VNA calibration / crystal measurements question

[antenna]

I have seen in videos where people will calibrate to a certain frequency range, but then to get a closer look, they change to a narrower span without recalibrating.  So lets say I calibrate my VNA for 28MHz to 29MHz to look at a variety of 10m crystals.  If I want to look at the S21 at 0° phase for Rs calculations or the 45° phase shift frequencies for measuring Lm and Cm, do I need to recalibrate every time I zoom in to a 10kHz span and look at a crystal within this 1MHz range, or will the results still be close enough if I leave the calibration for the 1MHz span loaded?

Thanks!

[Kim Christensen]

Depends on the VNA, but on some you can enable interpolation in the calibration menu. Then you won't be constantly prompted to re-CAL every time you change parameters within the range. Interpolation will add some error, but it's usually not an issue for most purposes.

[antenna]

I have a Siglent SVA1032x with latest update and a nanoVNA-F HW2.2.  I don't recal interpolation being an option on either, but I wasn't sure if they did it automatically or not

[Kim Christensen]

I'm not familiar with those models. With the HP VNAs I've used, if you don't have interpolation enabled, it will display uncal on the screen and disable the calibration factors if you change span after having calibrated it with a mechanical standard.
Is there any warning message on the screen when you do that? ie: Does it continue to indicate it's calibrated when shrinking the span for example?

[antenna]

The Cal indicator does disappear, but no warnings appear.  I did try a narrower calibration to see how it compared to a wider one that had its span made smaller without recalibration, and the results were surprisingly close.  I do not know that I can trust it in every situation though.