How To Characterize/Specify Impedance of 50 Ohm Coaxial Cable/Adapter? - Page 1
Forum section: electronics / rf-microwave

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2019, 01:58:47 pm
content: i made a BMA to BNC cable adapter so i can connect my DSO that has BMA inputs to probes with 50 ohm source BNC output. i have a bit of confusion, or the lack of standard reference or definition thereof. i downloaded some materials but they are too S-parm math intensive and not exactly what i'm asking in mind. i just need to know layman level where the boundary between ok or not ok spec cable...here is the BMA to BNC adapter cable that i've made:attach=655095S21 looks 7-flat good (approx flat up to max 7GHz):attach=655101but S11 Impedance/Return Loss/VSWR is a bit confusing, they are seem off starting 1GHz upward:attach=655107attach=655113attach=655119what confuses me is that there are -10dB to -20dB return loss at > 1GHz and somewhat unacceptable VSWR 1.5 - 2 at some freq spans, but.... those are not visible on S21 plot. i mean, if there is -10dB signal reflected (return loss), isnt that means there should be 10dB down visible on S21 plot on the affected frequencies?2nd but most important question. how to specify this adapter? if i base on S21, i can comfortably say this is a > 7GHz rated adapter. but when looking on S11 plot, how do i make "compliance" limit? should i spec at say ±10% from 50 ohm line? or lower than -20dB loss? or lower than 1.5 VSWR? is there any standard to this?ps: test setup is last attachment, the plot including some connectors/adapters to my VNA SMA setup from that BMA-BNC adapter (DUT) but they should give minimal effect as i've tested them in another setup, so far my conclusion the major contributor to S11 misbehaving is the china grade BNC to RG174 used in the first attachment. fwiw. thanks for your time.

author: dmills
author_rank: Super Contributor
timestamp: February 17, 2019, 02:40:57 pm
content: A 10dB return loss means you are reflecting 10% of the incident power, a 10dB loss in S21 means only 10% of the power is getting thru..... Not the same thing at all!If you have 10% mismatch loss then fully 90% of the power is still getting thru, so S21 would be ~-0.46dB.I would note that as ever with a VNA, the measurements are only as good as your cal kit.Regards, Dan.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2019, 03:10:48 pm
content: Quote from: dmills on February 17, 2019, 02:40:57 pmthanks for your explanation. is dB shown refering to dBW? i thought its dBV.Quote from: dmills on February 17, 2019, 02:40:57 pmi'm using Kirkby CAL kit (female version) so thats the best option i can afford. imho the best thing is the knowledge, but i'm very weak at those so far, esp the s-parm math. i tested the original cable BMA-BMA from where i got (stripped) the BMA connector from. the result is a lot better than my diy made BMA-BNC. see attached. comparing to above result, relatively speaking i think i can rightly say, the above result is much worse.

author: dmills
author_rank: Super Contributor
timestamp: February 17, 2019, 03:42:42 pm
content: dB is a power ratio by definition, for the purposes of figuring gain or loss it is unitless (Because gain is unitless).Regards, Dan.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: February 17, 2019, 05:15:15 pm
content: You possibly asking how to continue using VNA with unknown fixture cables. You can use very poor cable as long as you have quality standards to calibrate at the probing plane.If you dont have kit of standards for BNC side, then some vna software or external software can deembed the fixture if VNA was calibrated with different kit before fixture was added. Deembedded result will theoretically as good (for short period of time in between calibration).Short cable like this is totally not a problem. So the missing piece is either- accurate BNC kit- or accurate Sparams data for fixture cable plus original kit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2019, 06:17:31 pm
content: Quote from: unitedatoms on February 17, 2019, 05:15:15 pmi dont think so. prove me wrong though... i'm using huber suhner cable, i proved that to be a good cable, yes i agree on your statement on using poor cable "behind" the probing plane after calibration, i've done that as well on china coax cable. but for this setup, i'm using my best cable ie huber suhner cable. looking back at my test fixture... i'm probing at the sma end of huber suhner cable connected to VNA (the red vertical line on the left), thats where my probing plane...although my calibration using female load shown in the picture, it was connected to HS (that minibend tag) cable directly during calibration. the picture only shows it to terminate the DUT (assuming the DUT is 50 ohm Zc, but its not from the |Z| plot in OP, if it is we should see a straight horizontal 50 ohm plotted line). the DUT (red box) + the load (termination) is what characterized here. if i replace the DUT (my diy adapter + other adapters in the middle) with another HS cable (terminated with the same female load) i will get a flat 50 ohm characteristic cable impedance, that shows HS cable is a very good 50 ohm cable, but my diy DUT is not, to what degree? thats what i'm trying to find out. what you see, the impedance ripples, worsening in return loss and VSWR in the plots in OP are due to what is in the red box, my diy adapters + another adapters necessary for insertion to the plane.https://ws680.nist.gov/publication/get_pdf.cfm?pub_id=21880what i'm trying to do is somewhat similar to in page 2615 (Fig. 4)and in this articlehttp://projects.npl.co.uk/hf-circuits/docs/presentations/20160628-ruefenacht.pdfpage 11 (screenshot attached below)they are de-embedding technique but not completely described and what tool to the de-embedding calculation. but anyway, lets say i already got the s-parm for my DUT. so what next? all i have is numbers. what i'm asking is simply... from the plot in OP, how to characterize the adapter? or if its not the way, tell me how to characterize (specify) the adapter or cable of its bandwidth range? from scattering parameters numbers?Quote from: unitedatoms on February 17, 2019, 05:15:15 pmi dont mind characterizing the "sma female to bma female + diy adapter + BNC to sma male" (everything in red box). if i got that characterized its good enough already. so i dont think i will need another expensive BNC CAL kit.Quote from: unitedatoms on February 17, 2019, 05:15:15 pmmy VNA can produce calibration s1p file. so i just need someone to guide me to de-embedding software. so i can use method in the above article to subtract the DUT S-parm i want to characterize. but as i said, i'm not sure if i can bandwidth specify my adapter/cable from that scattering parameters anyway.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: February 17, 2019, 06:40:36 pm
content: Right. So the question is now clear. How to acquire S2P using single port VNA.You found the method and have pair of S1P files for identical termination (termination well matching 50), one with adapter, and one without adapter.What is the formula for extracting full quad of S params which will take pair of S1P and calculate the adapter. Uff. ... hmm..

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2019, 06:54:23 pm
content: if thats important i will try to figure that out or ask how. but i dont think retrieving S-parm characteristic is what i'm after here. put it another way, i would like to know how big brand like Gore, Huber Suhner, Astrolab, Amphenol, Tensolite cable etc characterize their cable/adapter/pigtail? without the need to provide corresponding s-parm. some cable/adapter/connector/pigtail are characterized as 8GHz, some 18GHz, some 24GHz etc, how do they get this numbers? thats what i'm after. btw the BMA-BMA cable i'm stripping off is Tensolite brand, but that Tensolite is not good enough compared to Huber Suhner cable i'm using (from induction logic)

author: unitedatoms
author_rank: Frequent Contributor
timestamp: February 17, 2019, 07:24:56 pm
content: From what I heard about cables the difference is in how smooth the metal surfaces are inside the cable. The higher the GHz spec, the lesser is S11 at highest frequencies. Someone should do the teardowns on scrap bits of cables.The method you are referring to seems to be named ML1P. I personally never heard of it before. May be it stands for multiple loads single port for low loss adapters. It looks like the idea is to create two virtual error adapters in error terms matrices and then remove one from other, the remainder becomes non virtual DUT full two port matrix. But I am not sure if I completely understood if it is that how this method works.

author: radiolistener
author_rank: Super Contributor
timestamp: February 17, 2019, 07:36:35 pm
content: Quote from: unitedatoms on February 17, 2019, 06:40:36 pmas I know, S1P is just a text file which contains record lines, each text line has frequency, magnitude and angle in degrees).If that S1P file contains S11 measurement, you can calculate S11 value in complex form in the following way:re = magnitude * cos( degree2radian(angle) );im = magnitude * sin( degree2radian(angle) );Then, if you're need S11 in dB, then you can calculate it in the following way:dB = 20 * log10( sqrt( re*re + im*im ) );For example, if S1P contains line:Code:142.3999940.298055-57.407345it means that for 142.4 MHz we have:magnitude = 0.298055angle = -57.407345 degreein complex form:re = 0.298055 * cos( degree2radian(-57.407345) ) = 0.160551136;im = 0.298055 * sin( degree2radian(-57.407345) ) = -0.251117733;S11 = 0.160551136 - j*0.251117733in dB:dB = 20 * log10( sqrt( 0.160551136*0.160551136 + 0.251117733*0.251117733 ) ) = -10.514From complex S11 we also can calculate SWR:SWR = (1 + sqrt(re*re+im*im)) / (1 - sqrt(re*re+im*im)) = (1+0.298055)/(1-0.298055) = 1.8492Also, from complex S11 we can calculate impedance:R = 1/((1-re)*(1-re)+(-im)*(-im)) * ((z0*(1+re))*(1-re)+(z0*im)*(-im)) =59.341X = 1/((1-re)*(1-re)+(-im)*(-im)) * ((z0*im)*(1-im)-(z0*(1+re))*(-re)) = -32.7089where z0 is a base impedance of vector analyzer (usually 50 Ohm).

author: TheSteve
author_rank: Unknown
timestamp: February 17, 2019, 07:52:42 pm
content: How does S11 of the scope input look with the adapter connected? Most 50 ohm terminated scopes are specified to have a VSWR of 1.5:1 or less. The BNC will not be doing you any favors into the GHz.

author: unitedatoms
author_rank: Frequent Contributor
timestamp: February 17, 2019, 08:05:04 pm
content: @Radiolistener. Right, you can derive magnitude from complex number. However the problem is about cost limitation (can you measure two port device S21 with single port reflectometer which only measure S11?).That how I explain it to myself:Say you have two persons with flashlights in dark room, and large transparent object. The task is to figure out how transparent it is. One person turns on flash light, other person, sees what light went through. Then they repeat without object and compare. And they get the measurement.Now you have single person, the rule is the flash light must stay with person. But you add 2 more not transparent objects and allow person to put them one at a time behind the test object. One of extra objects is white, another is black. So the comparative measurement will still work with single person still.And formulas perhaps exist, just need to figure them for practical case.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2019, 12:00:53 am
content: i think the end result of s-parm already laid out on the plot presented in OP, as Radiolistener demonstrated (need to check back that later), we will get things like Z, Return Loss and VSWR from s-parm, i saw some formula before in complex number notation form. imo s-parm will be needed if we have cascaded systems and need to do s-matrix operations such as addition or subtraction to get s-parm for a single system of interest. such as in the attached picture? (snapshot from a material i have here, google Network Analyzer Error Models and Calibration Methods, warning: math intensive there that i dont claim to fully understand)Quote from: TheSteve on February 17, 2019, 07:52:42 pmi will assume the input characteristics of the DSO (Lecroy made) is in "good" state, and similarly to the CAL load that i used (better spec needs super heavy pocket on DSO and CAL) that is the best i can get at a price, which is a compromise. i want to narrow down the investigation to the DIY adapter alone, which will be connected to the "assume to be good" Lecroy WM (or any other 50 ohm input) DSO.since you mentioned about BNC limitation, i also made BMA to SMA along with the BNC version (SMA is claimed to be good to 18GHz, so they say) below, i didnt want to include in OP to avoid diversion. i've made a solid solder to the ground wire of SMA connector to minimize discontinuity effect, but the S11 result is just as poor (actually worse i think) as BNC version here (for BMA-SMA adapter):so there must be something with the china/fleabay sourced RF connector here. i'm looking at places to get reputable BNC and SMA connectors now, currently from digikey. maybe i will make an order when i finalize my cart later, there are few other items i have in mind, ic etc. later to combine order for cheaper shipping cost penalty.

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2019, 12:28:01 am
content: Do not assume that BNC anything is 50 ohms; I am not sure anything commonly available is. When Tektronix started making 1GHz oscilloscopes and calibration sources, they had to have their own custom BNCs made.

author: Andrey_irk
author_rank: Regular Contributor
timestamp: February 18, 2019, 01:29:02 am
content: It can also be that you soldered the cable to the connector in a wrong way. I mean, sometimes there has to be an airgap between the connector and cable's dielectric and sometimes not. Maybe you used cable with a wrong dielectric.I'd suggest you have a look at the data in the time domain in order to see where the problem is and what kind of a problem it is. You can use the program for VNWA - it can do FD to TD conversion and is free (and you don't have to have VNWA itself).Alternatively you can have a look at the Smith chart. If there is only one big discontinuity then it will be easy to spot. First you find out to what connector it is closer and then by adjusting the reference plane position (phase, that is) you'll understand the type.Adjustment means that you change the phase until the curve on the smith chart the matches line of equal reactance. Then if you know the Vf of the cable - you can get distance from the phase.I think that the biggest problem with crappy connectors is that they don't comply the standard, which means they may not mate properly or even damage the mating connector. Also, durability won't be great. The insertion loss can be relatively high due to poor coating. But they have to work up to a few GHz (never seen ones that didn't).

author: radiolistener
author_rank: Super Contributor
timestamp: February 18, 2019, 05:09:36 am
content: Mechatrommer you're need to check the coax cable, I seen similar waves on impedance chart for Chinese RG316 cable.Also I had experience with 1 meter Chinese LMR195 pigtail with SWR=1.2 on 27 MHz!Here is my measurement of 10 meter Chinese RG316 from 1 MHz to 200 MHz

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2019, 06:18:38 am
content: Quote from: Andrey_irk on February 18, 2019, 01:29:02 ami think thats the best and last way i can do. connector has about 5mm collar protrusion, i only soldered on top of that collar, whatever gap is inside that metal collar with dialectric of cable where its inserted until meet the dialectric end of the connector. the end of collar directly meet cable gnd shield at where i cut the blue plastic sleeve. i did solderless method earlier, ie the cable gnd shield only pressed by cylindrical chrome plated metal sleeve (can be seen in picture above moved to the left BMA end after soldering cannot fit inside it anymore). the result of the solderless method is just as bad. thats why i make a irreversible dead end soldering joint there to get its best result. the connector is this type...https://www.ebay.com.my/itm/10X-SMA-female-nut-crimp-RG174-RG316-LMR100-RF-Coax-Cable-connector/191686834380?hash=item2ca16cf8cc:g:lW8AAOSwHmhV8N4y:rk:1:pf:0Quote from: radiolistener on February 18, 2019, 05:09:36 amthe cable is Tensolite (Carlisle) brand (came all the way from USA, not China) came already with BMA connectors soldered on both end. i cant find the datasheet though it has part number 473-872-03 Rev A1 Mfr 64023. it has double shielding, outer layer is stranded/woven wires, inner layer is metal strip wrapped in spiral, PTFE look alike dialectric and then thick solid single center signal cable, not like china cable which has loosely woven outer shield and thin wires strand in the middle. overall cable OD is around 2.5mm, similar to RG174 specification albeit not stated anywhere on cable's surface or tag. you can see the cable and its Z plot in my second post earlier. here again...in that picture, i'm characterizing the BMA-BMA (males) pigtail + 2 x BMA female to SMA female adapters for connection to VNA-SMA + the CAL load. the Z plot is a lot better, return loss is better than -20dB and VSWR is better than 1.2 almost to the end of the 7GHz sweep. the Z is near 50 ohm in 1GHz span, and acceptably to 2-3GHz span then it start to wobbles more after that. so i have no question on cable quality + the connected BMA male connector. my diy connection is on the cutted cable to china BNC and china SMA, thats where the majority the wobbles, discontinuity, reflection or what not came from i suspect, since my connection is i think good and i took much attention to it, i mean where else? fyi, currently the BNC adapter is solderless, only crimped gnd shield wire strand between metal sleeve and metal collar of the BNC connector. i think if i have to solder it the same way as the SMA, i wont get much improvement, apart from the well known BW limitation inherent to BNC connection. here is the BNC connector used... claimed as 50 ohm impedance well, china claim can be anything claim as stated by David.https://www.ebay.com.my/itm/10Pc-BNC-Crimp-Female-Jack-Bulkhead-O-Ring-Coax-Connector-For-RG179-RG174-RG316/273599521012?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2019, 06:38:19 am
content: but anyway. this thread is about how in characterizing/specifying a cable bandwidth or an adapter or pigtail alike, however crappy they are, not about manufacturing process nor part quality nor about how to improve it. that discussions deserve in another thread. fwiw... thanks for replies so far, but the main question still not answered yeti guess standard to cable/adapter specification is not easily available for free?

author: TheSteve
author_rank: Unknown
timestamp: February 18, 2019, 07:13:06 am
content: Well the proper way would be to have two cal kits that mate to the cable you've made would it not?

author: Andrey_irk
author_rank: Regular Contributor
timestamp: February 18, 2019, 08:48:12 am
content: Quote from: TheSteve on February 18, 2019, 07:13:06 amThe only problem is that one may not be able to find a BNC cal kitMechatrommer,Bandwidth of a cable assembly or adapter is restricted by the performance one can put up with.The method of characterization depends on accuracy you need to achieve.A set of S parameters describing RF performance of a cable is sufficient in most of the cases. So, you measure 2-port S-parameters of an assembly and Bob's your uncle.What do you expect to hear from us as what we've told you didn't answer the question?

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2019, 09:29:57 am
content: Yes for complete 2 ways charaterization i'll need few cables and cal kits for this only one project, this is simply not economical, so i will accept cheap hack.Thinking about it, for ideal adapter/connector, characteristic should be identical or symetric if seen from either direction, ie s22 = s11, s12 = s21, but non ideally, for better and well built adapter, they should be very closely matched, no? Since my adapter will only be used in one direction, characterization from only one end will be enough, as seen in the OP, character from the other direction is irrelevant, imo. even if i have to, i'll just turn it around and test from another direction.its just where i should draw the line is the question. Maybe i'll just make my own spec.. Or just not provide any.. maybe that gore cable speced at 24GHz has vswr of 3, loss of -5db, and impedance 500 ohm at that edge. With multi K$ price of the cable, the s-parm or vna plot never been published, unless someone with a better vna can buy and find out himself and tell the story. I think i can do the same, ie not providing any details since i guess there is no standard definition to this.I will check later how they perform in real condition, ie connecting it to probe and dso, and see how they affect the signal's shape displayed, with and without. I should be able to see something as s21 is almost perfect no insertion loss.

author: tggzzz
author_rank: Super Contributor
timestamp: February 18, 2019, 12:26:35 pm
content: Why not just use a TDR? The magnitude of a reflection can easily be translated into VSWR.You can use just about any crap cable and see how the cable performs independently of how the connector performs.Even my 50yo Tek 1502 (not A/B/03/04 etc) is up to that task.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2019, 02:35:21 pm
content: i know what a TDR is but what actually is in real life? i came across a TDR capable scope iirc but it cost an arm and leg. and i'm not sure why you ask me to get VSWR from a TDR, its already plain and clear in the OP, cheers

author: tggzzz
author_rank: Super Contributor
timestamp: February 18, 2019, 04:31:34 pm
content: Quote from: Mechatrommer on February 18, 2019, 02:35:21 pm?QuoteI have aworkingTek 1502 to sell for £400. 140ps system rise time. Manuals on tekwiki.QuoteI didn't ask you to get the VSWR from a TDR; I said you could. Many people think in terms of VSWR, particularly those that are familiar with S parameters.

author: virtualparticles
author_rank: Regular Contributor
timestamp: May 29, 2019, 01:23:20 pm
content: Full two port S parameter files can be extracted from S11 measurements using the SOLR method. Here is the basis formula:Input Reflection Coeff = S11 + (S12*S21*LoadReflCoeff)/(1+S22*LoadReflCoef)Now if you assume that the device under test is symmetrical then you can assume that S12 = S21 and replace S12*S21 by S21^2 in the above formula.Three loads are applied: A Short, an Open and a Load. Actual reflection coefficients over frequency for the Open and Short must be known. The Load can be assumed to be perfect with LoadReflCoeff = 0Make the three one port measurements looking into the other side when you apply the Open, Short and Load. At each frequency you now have three equations and three unknowns and you can solve for S11, S21 and S22. Then S12 is set to equal S21.The calculation is done using complex numbers. I use Excel or Python for this.Good Luck!BW