Hi Jeff,
Ok, I've finally fired up the 8753 and done a test.
This was done a bit differently to yours, as the calibration is done
offline on a PC. Therefore it eliminates all of the issues around cal kit
definitions on the two devices.
It also eliminates the calibration code on the NanoVNA, there could be a
subtle bug in there.
Conversely your test is more comprehensive and tests more of the NanoVNA
than I have.
Outline of the process I followed:
3MHz - 300MHz sweep on both
101 data points on both
I set the 8753 to 100Hz IF filtering to drop the noise a bit.
Warm up time of about 1 hour for both devices.
NanoVNA was kept flat on a surface. My thinking was that would reduce air
flow due to convection.
NanoVNA was connected to the PC USB port. Again, trying to keep the system
static. Hoping that any noise introduced will not be significant.
Sweep each of the NanoVNA calibration standards and save the s1p files for
each one.
Sweep the reference attenuator from my Kirkby cal kit and save the s1p file.
Repeat the process on the 8753 and save all of the s1p files.
Sweeps were taken as quickly as possible to reduce drift. Time was about 5
minutes.
Using the sci-kit rf library I did two calibrations on the s1p files -
using perfect ideals. One for the 8753 data, and one for the NanoVNA data.
The results and code are in a jupyter notebook here:
The results are excellent I think, and show that the NanoVNA hardware and
firmware involved in extracting the data is capable of good performance.
[image: image.png]
Roger
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On Thu, 22 Aug 2019 at 03:13, Jeff Anderson <jca1955@...> wrote:
On Tue, Aug 20, 2019 at 05:32 PM, Roger Henderson wrote:
However since it is a comparison, would still need to consider what the
cal
kit definitions are in your 8753. I expect they are also tiny, but the
question remains.
And on Tue, Aug 20, 2019 at 06:51 PM, Dr. David Kirkby from Kirkby
Microwave Ltd wrote:
Unless you know the properties of the calibration kit supplied with the
NanoVNA, and set up the 8753 to have a user kit with those parameters,
then
the 8753, will not be calibrated properly. I would suggest that you use
1) The supplied kit with the NanoVNA.
2) A professional calibration kit with the 8753, with the 8753 being
configured properly to use that kit.
Good morning, Roger and David,
Both of you raise an interesting question, which is "how much does a
calibration kit's parameters affect measurements at very low frequencies?"
I don't have a quantitative answer for that, but in my case of measuring
at 4 MHz, I believe it to be so minimal as to be visually imperceptible on
a full-size (rather than a significantly zoomed-in) Smith chart. Here's my
reasoning...
First, HP, in their app note: "Specifying Calibration Standards for the
Agilent 8510 Network Analyzer" (
-- a document referenced by the 8753C Network Analyzer Reference manual),
state:
"At microwave frequencies however, the magnitude and phase of an ¡°open¡±
are affected by the radiation loss and capacitive ¡°fringing¡± fields,
respectively."
The app note states essentially the same thing, but in terms of residual
inductance, for a reference "short."
Note their use of the phrase, "At microwave frequencies". In my opinion,
4 MHz is so far away from "microwave frequencies" as to be essentially DC.
Next, if one looks at the terms of HP's third-order polynomial equations
for defining the residual capacitance and inductance of cal kits, the two
terms that will have the greatest impact on these definitions, C0 and L0,
are in terms of femtofarads and picohenries, respectively.
Anyway, I'm having a (very) difficult time believing that
standards-residuals in the femtofarad or picohenry range have any
significant effect on my measurements at 4 MHz, where, for my DUT, gamma's
phase delta between the 8753C measurement and that of the NanoVNA is 1
degree.
Well, such are my thoughts, which brings me back to the question, "how
much does a calibration kit's parameters affect measurements at very low
frequencies?". Do either of you (or anyone else) have an answer to this
for, say, frequencies in the HF range -- i.e. up to 30 MHz?
Best regards,
- Jeff, k6jca
P.S. David, per your phase-shift measurement of the supplied "open" load.
Roger points out that there seems to be an "open" compensation factor of 50
femtofarads written into the NanoVNA code (e.g. the C0 term in the
third-order polynomial) -- would this compensate for the additional phase
shift you're measuring? And would it therefore be better to cal using the
supplied standard, rather than leaving the connector open?
