how temperature sensitive the NanoVNA is, but things like the bridges and detectors will change somewhat with temperature.? The oscillator frequency also changes, although I would expect that to be a 10s of ppm sort of thing, so unless you're measuring crystals or 2kHz wide filters at 30 MHz, you probably wouldn't see it.
Think of the NanoVNA as being somewhat like a RF Ohmmeter.? When you turn on the ohmmeter you check to see that it reads infinity with the leads apart, and zero with the leads shorted.? Same sort of thing.? Check to see that it reads appropriately with open and load, and if it looks ok, it probably is.? (If you store it with the load on CH0, then when you turn it on, you'll see the good match, and then when you disconnect the load to connect the DUT, you'll see the reflection match, so that's a easy way to check.
