On 10/17/21 1:48 PM, Kenneth Hendrickson via groups.io wrote:
--- On Sunday, October 17, 2021, 09:28:17 AM EDT, Joe WB9SBD <nss@...> wrote:
When you do a calibration open/short/50, Stimulus from 3 to 30 Mhz.
it should then be accurate between 3 and 30 Mhz correct?
OK you calibrated with the 3 to 30 Mhz scan width.
Now if you change the stimulus width to like 14 to 14.5 Mhz and not do a new calibration,
is the calibration still good since this narrower window is still inside the original scan width?
When you choose a set of frequency limits, and a set of frequencies in that span (number of frequency steps), then only and exactly that set of frequencies is calibrated. If you then make a measurement on that range, the perfect calibration is used for each frequency.
If you change the frequency range, or the number of frequency steps, then interpolation must be performed, and the calibration is not as good as it could be.
Barring pathological cases (you've got things with narrowband phenomena, inside the calibration plane, like a stub that's a significant fraction of the wavelength),? I don't think the cal performance is degraded very much.? The cal parameters don't vary in a radical or fast way, most of the time, and the calculations are done with sufficient precision, that the limit on accuracy is something else (SNR of the measurement).? The interpolation is done in floating point, as are the calibration calculations.
There are places where I'd watch out - if you cross a harmonic switchover boundary, there's likely to be a glitch, because gains, etc. all change, and interpolation might not be valid.
If you want the best results, always calibrate every time you change frequency range or the number of frequency steps.
