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On Jun 14, 2022, at 1:49 PM, Jim Lux <jim@...> wrote:

On 6/14/22 9:24 AM, W0LEV wrote:

Thank you, Jim!!!

Again, we are not running a metrology lab nor do our measurements approach
those of HP, R&S, Tek, and others. Something my PhD friends tell me is
that error analysis and assignment of error bars is no longer taught - not
even at CU/Boulder. I had a required course dedicated to that musing to
obtain my Physics degree some 50+ years ago at Michigan State U.

The students today have no idea how the error bar is established or even
what it truly indicates. Several of us have tried introducing our STEM
students at LTO (Little Thompson Observatory <starkids.org>) to the concept
of measurement errors and how they affect final outcomes. We usually get
wrinkled foreheads and "why".

Yes, 5 parts in E-5 is -86 dB. Yes, the HP 8753C noise floor can measure
below that. But, not our inexpensive NANOVNAs. Mine typically shows a
noise floor of -60 dB, depending on frequency and measurement type.

Again, thank you to those who put these VNAs at a reachable price in the
hands of us amateurs and those who want to learn the "fine points" of RF
engineering.

Never measure the temperature with more than one thermometer.

Never determine the time of day with more than one Cesium clock.

Never determine _________with more than one _________.

Dave - W?LEV

"sig figs" is taught in high school and undergrad.

If you do any sort of hard science classes they cover measurement uncertainties as part of the class (e.g. in lab) - Most lab classes discuss this (Undergrad chem lab certainly does).

The more sophisticated stuff is covered in classes like numerical analysis - if you're doing signal processing, for instance, round off and error propagation are a big thing. Same with classes on numerical solutions of differential equations, in connection with things like Runge-Kutta. I doubt there's many classes that specifically care about "calibration uncertainty" - you're on your own with mfr notes and the professional literature.

As a grad student, you'd be expected to get this knowledge in some way - there are a variety of short courses offered by various government labs as well as industry. For instance NIST has annual meeting in certain fields, and there's often some short courses associated with it.

I'm sure that at least a few here are familiar with the following section
from the STMicro application note "AN4974: Antenna matching for
ST25R3911B/ST25R391x devices", since it calls for the use of a VNA and the
ST25R391x is an RFID reader chip, supporting several standards. What I'm
confused about is line 4. And I'm not sure of the nanoVNA setting for line
5 (Q factor measurement). Can anyone brief me on what the author is saying
there exactly? I'm by now familiar with the open/short/50ohm calibration
procedure of course, but intermediate level in re-exploring the many curvy
zen mysteries of... the dreaded Smith Chart and applying the procedures
within AN4974.

Also, does anyone here have actual experience with the design of reader
impedance matching and tag antenna design for those chips? In other words,
has anyone ever survived AN4974 and lived to tell about it? Finally, I
need to design an RFID system (I chose the ISO-15693 standard for its
relatively long range, although I would LOVE to hear about any other
standard for which cheap front end chip solutions exist). Requires a 3"
diameter tag antenna and any diameter below 5.5" for the reader antenna and
a read range of up to 10" and must merely read out its unique UID code when
detected. Am I dreaming? Sound feasible? Many thanks to anyone who might
be willing to offer a little experience and advice, and I'd discuss
compensation if an expert is willing.


7.3 Verification of the Q factor in the frequency domain

The Q factor can be measured using a vector network analyzer and an
ISO10373-6 Class 1-3 calibration coil. The following steps should be
carried out:

1. The network analyzer shall be calibrated for a frequency sweep from
about 10 to 20 MHz
2. S11 measurement in log mag format shall be displayed.
3. The calibration coil is connected to the VNA.
4. “Short” calibration of the coil and conversion to “Z: Reflection”
5. Set marker 1 and enable the bandwidth/Q factor measurement
6. Place the PCD antenna on the measurement coil Note: If the reader is
plugged and powered, ensure that register 0x27 is set to 0xFF to avoid an
high power transfer to the VNA ports, which can damage the VNA.
7. Adjust the suitable trim value via the register map (register 0x21) in
the GUI of the reader
8. Place a 3 ? resistor between the RFO pin to simulate the chip
resistance during operation.
9. Press “max search” to align the marker on the resonance frequency peak
of the PCD antenna Figure 33 shows the results of such a measurement.

#applications #coils #design #matching #nanovna

Measurement of a short at the VNA connector after clearing cal:

49002000 -0.869956992 0.080768176
49126750 -0.869913280 0.080994536
49251500 -0.869870784 0.081221216
49376250 -0.869828352 0.081447216
49501000 -0.869784000 0.081672776
49625750 -0.869736192 0.081898376
49750500 -0.869684800 0.082123680
49875250 -0.869630720 0.082347872
50000000 -0.869575232 0.082570920

After calibrating:

49002000 -1.000384927 -0.000153716
49126750 -1.000387430 -0.000148167
49251500 -1.000389814 -0.000142248
49376250 -1.000392199 -0.000138649
49501000 -1.000394344 -0.000137316
49625750 -1.000395775 -0.000135393
49750500 -1.000395656 -0.000129219
49875250 -1.000393629 -0.000117017
50000000 -1.000389934 -0.000100459

After switching to MA mode. File stayed in RI mode but data changed:

49002000 -0.999972800 0.000051251
49126750 -0.999974336 0.000058021
49251500 -0.999975360 0.000064329
49376250 -0.999976000 0.000067545
49501000 -0.999977344 0.000067800
49625750 -0.999980672 0.000067987
49750500 -0.999986496 0.000071876
49875250 -0.999994304 0.000081417
50000000 -1.000003099 0.000095119

These are excerpts of a 401-point file from 0.1 to 50 MHz. At lower
frequencies the last data set is beyond -1 like the second one.

Observations:

1. |S11| > 1 after calibration.

2. MA mode leaves file in RI mode but affects the data.

Brian

On 6/14/22 9:24 AM, W0LEV wrote:

Thank you, Jim!!!

Again, we are not running a metrology lab nor do our measurements

approach

those of HP, R&S, Tek, and others. Something my PhD friends tell me is
that error analysis and assignment of error bars is no longer taught -

not

even at CU/Boulder. I had a required course dedicated to that musing to
obtain my Physics degree some 50+ years ago at Michigan State U.

The students today have no idea how the error bar is established or even
what it truly indicates. Several of us have tried introducing our STEM
students at LTO (Little Thompson Observatory <starkids.org>) to the

concept

of measurement errors and how they affect final outcomes. We usually get
wrinkled foreheads and "why".

Yes, 5 parts in E-5 is -86 dB. Yes, the HP 8753C noise floor can measure
below that. But, not our inexpensive NANOVNAs. Mine typically shows a
noise floor of -60 dB, depending on frequency and measurement type.

Again, thank you to those who put these VNAs at a reachable price in the
hands of us amateurs and those who want to learn the "fine points" of RF
engineering.

Never measure the temperature with more than one thermometer.

Never determine the time of day with more than one Cesium clock.

Never determine _________with more than one _________.

Dave - W?LEV

"sig figs" is taught in high school and undergrad.

If you do any sort of hard science classes they cover measurement
uncertainties as part of the class (e.g. in lab) - Most lab classes
discuss this (Undergrad chem lab certainly does).

The more sophisticated stuff is covered in classes like numerical
analysis - if you're doing signal processing, for instance, round off
and error propagation are a big thing. Same with classes on numerical
solutions of differential equations, in connection with things like
Runge-Kutta. I doubt there's many classes that specifically care about
"calibration uncertainty" - you're on your own with mfr notes and the
professional literature.

As a grad student, you'd be expected to get this knowledge in some way -
there are a variety of short courses offered by various government labs
as well as industry. For instance NIST has annual meeting in certain
fields, and there's often some short courses associated with it.

On 6/13/22 1:31 PM, Brian Beezley wrote:

I've been analyzing Touchstone files Rudy Severns, N6LF, has recorded

with his NanoVNA-H4. The magnitude of S11 is greater than 1 for many files.
For example, after calibration right at the VNA connector, |S11| was 1.0007
maximum for the open cal part itself. For the short the maximum was 1.0005.
For both, |S11| > 1 for all 401 points from 0.1 to 50 MHz.

The images show calculated permittivity and conductivity for a ground

probe with the rods in air. The image with most of the points missing used
uncorrected data. The other image is for normalized data where |S11| = 1
maximum.

Although normalization solves the problem for my software, I'm curious

why |S11| is ever > 1.

Brian

Measurement uncertainty? You're essentially measuring (V refl)/(V
incident) with a noisy sensor. 1 part per 1000 (1.001) is 60dB.. the
SNR of the measurement is in that ballpark.

Calibration peculiarities - you determine the cal coefficients with
noisy measurements, so the combination of cal coefficient high, and
measurement high,.

The ADC measuring the output of the mixer has an ideal SNR of ~90 dB.
It's a 16 bit adc, so there's some quantization uncertainty. Then
there's the arithmetic aspect. The basic software multiplies the ADC
numbers by sin and cos, then sums to get I/Q. That's done with 16 bit
signed integers, 32 bit products, but then truncated before integrating.

There's also the single precision floating point calculation of the
various calibration coefficients using single precision float (32 bit,
with 24 bit mantissa/significand).