I've got nanoVNA on order, but don't have it yet. It's not due to at least
4th July, so that's no big surprise.
Can anyone tell me if one can change the coefficients of the
open/short/load kit? I just have done a test with 4 sorts of opens. The
connector was a male N, so the calibration standard a female N. But
anyway, here are the results at 902.625 MHz, which is the nearest frequency
I have any data. (I had calibrated to 18 GHz for another job, so don't have
data at exactly 900 MHz, but this is as close as makes no practical
difference)
*Male N plug just open. No female standard*. Phase = +4.64722 degrees @
902.62500
Female open standard from HP 85032B open standard, used without any
extender, as it is NOT supposed to be. Phase = +4.64014 degrees @ 902.62500
MHz
Female open standard from HP 85032B calibration kit, with the extender, as
it SHOULD be used -3.65161 degrees @ 902.62500 MHz
Female Open standard from HP 85054B 18 GHz calibration kit. Phase =
-18.18457 @ 902.62500 MHz
IMPORTANT
The male N plug was a metrology grade male-male adapter from an HP 85054B
VNA calibration kit. Since the shape of the male pin is not well defined,
and commercial grade connectors are recessed by various amounts, I would
not expect this result to be reproducible with different connectors, but
fairly substantial variations
Anyway, the conclusion is that for the female N calibration standards, the
phase varies from +4.6 to -18.2 degrees at 902 MHz. So a phase variation of
22.8 degrees depending on what calibration standard one uses. The variation
would be smaller at lower frequencies, but depending on what the nanoVNA
assumes about the calibration standard, one could get quite different
results.
Ideally, one needs to be able to enter the offset delay of the calibration
standards as an *absolute minimum.* The fringe capacitance would be nice
too, but could just about get away without that, as one could do a rough
job of compensation via changing the offset delay. Better still would be a
third order polynomial, which would make entering coefficients from a
commercial calibration kit easy, without trying to work out any
compensation values.
If the firmware is dumb enough to assume the open standard is ideal (phase
= 0 degrees), and the short is ideal (phase = 180 degrees), then it would
cause significant errors.
Likewise, if the firmware made assumptions about the SMA kit supplied,
those assumptions would be wrong if a different SMA, 3.5 mm, N or APC cali
kit was used.
Even if you are not interested in phase measurements, be aware of the fact
the point of the vector correction is to correct for amplitude variations
too. Anyone that believes that they don't need to worry about vector
correction, as they are only interested in amplitude measurements, is
seriously mistaken.
Dave
--
Dr David Kirkby Ph.D C.Eng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, CHELMSFORD,
Essex, CM3 6DT, United Kingdom.
Registered in England and Wales as company number 08914892
Tel 01621-680100 / +44 1621-680100
