Mini-Circuits KARN-50-18+ as the Reference Termination for a VSWR-Bridge - Page 1
Forum section: electronics / rf-microwave

author: TimYTTT
author_rank: Contributor
timestamp: August 15, 2024, 01:41:37 pm
content: Recently I was looking for a 50 Ohm Reference Termination for my VSWR bridge. Someone on the EEV-Blog forum recommended Mini-Circuits KARN-50-18+ termination so I ordered from Mouser Electronics 3 pieces, just to be sure to get proper stuff.After order arrival I checked DC resistances of the terminations with my 5 1/2 digit multimeter and found out to my greatest surprise that their resistances are 51.90 Ω, 51.78 Ω and 51.86 Ω. I was expecting the values very close to 50 Ohm.The Mini-Circuits data sheet specifies impedance of 50 Ohm, but provides also information that the minimal return loss for KARN-50-18+ is 33 dB for the range DC-0.5GHz . The 33dB return loss corresponds approximately to VSWR=1.0458. Calculating VSWR for value of Termination with 51.90 Ω ( comparing to hypothetical 50 Ω reference) results in the VSWR = 1.0379. This complies also with the data in Mini-Circuits Data-sheet. However I do have understanding problems and therefore following Questions:-Is it so difficult to produce the exact 50 Ohm termination?-Why Mini Circuits specifies 50 Ohm termination and delivers 51.9 Ohm Termination?-What happens if I use this termination as the reference termination in my VSWR bridge?Thanks for all inputsRegards, Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 15, 2024, 03:05:45 pm
content: If Mini Circuits are following common convention (and ISO 5725) in the data sheet:The margin of error is one half of the last significant place.50 Ohm as shown the data sheet with no decimal places, can legitimately be between 49.5 and 50.5 OhmTo check for systematic errors (bias) in you measurements (of around 51 Ohm), you could useMini Circuits Type N calibration (short and open and load) Part number SOL-63-NM+The parameters with their decimal places are shown in the datasheet.You may be able to find a more accurate type N termination, I did not look.

author: RoV
author_rank: Regular Contributor
timestamp: August 15, 2024, 04:02:47 pm
content: Quote from: TimYTTT on August 15, 2024, 01:41:37 pmI would be tempted to say that they sacrified a bit the response at low frequency (although remaining within specs) to get a better overall response up to 18 GHz, where a bit of internal capacitive loading, combined with the pin electrical delay, will tend to lower impedance seen from outside.However, this interpretation conflicts with the "typical" response table, where they claim >50 dB RL below 0.5 GHz.

author: TimYTTT
author_rank: Contributor
timestamp: August 15, 2024, 06:36:17 pm
content: Quote from: mag_therm on August 15, 2024, 03:05:45 pmWell, that is what I wanted to avoid. Such calibration kits are quite expensive. Thanks for yourvaluable comments.

author: LM21
author_rank: Regular Contributor
timestamp: August 15, 2024, 10:20:28 pm
content: How much does that MiniCircuits SOL-63-NM+ cost. I can't find a price for it.Is it the same as in SDR-kits page, which has other calibration kits, toohttps://www.sdr-kits.net/index.php?route=product/category&path=66_68_69

author: TimYTTT
author_rank: Contributor
timestamp: August 16, 2024, 06:14:33 am
content: Quote from: LM21 on August 15, 2024, 10:20:28 pm- I paid CHF 19.76 pro termination. This is the price in Switzerland.- I do not actually know if this termination is also included in the SDR-Kits offer. Certainly you can use it as a 50 Ohm calibration standard, but you have to calculate/measure yourself other parameters relevant for this use (delay, L, C).

author: Ringmodulator
author_rank: Regular Contributor
timestamp: August 16, 2024, 05:37:02 pm
content: Hi Tim,at first, you need to get an accurate measurement of the resistance. For such a low resistance, a 4-wire measurement with kelvin probes is highly recommended.If you use a 2 wire measurement, you will likely err on the high side.Second: It is not a problem to build a high precision 50 ohm resistor for dc al low cost.But the issue is, to create a tremination, that is 50 ohm over the specified frequency range. The higher the frequency, the more the parasitic capacitance and inductive components come into play.The KARN50-18 is specified up to 18 GHz. This is quite something.The sdr-kits calkits have each 50 ohm load specified with the resistance measured individually with the kelvin measurement.The value is entered in the vnwa3 sotware and therefore taken into account for each masurement after calibration.RegardsChris

author: shabaz
author_rank: Frequent Contributor
timestamp: August 16, 2024, 06:11:33 pm
content: I've noticed that, too. My Mini-Circuits 50-ohm terminators have a DC resistance approx 51.3 ohms, whereas the Huber & Suhner ones that I have are approx 50.3 ohms. However it's not a large sample (I only have two Mini-Circuits ones, ballpark $20 value, and two H+S, one was approx $30, the other is a 15W load, $$$).

author: TimYTTT
author_rank: Contributor
timestamp: August 16, 2024, 08:17:20 pm
content: Quote from: Ringmodulator on August 16, 2024, 05:37:02 pmHi Chris,Thanks for your comments. I do not believe that 4-wire measurement will so dramatically improve the values I have measured. Having said that, my measured values of 51.90 Ω, 51.78 Ω, 51.86 Ω can differ by 4-wire measurements only in the places beyond the decimal point. That means that I still do not have answer why Mini-Circuits are selling this product with the value higher than 1.5x Ohm from the nominal value. I think that they have a good reason for that, but this is still hidden to me. Maybe I have to ask them directly.Regards,Tim

author: TimYTTT
author_rank: Contributor
timestamp: August 16, 2024, 08:30:13 pm
content: Quote from: shabaz on August 16, 2024, 06:11:33 pmHi Sahbaz,Thanks, I am obviously not alone! I noticed this discrepancy too, comparing the Mini-Circuits KARN-50-18+ values with other 50 Ohm terminations I have (Rosenberger, Huber Suhner and even Mini-Circuits (BTRM-50+->50.27 ohm) terminations ).Regards,Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2024, 04:20:33 pm
content: Quote from: TimYTTT on August 15, 2024, 01:41:37 pmI'm a bit confused by your post.QuoteI would suggest exact is impossible to achieve but I don't know why you would need that for measuring VSWR.QuoteThey provide a tolerance. Your comment of their specified 33dB return loss corresponds approximately to VSWR=1.0458 is correct. Assuming no errors in your measurements, your worse case of 51.9 falls within the specified 52.3 & 47.8. I don't understand why you would ever "expect" them to be closer.QuoteYou calculated the error correctly. Guessing you read the datasheet prior to purchase, knew the errors and found them acceptable for your needs. You measured it and found it in spec, so I am not sure what you are asking.Quote from: TimYTTT on August 15, 2024, 06:36:17 pmAssuming you are are cheap ass like myself and really want (from your posts, it seems you don't actually need it) that higher accuracy, you can always order several parts and sort them.***Of course, sorting them means having a way to measure them, meaning you need standards. In my case, I borrowed a couple of sets. You may be able to rent a set.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 17, 2024, 05:14:41 pm
content: Quote from: joeqsmith on August 17, 2024, 04:20:33 pmIn eng school 55 years ago in Australia we had a Very British supervisor "Dr Chris" for first year physics.If a report was submitted that did not have correct number of places to match the precision of the measuring instruments, and the experiment calculations, (including slide rule and sometimes log table precision) he would roar expletive at the poor student and sometimes throw the report on the floor.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 17, 2024, 05:59:19 pm
content: Quote from: mag_therm on August 17, 2024, 05:14:41 pmFunny, but I don't see the point you are trying to make, if any.QuoteThere is no need to guess. The manufacture's datasheet provides the return loss down to DC. From there we can calculate the acceptable range of DC resistance.

author: TimYTTT
author_rank: Contributor
timestamp: August 18, 2024, 03:24:51 pm
content: Quote from: mag_therm on August 17, 2024, 05:14:41 pmHi mag_therm,Thank you, I perfectly understand what you wanted to say.A long time ago I had also at Uni a math professor who was looking just for our faults and didn't bother very much if we understood his lectures. That was not quit good experience for all of us, but at the end we all survived.Kind regards,Tim

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 18, 2024, 11:52:14 pm
content: Hi, TimYTTT Yes, I did not keep fully up to date with electrical metrology, but was always aware of the basics of uncertainty of measurement after that experience!Reading again your o/p, how did you zero and then measure the DC R? . You did not have a type N short (post #3)?Did you screw the 50 Ohm, fully compressing the gasket, into (for example) a bulkhead jack Amphenol 082-97-RFX ?If the DC measurements (51.78 Ohm etc) were with a calibrated and zero-ed DC Ohm meter, it might be appropriate to raise a "Tech Support and Satisfaction" ticket with Mini Circuits.

author: TimYTTT
author_rank: Contributor
timestamp: August 19, 2024, 02:14:45 pm
content: Quote from: mag_therm on August 18, 2024, 11:52:14 pmHi mag_therm, thanks for feedback! Here are my comments:For the measurements I used a DIY fixture (Photo) with N-,BNC- and SMA-connectors. The N-female connector on the fixture (bulkhead jack) comes from Huber+Suhner.The first measurement is with "N-short" termination in order to find out what is the "zero-level " resistance of the fixture + N-Short termination (000.10 Ohm).Than I press "zero" button on the multimeter. Now I have reference zero for the KARN-5-18+ termination measurement (000.00 Ohm)I replace the N-short connector with the KARN-5-18+ termination . The multimeter displays now the KARN-5-18+ termination resistance (51.90 Ohm).I do not think that there is a substantial measurement error in my measurement procedure. Most probably joeqsmith is right telling that Mini-Circuit specification shall be interpreted using their Return Loss specification. However I measured various terminations from other known and unknown vendors (10+ samples) and all of them had quite close values to 50 Ohm. I am going to ask Micro-Circuits for their opinion on this.Kind regards,Tim

author: tszaboo
author_rank: Super Contributor
timestamp: August 19, 2024, 02:43:36 pm
content: Quote from: mag_therm on August 17, 2024, 05:14:41 pmWell, he wasn't an engineer.We had other things written down (especially for telecommunications class) for example:sqrt(2) =1sin(x) = xg = pi21/2 = 1Because we were engineers, and it's usually good enough, especially when you have to calculate stuff in decibels.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 19, 2024, 04:25:39 pm
content: Check my math but Mini-Circuit's specified worse case 33dB return loss at DC = +/-0.0224 +0j = 52.29/47.81 ohms or a VSWR of 1.046. Expecting it to be tighter than what is specified makes zero sense to me.As was mentioned, we don't normally measure VSWR at DC.A similar discussion about the tolerance:https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2978858/#msg2978858QuoteI agree. Assuming the meter is in working order, what you show should be fine.Shown is my meter with various adapters, no null, with a decent fixed resistor along with a Narda and HP terminator.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 19, 2024, 04:38:42 pm
content: Sure the 4W and null is going to remove some of those errors but I doubt anyone measuring the VSWR of their antenna would care.

author: TimYTTT
author_rank: Contributor
timestamp: August 24, 2024, 08:52:29 pm
content: Hello all,I contacted Mini-Circuits sales and asked them about the DC resistances of KARN-50-18+ terminations. Here is their response:"Our KARN-50-18+ and BTRM-50+ terminations designed to use for commercial – testing application, they are cost effective options, andthey are not precise terminations. Due to internally built structure for KARN-50-18+,as this contains more resistorsdue to higher power handling, so more variations occur."After that I asked them to suggest me a suitable precise termination from their portfolio. Again, here is their response:"Unfortunately, we do not have any precise terminations available in our standard catalogue."By the way, does someone can provide me description or specification for the "Precision RF Connectors and Terminations"? Are these specifications for mechanical or electrical or deployment characteristics of RF connectors/terminations?Thanks, Tim

author: Bud
author_rank: Super Contributor
timestamp: August 24, 2024, 09:15:43 pm
content: Nobody measures VSWR to 4 decimal points, it makes no sense for any practical purpose. What is your use case?

author: shabaz
author_rank: Frequent Contributor
timestamp: August 24, 2024, 10:50:39 pm
content: If you need 50 ohm for DC or near-DC applications, you'd be better off making a DIY one. Sometimes it can be useful, for instance, I sometimes use 50 ohm components to deliver a low frequency signal to a 'scope for convenience, rather than 'scope probes. But usually I don't mind a few percent error.Otherwise, for $20-$30 ballpark the Huber & Suhner ones appear to be a lot more close to 50 ohm at DC than the MiniCircuits parts, but that's just an observation based on a limited sample, and would be easy to improve on with a DIY load, at DC. If you search for 'DIY OSL' or similar, you'll see lots of examples of how those can be assembled. I have a few DIY ones, and personally I measure a dozen or two resistors (2-wire, I don't bother with 4-wire for this use-case), to pick the ones closest to 50 ohm (when combined; usually I'll use two 100 ohm resistors in parallel), simply because it's easy to do.Even calibration kits (3- or 4-digit $) might not come with any measurement at DC. My fairly decent (but not metrology grade) OSL 'T' cal tool doesn't come with any measurement at all for the 50 ohm load below 50 MHz.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2024, 01:08:32 am
content: Quote from: TimYTTT on August 24, 2024, 08:52:29 pmWere you expecting something else? Keysight has a similar note for some of their terminators, for example:QuoteQuote from: TimYTTT on August 24, 2024, 08:52:29 pmI'm sure it came as no surprise that Mini-circuits does not produce standards. If you want some data for actual standard, the best thing would be for you to look up something that meets your requirements (something I am still not sure of).***If your looking for something low cost:https://www.ebay.com/itm/266817014023LOADDC-resistance: 50 Ω ± 0.5 ΩReturn loss, min.: 42 dB @ DC to 6 GHzPower rating, max.: 0.5 W

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2024, 05:51:43 pm
content: Looking up a few N male standards.Again, double check my math and datasheets.Mini-Circuits KARN-50-18+ 33dB DC-1GHz. +/-0.0224 +0 i = 33dB RL = 52.3 / 47.8 ohms = VSWR 1.046Maury Microwave 2510B6 VSWR 1.02 DC-2GHz, +/-0.01 +0 i = 40dB RL = 51.01 / 49.01 ohms = VSWR 1.02Copper Mountain N1802 50 +/-0.5 >= 42dB DC-6GHz, +/-0.00794 +0 i = 42dB RL = 50.8 / 49.21 ohms = VSWR 1.016Keysight 85032F >= 48dB RL DC-2GHz, , +/-0.00398 +0 i = 48dB RL = 50.40 / 49.60 ohms = VSWR 1.008You should also consider the mate cycles, does the company offer service / calibration in your area.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2024, 06:17:55 pm
content: I thought I would have a look at what Kirkby Microwave offers. If you check out their website, there is no mention of the RL.https://kirkbymicrowave.co.uk/Sales-and-Services/Vector-network-analyzer-calibration-kits/N-calibration-kit/Someone made a review for it, but it was just a typical blab. No measurements or verification.I also tried to look at what Pico has. Their website is so bad now, I can't seem to get to any information without the top menu popping up.https://www.picotech.com/R&S, ZV-Z270https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/ZV-Z2xx_dat_sw_en.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: August 25, 2024, 10:18:57 pm
content: Here are some nice terminations:https://www.telegaertner.com/en/online-catalogue-coax/n/n-termination-plugThe model number 100024181 has -47dB return loss and goes to 6GHz. There is a 18 GHz version of it, which only has -40dB at 1GHz, but it goes to higher frequency. So likely there is some tradeoff of high and low frequency performance.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2024, 11:57:20 pm
content: Quote from: tszaboo on August 25, 2024, 10:18:57 pmFrom what I saw, they don't provide a worse case return loss, only typical. Note the typical for that Mini-circuits was in that ball park but as the OP found out, that's not a guaranteed spec.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 26, 2024, 04:24:25 pm
content: It appears Mini-Circuits does offer a low cost N calibration standard. RL is 36dB compared with your 33dB terminator.https://www.minicircuits.com/pdfs/SOL-63-NM+.pdfI tried to find a price but it seems you would need to contact them:https://www.minicircuits.com/WebStore/dashboard.html?model=SOL-63-NM%2BThe following is a good introduction. Quotes taken from the article. Member virtualparticles works for Copper Mountain and may be able to chime in if you have questions about the article.https://coppermountaintech.com/introduction-to-the-metrology-of-vna-measurement/QuoteQuoteQuoteQuote***Article on the sliding load:https://www.microwaves101.com/encyclopedias/sliding-loadArticle on TRL:https://www.microwaves101.com/encyclopedias/trl-calibration

author: TimYTTT
author_rank: Contributor
timestamp: August 26, 2024, 07:05:07 pm
content: Thanks a lot to all of you who provided me quite a lot of valuable information! Actually I wanted to get hands-on experience on measuring low VSWR values of components like 2R power-dividers or connector adapters using not too expensive measuring equipment in the range of 50 to 500 MHz. I do have some theoretical know-how for this, but as some of you noticed, I am missing a real practical know-how in his area. I am fully aware that there are high-end solutions for such measurements, but I think I can learn more if I go other ways.Kind regards,Tim.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 26, 2024, 08:09:36 pm
content: My advice would be to buy one of the low cost VNAs. Specifically, because you want to experiment above 300MHz, get the LiteVNA64. The original NanoVNA will use harmonics above 300MHz and the performance suffers. The LiteVNA 64 does not use harmonics up to 6GHz. Cal kits they provide arealrightbut they use SMA rather than N.***Alright meaning for experimenting, not for precision measurements.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 27, 2024, 12:09:46 am
content: This is some old tuner that I got as part of some military surplus scrap. Other parts were characterized to 10KMC(GHz), which dates it to the 50s.This device is something like a load with a variable length (similar to a sliding load but all sorts of parasitics)Connected to the VNA you can see how it starts at 51.5 ohms. I then adjusted it to give me 54 ohms at 4GHz. The parts you are playing with provides 50ish ohms over the entire range.The micrometer was made by Starrett and I'm sure this cost the government (tax payers) a fair bit of coin. I've been tempted to steal the micrometer off it or just toss it as it has never found a use outside of a odd conversation piece.***Added a few pictures if your curious. The pin slides into the center of the resistive element. The lower pin/load element threads into the upper block to lock everything into place when the pin is moved in and out. The outer body of the N connector has an insert to maintain some fixed diameter around the element.I have no idea who made the part. The only part numbers are on the Starrett micrometer and the 91737ug-21d/u ms91236-210 marked on the Amphenol connector.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 27, 2024, 01:46:11 pm
content: Shown is what I use to calibrate the NanoVNAs when working below 1GHz. 2X1205 100 ohm, sorted. Load was attached to an SMA-N and connected to the meter the same way.My N standards are homemade and while I did characterize these with a set of Agilent standards, I am using the ideal model (treating everything as perfect), sweeping to 2GHz. Red is the home made standard, Yellow is that old HP terminator.

author: TimYTTT
author_rank: Contributor
timestamp: August 27, 2024, 04:06:58 pm
content: Quote from: joeqsmith on August 27, 2024, 12:09:46 amThanks, this is quite interesting. I was reading in some old books about sliders and tuners, but have not until now seen any of them! It seems that you are well equipped with all possible electronic components.

author: TimYTTT
author_rank: Contributor
timestamp: August 27, 2024, 06:30:56 pm
content: Quote from: tszaboo on August 25, 2024, 10:18:57 pmHello tszabo,Thanks for this info. According to datasheet the termination you mentioned has better minimal return loss than KARN-50-18+: 38 dB@1G, 35 dB@2G, 30 db@4G (here is the minimal RL same as by Mini-Circuits). It is a 6G termination comparing to 18G with KARN-50-18+, but that is not relevant for me.Have you ordered something from Telegärtner? Do they have a direct sales or they go thru distributors?Kind Regards, Tim

author: joeqsmith
author_rank: Super Contributor
timestamp: August 27, 2024, 06:49:12 pm
content: Just be aware that the datasheet that they link for PN# 100024181:https://www.telegaertner.com/fileadmin/pdms_files/J01026B0012kp.pdfreferences the old part, J01026B0012, not the current. If you select read more, then scan to the bottom, you will find:QuoteIs it 30.5 minimum per what they state for the new part number or 38 per the old datasheet? My advice is at least take the time to contact them before making a purchase. Explain what you are looking for and they should be able to help.

author: tszaboo
author_rank: Super Contributor
timestamp: August 28, 2024, 11:09:32 am
content: Quote from: joeqsmith on August 27, 2024, 01:46:11 pmHow do you compensate for the electrical delay/port extension? Do you place your calibration plane to the SMD pad, or do you calculate it back to the SMA connector? Or just ignore it because it's 1GHz.I have a trick, I'm just wondering how others do it.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 28, 2024, 12:32:55 pm
content: That fixture shown was designed for not only creating the standards, but also for testing the parts. Because the boards and connectors are both part of the calibration, we basically remove those errors. These standards are only used for measuring parts that are mounted to them. If I need to compensate for the phase, I do it with software. There are two ways. You can enter the offsets for each of the standards used, or you can trim the overall offset after cal.I went over the basics in that very first video I made for the NanoVNA, time stamped here:https://youtu.be/mKi6s3WvBAM?list=PLZSS2ajxhiQD2gftdurGQoyGpUM_HobNI&t=850***See AN 1287-9 In-Fixture Measurements Using Vector Network Analyzershttps://www.testunlimited.com/pdf/an/5968-5329E.pdf

author: tszaboo
author_rank: Super Contributor
timestamp: August 28, 2024, 01:39:12 pm
content: Quote from: joeqsmith on August 28, 2024, 12:32:55 pmRight, I have a variation of this board. I think it's a concept that people will naturally reach when working with VNAs.Yours is more functional, it can measure more setups, like shunt through measurement.https://www.eevblog.com/forum/rf-microwave/measuring-passive-parts-with-vna/msg5017483/#msg5017483I was asking about how you would find out the electrical length.So I was thinking about making my next setup with a Y connection in the middle. You can mount a 50 Ohm SMA terminator at the end. The other end of the Y would be open circuit. Then do TDR measurement with the LiteVNA to figure out the electrical length of the SMA connector and trace. Or measure it as a notch filter and use the frequency for the calculations.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 28, 2024, 02:09:09 pm
content: That's a bit more difficult. For the fixture shown, I don't care as it all comes out in the wash. I have slowly turned up a few waveguide standards but I had to make a few of them. Assuming the short is a flat surface, that's pretty easy to measure. Of course, there is no such a thing as an open standard, so we need to slide the short over by a 1/4 wavelength. We can add a shim or make an offset short. Shown is a Maury Microwave 3/8 offset short along with my homemade 1/4 wavelength. That was measured with a depth gauge when I polished the end to get final dimension. The shim was also measured with a micrometer during polishing.If I know the delay of the calibration standards (typically supplied by mfg) and am using this as part of the model (which I never do with the low cost VNAs, but certainly could), I can measure the fixture on the VNA and trim the offset while looking at the Smith chart.If I measure the length of my homemade FR4 standards using the LiteVNA /TDR, the first problem is it's FR4 with poor control. Then there is the limited frequency range of the LiteVNA. I was going over that with another member recently who is attempting to make a standard test cable:https://www.eevblog.com/forum/rf-microwave/vna-for-cable-characterization/msg5591717/#msg5591717Here I was showing a linear slide with a section of airline. This allows me to adjust the length and measure it. You can get some idea of the variance and resolution.https://www.eevblog.com/forum/rf-microwave/nanovna-custom-software/msg3337248/#msg3337248

author: tszaboo
author_rank: Super Contributor
timestamp: August 28, 2024, 05:08:51 pm
content: Quote from: joeqsmith on August 28, 2024, 02:09:09 pmActually, I just had an idea. Calibrated the LiteVNA with a T splitter at the end, then placed my Through calibrator (twice the electrical length as the length to the DUT) on the splitter. It made a beautiful notch at 2.85GHz. Now I'm of to look up the formulas, if I need any other variable.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 28, 2024, 10:17:29 pm
content: If you just wanted a ballpark, my boards for example are FR4. AppCAD lists the dielectric constant as 4.6. Velocity factor = 1/sqrt 4.6 = 0.46625I enter that into Solver64. I then run a cal over the VNAs entire frequency range. I then remove all the standards and make a relative measurement. Next I insert the test through PCB with nothing attached on the backside, to give us a high reflection.Solver measures the peaks location at44.364mm.I then measured the total length from the ends of the two SMAs and got 46.98mm.From the following:https://www.amphenolrf.com/rf-connectors/sma-connectors.htmlthe reference plane should be between 0.074 and 0.078" or 1.8796 to 1.9812mm. 46.98mm - (2 connectors) = 43.2208mm to 43.0176. I remeasured the board a couple more times but there is about 1mm of error.Keep in mind these measurements are a ballpark and we really don't know the dielectric constant. And of course, the biggest problem is the VNAs limited BW. If I zoom into the peak, you can see we just don't have the resolution to resolve it.Lots of errors. Good to have known standards if you really need them. Fortunately, for my hobby use, absolute accuracy is seldom a concern.****From:https://resources.altium.com/p/fr4QuoteThis equates to a velocity factor of 0.456435 to 0.512989. Plugging these into solver and measuring the test board, the length measures from 43.43 to 48.811mm.***I was curious why it reads so short and gaged the connectors.As you can see, this isn't one of my normal test boards with the proper end launch connectors. These are some junk I had pulled from some cheapo something or another that I snipped two of the mounting pins on and laid them sideways. Both the dielectric and pin depth are way off (sunk in).Still, I hope it is clear that the LiteVNA really isn't a good tool to make this measurement.***The female pin should be between 0 and 0.010". Pin 1 measure 0.0205" and pin2 measures 0.0125". I think these came off of one of those $10 PCB attenuator sets so no big surprise.I have a cheap set of adapters attached to my Agilent VNA for when I used these low end connectors.

author: Robert763
author_rank: Super Contributor
timestamp: October 01, 2024, 03:54:18 pm
content: Lets put some perspective on this. In the UK is about £20 including tax. A Keysight 909F N Male is about £1320. The precision load is 66 times more expensive.You pay your money and takes your choice.I've had good results with Dr Dave's kitshttps://www.kirkbymicrowave.co.uk/Robert.