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Re: |S11| > 1
Thanks to DAve W0LEV and others for bring up the subject of error analysis and significant figures.
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My own background is like Dave¡¯s; a physics degree in 1965. Our professor in senior labs was very big on error analysis. It¡¯s a hard subject. I asked at our local hospital regarding uncertainty and error in lab tests. I never got a good answer. A friend of mine, an engineering professor, made a joke ¡. ¡°If you want to be absolutely certain about a measurement, only measure once.¡± Do I need to explain? Chuck KF8TI On Jun 14, 2022, at 1:49 PM, Jim Lux <jim@...> wrote: |
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Re: |S11| > 1
I do not have a VNA. I rely on Rudy Severns, N6LF, for data. He recalibrated his VNA at the SMA connector, measured a short there, and now MA mode does not affect the file data. In addition, |S11| is no longer greater than 1.
Sorry for the confusion. I would delete my original post if I could. Brian |
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Re: NanoVNA for RFID design
I am one of several engineers who worked for the company that originally
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introduced RFID to the world, AMTEK. They no longer exist as many start-ups go. The company was fresh off "The Hill", Los Alamos, where RFID was originally developed to log and to some extent, track, Pu and other radioactive carrying trucks. Dr. Gary Seawright purchased the patent rights from Los Alamos and formed the company. Those systems operated in the 915 MHz and 2.45 GHz ISM bands with an experimental license from the FCC. When AMTEK was bought out by the Texans (that was the end of a good beginning!!), Gary resigned. Another engineer, Dr. Jerry Landt, also left shortly after. Jerry wrote and still participates in authoring most of the RFID standards we have today for the RFID industry. The rest is history........ So much for history. I was there. However your system is a low-frequency inductively coupled system that likely operates below 150 kHz. Even though the coupling element is referred to as an "antenna" is it not. You are dealing with inductive coupling between the tag or badge and the reader coils. I did not work on these systems, but am quite familiar with them from much later work before I retired. I note the required equipment lists only a Network Analyzer. There is a big difference between a Scalar and Vector network analyzer, both in performance and cost. The NANOVNAs are vector analyzers, but can be used as a scalar analyzer as well. A scalar analyzer can not produce Smith Charts as it does not measure the angle of the measurements and, therefore, can not represent complex impedances or deal with Smith charts. That is accomplished with a VECTOR network analyzer which is far more complex and dwells with the Smith Chart. So,........., if there is no need for measuring complex portions of the impedances of the antennas (inductors) (which I do not read in AN4974) you're off the hook for a lot of complex arithmetic and the Smith Chart! Dave - W?LEV On Tue, Jun 14, 2022 at 8:20 PM tjackson382000 <tedj1@...> wrote:
I'm sure that at least a few here are familiar with the following section-- *Dave - W?LEV* *Just Let Darwin Work* --
Dave - W?LEV |
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Re: |S11| > 1
Brian,
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As a reference point, when calibrated, the HP 8753D has a linear magnitude reflection uncertainty of 0.015 or so when the reflection magnitude is near 1. I am not sure if that means the repeatability is limited to that or if that is due to imperfections of the standards used. (Source: Quick Reference Guide, 08753-90259, page 7-5) --John Gord On Tue, Jun 14, 2022 at 02:45 PM, Brian Beezley wrote:
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Re: |S11| > 1
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 |
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NanoVNA for RFID design
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 |
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Re: |S11| > 1
Jim, I can assure you our STEM students know nothing about significant
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figures! Terry (Dr. Terry Bullett) and I have tried, but no knowledge of the concept. They are Juniors going into their senior year and have had Chem. and Physics (not "advanced" phys which is seldom taught due to low demand). Dave - W?LEV On Tue, Jun 14, 2022 at 5:49 PM Jim Lux <jim@...> wrote:
On 6/14/22 9:24 AM, W0LEV wrote:--Thank you, Jim!!!approach *Dave - W?LEV* *Just Let Darwin Work* --
Dave - W?LEV |
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Re: |S11| > 1
On 6/14/22 9:24 AM, W0LEV wrote:
Thank you, Jim!!! "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. |
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Re: |S11| > 1
Thank you, Jim!!!
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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 On Tue, Jun 14, 2022 at 12:02 AM Jim Lux <jim@...> wrote:
On 6/13/22 1:31 PM, Brian Beezley wrote:--I've been analyzing Touchstone files Rudy Severns, N6LF, has recordedwith his NanoVNA-H4. The magnitude of S11 is greater than 1 for many files. *Dave - W?LEV* *Just Let Darwin Work* --
Dave - W?LEV |
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Re: |S11| > 1
On 6/14/22 7:26 AM, Brian Beezley wrote:
I've asked Rudy to remeasure the short and open using MA (magnitude/angle) mode. That might shed some light on where the inaccuracy lies. Also, I think he was using averaging. It might be interesting to disable it. what would be interesting is to measure it without calibration, and with, and make sure there's not some mis calibration going on somehow. |
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Re: |S11| > 1
On 6/14/22 4:29 AM, Brian Beezley wrote:
15070000 -1.000470519 -0.000045675 Interesting - I wonder if it's a "round off" or truncation error of some sort.? The "detector" mixes with I/Q 5 kHz, summing, and there could be a 1/2 LSB bias or something like that. here's the raw I/Q calculation code: void dsp_process(int16_t *capture, size_t length) { uint32_t *p = (uint32_t*)capture; uint32_t len = length / 2; uint32_t i; int32_t samp_s = 0; int32_t samp_c = 0; int32_t ref_s = 0; int32_t ref_c = 0; for (i = 0; i < len; i++) { uint32_t sr = *p++; int16_t ref = sr & 0xffff; int16_t smp = (sr>>16) & 0xffff; int32_t s = sincos_tbl[i][0]; int32_t c = sincos_tbl[i][1]; samp_s += smp * s / 16; samp_c += smp * c / 16; ref_s += ref * s / 16; ref_c += ref * c / 16; } acc_samp_s = samp_s; acc_samp_c = samp_c; acc_ref_s = ref_s; acc_ref_c = ref_c; } raw (uncalibrated) gamma is calculated here void calculate_gamma(float gamma[2]) { #if 1 // calculate reflection coeff. by samp divide by ref float rs = acc_ref_s; float rc = acc_ref_c; float rr = rs * rs + rc * rc; //rr = sqrtf(rr) * 1e8; float ss = acc_samp_s; float sc = acc_samp_c; gamma[0] =(sc * rc + ss * rs) / rr; gamma[1] =(ss * rc - sc * rs) / rr; #elif 0 gamma[0] =acc_samp_s; gamma[1] =acc_samp_c; #else gamma[0] =acc_ref_s; gamma[1] =acc_ref_c; #endif } this is the code that applies the calibration: if (cal_status & CALSTAT_APPLY) apply_error_term_at(i); static void apply_error_term_at(int i) { // S11m' = S11m - Ed // S11a = S11m' / (Er + Es S11m') float s11mr = measured[0][i][0] - cal_data[ETERM_ED][i][0]; float s11mi = measured[0][i][1] - cal_data[ETERM_ED][i][1]; float err = cal_data[ETERM_ER][i][0] + s11mr * cal_data[ETERM_ES][i][0] - s11mi * cal_data[ETERM_ES][i][1]; float eri = cal_data[ETERM_ER][i][1] + s11mr * cal_data[ETERM_ES][i][1] + s11mi * cal_data[ETERM_ES][i][0]; float sq = err*err + eri*eri; float s11ar = (s11mr * err + s11mi * eri) / sq; float s11ai = (s11mi * err - s11mr * eri) / sq; measured[0][i][0] = s11ar; measured[0][i][1] = s11ai; // CAUTION: Et is inversed for efficiency // S21m' = S21m - Ex // S21a = S21m' (1-EsS11a)Et float s21mr = measured[1][i][0] - cal_data[ETERM_EX][i][0]; float s21mi = measured[1][i][1] - cal_data[ETERM_EX][i][1]; float esr = 1 - (cal_data[ETERM_ES][i][0] * s11ar - cal_data[ETERM_ES][i][1] * s11ai); float esi = - (cal_data[ETERM_ES][i][1] * s11ar + cal_data[ETERM_ES][i][0] * s11ai); float etr = esr * cal_data[ETERM_ET][i][0] - esi * cal_data[ETERM_ET][i][1]; float eti = esr * cal_data[ETERM_ET][i][1] + esi * cal_data[ETERM_ET][i][0]; float s21ar = s21mr * etr - s21mi * eti; float s21ai = s21mi * etr + s21mr * eti; measured[1][i][0] = s21ar; measured[1][i][1] = s21ai; |
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Re: New to NanoVNA - HAM wanting to play w. antennas
#edy555_nanovna
Carsten, my experience is all antennas, even those off the shelf one, need some finetuning.
1. Tuning - I would recommend the minimal height for tuning purposes is 3-4m over the ground at the end of the antenna and 1.5-2m at the transformer side. The higher side could be done such you simply put a stone at the end of the parachute cord, and throw it over the tree - you may then move the end of the antenna up and down easily by pulling the stone. The tuning of the 40/20/15/10m band is easier, as the "tuning sensitivity" there is lower (do not cut off more than 5cm of wire at once), the 80m piece of wire after the 110uH coil is extremely sensitive though - like 5kHz/cm, and, most important information - the Bandwitdh of the 80m band portion with that specific antenna is pretty narrow - like 40-50kHz, this is what you have to decide upfront - where you want to place that 80m segment (it is from 3500-3800kHz and you will place yourself into a 40-50kHz wide segment of it). That is caused by the resonant LC behavior of the 110uH coil and the aprox 2m long wire (acting as the parasitic Capacitor).. The antenna de-tunes itself a little bit when pulled into higher height after the tuning, how much - it depends on many factors, however. The transformer - its mounting is also important - as the output of the transformer (where you connect the antenna wire) is extremely sensitive to parasitic capacitancies, it should be at least 20cm off any other parts, especially metal ones. 2. The shape: the "V" horizontal shape will not hurt the swr too much, afaik, but the radiation pattern diagram (which is difficult to plan either - as it depends heavily on the band and the surrounding objects). |
tjackson382000
?lvaro Felipe Hern¨¢ndez
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