How to Better Wire the BNC Connector and GHz issue. - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 02, 2011, 09:05:26 am
content: i just got my male BNC connectors and female-T to interface my future project with my newly bought function generator. I bought quite many (10 for the male) so i can further interface to my rigol dso or do some funky BNC connections stuff. Since its only the head/connector, so i'm going to wire it. I can just wire it with simple 2 black and red wires and ignore about anything else, but for the sake of information and better knowledge, i want to make a public query. Since there are issues such as data integrity, attenuation, reflection at high/RF frequency etc which i dont really fully understand.Variaty of cables, even coax alone. My satellite TV cable is very hard to bend. My FG cable quite flexible. My Rigol, i'm not sure if its even coax, you know? that thin Rigol's probe cable. So the question...1) What is the best (or better) coaxial cable around and preferebly easy to get everywhere? if there is difference, what makes one coax better from another?2) Whats with normal 2 wire setup? or twisted type? for RF design/analysis/probing?3) How do i define and measure an impedance of a wire/transmission line (or two close together, coaxed or twisted) with relation to their length? (or is this just a stupid question to ask?)hope to hear some answer. Thanx.

author: jahonen
author_rank: Super Contributor
timestamp: March 02, 2011, 09:31:11 am
content: 1) Generally, stiffer the coaxial cable, better it is. Best "cables" are actually solid copper tubes (usable to 100 GHz or so), so called rigid coax. Then there is semi-rigid coax (usable to 30 GHz or so) and finally, the ordinary "flexible" coax, usable maybe up to 1-2 GHz, depending on the length. Flexible coax exhibits changes in cable geometry when moving it around, so it might be unusable in reflection loss measurements if you need repeatability (see below). Since you mentioned BNC, that probably itself limits usable frequency range to less than 1 GHz, so you might get away with something like RG-179 (about same than average scope probe coax), or RG-58 (a bit thicker), if your cables are short. See some datasheets of the cables to get idea of the losses involved. Basically, thicker cables tend to have less loss.2) 2-wire setup (i.e. loose wires) is usually not an option since impedance is completely unpredictable. For RF-design, things you want to measure are S-parameters, like input/output reflection loss (S11, S22) and transmission loss (S21, S12), For amplifiers, noise figure, intermodulation distortion (IP3, ACLR etc) and 1 dB compression point are common measurements. For this, you'll need tightly controlled impedance all the way. Probing is much more difficult with RF, since you really can't stick just about any kind of probe to your circuit without affecting internal matchings. Yes, there are microprobes and such things with controlled impedance all the way to the tip of the probe but these are not really things that average hobbyist will get.. So you must usually provide controlled impedance test points with proper coaxial connectors. Of course this depends your requirements, but that probably is more like worst case.3) Measuring the impedance is somewhat difficult, and would involve a VNA. Basically, it is determined by the conductor geometry (thats why twisting the coaxial cable will change its impedance!) and permittivity of insulator dielectric.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 02, 2011, 02:01:14 pm
content: Thanx master Janne. the description is really serious, but i dont think i will ever afford to design up to GHz system/project before i can collect substantial amount of fund. I just wondering. So let put the spec down a bit. Say for designing 100-400MHz system, is loose wire acceptable? or any rough figure of what MHz loose wires will still be applicable? but you already gave me a rough figure on the limit of "ordinary" coax cable capable of, so i should find the coax cable instead of loose wires, thanx. Will search my collection if i got any spare coaxial type cable, or even cut my satellite TV cable, its still got more length than what the TV needs.Having learnt from this blog that we have to take harmonics into consideration, then for 400MHz system, we should consider for cable thats capable of 10th harmonics ie 4GHz, ie semi-flexible-coax, right? about the length and impedance measurement need vna to analyze which is expensive for me, any general rule about the length, my mind saying so far that shorter cable is better, or some multiple of frequency? a quarter of wavelength? isnt it?

author: Wim_L
author_rank: Regular Contributor
timestamp: March 02, 2011, 03:45:51 pm
content: Be careful with using TV cable for electronics. Most TV signals are sent over 75 ohm coax, while a lot of electronics equipment is matched to 50 ohm. Mixing the two is undesirable. The connectors usually are specified for one or the other too, not both. There's a table showing some common cable types athttp://en.wikipedia.org/wiki/Coaxial_cable

author: jahonen
author_rank: Super Contributor
timestamp: March 02, 2011, 04:36:54 pm
content: Quote from: Mechatrommer on March 02, 2011, 02:01:14 pmDo not use that TV coax, get 50 ohm coax. As for "loose wire", I suggest that you make some coaxial cables with BNC connector in the other end and leave other end free (or get some pre-made cables and then cut them to half, you get two cables then), you can then solder the other end to your board. Not perfect but it works quite often, especially for frequencies you are aiming.Yes, harmonics must be considered, but it is not hugely important thing unless you want to measure harmonicsAll the measurements I mentioned are done in vicinity of the main frequency. The limits are not strict, so you can still see higher frequencies, but attenuation will increase, so upper harmonics will appear in lower level. It again depends how accurate measurements you want to make. Cable (and connectors) matter more when measuring those reflection losses, but if you are not going for a VNA, then don't care about it.The cable length is not very significant, as long you have properly terminated system, i.e. your measuring equipment has 50 ohm input impedance, connected via 50 ohm connectors to 50 ohm cable. Of course, the attenuation will increase at higher frequencies, but is not usually prohibitively high for usual lengths for these kind of purposes. It only matters greatly if you do some tricks with it (like open end cable with length of 1/4 wavelength will look like a short circuit from feed end, due to reflection).Regards,Janne

author: saturation
author_rank: Super Contributor
timestamp: March 02, 2011, 05:04:10 pm
content: Just my 2c to those already posted. Yes, make sure cable used has a terminated impedance of 50 ohms; RG59 is 75 ohms and can look fairly close, specify the cable as 50 ohms RG58 which is as cheap and flexible, and typical test leads of 3' or 1m will not be significant.Compare against RG8.http://www.bcar.us/cablespec.htmThe differences between other 50 ohms cables is cost, size and stiffness; you'll pay a little more for reduced signal attenuation per foot.Use pure coax whenever possible. You'll note some have adapters for BNC to alligator or minigrabbers, or as in your photo, for basic AWG wire. These can cause issues, so if you make your own keep the non-coax segment as short as is practical; like in those photos a good portion of the cable is still coax. Its flexible to use them in lower frequency applications so long as you are aware of them; jahonen's recommendation is best; direct soldering of cable into the board.One thing you can do with your signal generator, is actually test the effect of various connectors [ banana, clips, alligators, BNC splitters etc., ] against one that is just purely coax; then you'll have some idea what artifacts its causing. Also, the BNC connector and BNC adapters can produce artifacts too at higher frequencies. For example, BNC is useful to about 4-11 GHz, and how high you can take them before reflections become substantial can differential a brand name connector from Amphenol versus no-name Chinese brands that 'look' them same.http://www.amphenolconnex.com/products/bnc.aspFor practical purposes, about ~ 2GHz is the limit of BNC.Quote from: Mechatrommer on March 02, 2011, 09:05:26 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 02, 2011, 11:41:54 pm
content: thanx guy. i got some picture now. and thanx Wim for reminding me of 75ohm vs 50ohm impedance cable. i know there are sort of thing, but it seem i just didnt get it. one last noob question... whats exactly the terminated system, terminated impedance? hihi.

author: saturation
author_rank: Super Contributor
timestamp: March 03, 2011, 01:53:09 am
content: Terminated impedance meant the final transmission line impedance from the effects of, source, line and load are matched. I guess I should have said the cable has a 'characteristic impedance' of 50 ohms to refer to the cable only, my error, see below.http://en.wikipedia.org/wiki/Electrical_terminationZs and Zl can be resistors of 50 ohms each, but the cable's Zo, called thecharacteristic impedance, depends on the inductance and capacitance of the cable, per unit length, and bypassing all the math to summarize, its units are in ohms too.http://en.wikipedia.org/wiki/Characteristic_impedanceIf you use the wrong cable type, say Zo = 75 ohms, RG59, it will create an impedance mismatch even if you terminate each end as 50 ohms, because the cable is part of the system.By example, the Hantek 3x25 has an source impedance of 50 ohms, Zs, and using '50 ohm' cable, Zo, the load end must also be 50 ohms, Zl, for the most efficient signal transfer. If the Rigol 1052e is the load, you need to put a 50 ohm terminator, Zl,in parallel to the 1 megaohm so the terminated impedance is ~ 50 ohms.Quote from: Mechatrommer on March 02, 2011, 11:41:54 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 08:04:58 am
content: Janne, the RG-179 is 75ohm i checked in wiki, i dont get why you make the suggestion as most device is 50ohm.for my own later reference...http://en.wikipedia.org/wiki/Impedance_matching.thanx saturation for the links, gave me some direction on what to read.a.jpg) ok, made my first quick "invention" with bnc's, tried to make a 50ohm terminator (2x100ohm)b.jpg) connected it from my Hantek 3x25 AWG to Rigol 1102E(mod) directly without terminator, 100Hz 3.5Vpp sine is generated, Rigol showed what is set in 3x25 Software. everything is fine!c.jpg) with the terminator installed, then the peak went wrong. any explanation with this confusing 50ohm termination stuff?

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 08:37:40 am
content: well i think what i got is RG59 (or RG6) 75ohm connector, this is closest i found in ebay saying its RG59 (for video stuff)http://cgi.ebay.com.my/6x-BNC-Male-RG59-cable-Crimp-75-OHMS-Connector-131-/300530023762?pt=LH_DefaultDomain_0&hash=item45f8fc3d52i bought is local web here (no description whatsoever)http://www.lelong.com.my/bnc-connector-10-pieces-pack-78810266-2011-03-Sale-P.htmbut it looks damned identical to the 3x25 bnc cable/connector. i dont know where is the physics that tells they are different.from left (bnc to wire (screw type), bnc to coax (screw, solder/crimp type, 3x25 cable)ps: i hate to buy another stuff, i'm not this professional. so i just assume they will not make any difference (RG6/RG59 75ohm to RG58 50ohm) and become ignorant

author: jahonen
author_rank: Super Contributor
timestamp: March 03, 2011, 09:11:44 am
content: Quote from: Mechatrommer on March 03, 2011, 08:04:58 amSorry, I meant RG-174.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 01:11:49 pm
content: thanx janne

author: deephaven
author_rank: Frequent Contributor
timestamp: March 03, 2011, 01:25:57 pm
content: What sort of systems do you wish use with this high frequency stuff? This will have a large effect on how many harmonics (i.e. bandwidth) you should be considering.If it's a modulated radio frequency system, then you don't need much more than the carrier frequency because the spectrum will be the carrier frequency +/- the frequency component of any modulation (with a multiplication factor on the modulation component depending on the form of modulation and it's spectrum).If it's a data system, then you need to consider a higher bandwidth to accommodate some of the harmonics. Unless you really want to see the true nature of the signal, you can probably get away with a lot less than a 10X sample rate. This is because practical systems don't have vertical-sided square waves, but more curved looking waves. What is important here is the 'eye' which you may have seen elsewhere. What this means is that the 'eye' should have a good vertical and horizontal window relative to the (possibly recovered) clock so that the circuit can make a reliable decision on whether the signal is a logical 'high' or 'low' at that point. Also in a data system, matched impedances are important otherwise you will get reflections which can cause ringing and overshoots.On the waveforms you showed, the signal will drop 50% when you terminate it.

author: saturation
author_rank: Super Contributor
timestamp: March 03, 2011, 01:29:16 pm
content: The rated output of the 3x25 is 3.5Vp into 50 ohms, it will however produce more Vp into 1megaohm, but the amplitude will not be stable nor the waveform quality be maintained throughout its frequency range, to at least 20 MHz. It depends on the reflections being sent back and the degree of interference from it, and it will vary by type of waveform and frequency. At 100 Hz, that's pretty much just DC as far as transmission lines are concerned.Set the output of the Hantek to its max, 3.5Vp, without the terminating resistors. On the Rigol the output will be more that 3.5Vp. Now put the terminator, it should now be 3.5Vp.Now for the acid test, use the square wave output of the SYNC port, which as we discussed, is very sensitive. You asked about harmonics earlier, well here is a way to test as the square wave Fourier components will push at least 9x the fundamental.Check the output, unterminated at 100 Hz, note the Vp, it will be a high value, may near 4Vp. Do the same, with terminated. I think it should be 1Vp.Now ramp the frequency up to 10 MHz. You will see better edges as part of 90 MHz component of the fundamental, the limits of your modified Rigol. You should have a stable square wave at 1Vp with some ringing depending on you cables and connectors but it will be noticeably square. Now pull your resistors out and see what happens.Quote from: Mechatrommer on March 03, 2011, 08:04:58 am

author: saturation
author_rank: Super Contributor
timestamp: March 03, 2011, 01:31:01 pm
content: [ edited for clarity]BNCconnectorswill work for any cable you can stick it into. The problem is more limited by the characteristic impedance of thecable, and less the connector, you choose. However, since the characteristic impedance is really a property of inductance and capacitance, better manufacturers make separate 75 ohm vs 50 ohms BNC connectors, so they are not identical.I posted a link for details earlier, but here it is again:http://www.amphenolconnex.com/products/bnc.asp#specs75I don't think it will make a big difference for under 100 MHz, using either connector, but whenever possible, buy the right spec.Quote from: Mechatrommer on March 03, 2011, 08:37:40 am

author: jahonen
author_rank: Super Contributor
timestamp: March 03, 2011, 02:27:19 pm
content: For this context, perhaps I can refer to"1k probe"I once studied. That certainly gives idea how good such simple connection can be, if terminated properly.Double termination is not absolutely necessary, but it matches both ends, suppressing reflections from either direction. The 1k probe is an example of the system where only load is matched. That means 1k resistor end is not matched, but in theory it does not have great significance since load (your 50 ohm terminated measuring equipment) absorbs the signal fully. In practice, there is some mismatch, thus there will be some ripple in the frequency response, as can be seen from my measurements.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 05:10:49 pm
content: Quote from: saturation on March 03, 2011, 01:29:16 pmno, the reading is less than 3.5, somewhere 3.3V (b.jpg)Quote from: saturation on March 03, 2011, 01:29:16 pmno, the reading is halved, somewhere 1.8V (c.jpg)the 50ohm i was making is parallel to dso, maybe thats a mistake?i tried series 50ohm to dso (d.jpg), the reading is the same as direct connection, ie no terminator (~3.3V)i tried 100Hz, assuming (or imagining) to minimize the reflection/loss/ringing effect.Quote from: saturation on March 03, 2011, 01:29:16 pmno, still the same at ~3.3VQuote from: saturation on March 03, 2011, 01:29:16 pmno with series 50ohm, still the same at ~3.3V (e.jpg). but yes with parallel 50ohm (1V). with both parallel and series 50ohm installed, its ~0.8V

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 05:19:18 pm
content: now i think series 50ohm resistor is not the way. so i stick back to signal to ground 50ohm (parallel) and do some measurement... more confusing stuff... 10MHz square (sync out) and sine (output) using 3x25 cable ...

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 05:22:43 pm
content: more confusion... the same signal (10MHz square sync out) as above using rigol probe...

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 06:20:08 pm
content: great! with rigol probe, its getting more and more complicated. now i dont know which reading to trust.10MHz square synch out from 3x25. rigol set at 1V/div. notice the termination placement (ie close to the rigol and close to the 3x25)the most notable difference is at 10X setting with 50ohm resistor termination, i dont know what the hell is happening.maybe this is called ringing (or the reflection), but it will make it different if we put the termination on the different side, i dont know, maybe i'm just wasting time here.ps: with 3x25 cable, whether the termination is close to 3x25 or rigol, the shape and amplitud is something like e&f in top part of efgh.jpg

author: jahonen
author_rank: Super Contributor
timestamp: March 03, 2011, 06:28:41 pm
content: That BNC with screw terminals looks just horribleIt is probably meant to be used for DC-stuff. Do not put it even in same room where high-frequency measurements are being doneRegards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 06:38:22 pm
content: Quote from: jahonen on March 03, 2011, 06:28:41 pmmaybe i should make proper cable connection next time (bnc to bnc), proper coax cable, soldering, crimping and stuff. instead of that quick screw in connection. sigh

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 06:52:11 pm
content: Quote from: deephaven on March 03, 2011, 01:25:57 pmi want to make bnc connection from/to my hantek 3x25 signal generator or rigol dso to/from any project that i will be doing. anykind of project, i dont know, but i'm sure it not going to be GigaHertz, just a few or ten or hundred MHz. but i want to get some rough picture/advice on high quality cable connection diy stuff.Quote from: deephaven on March 03, 2011, 01:25:57 pmi guess my low tech tinkering/test/pictures earlier proved that.

author: saturation
author_rank: Super Contributor
timestamp: March 03, 2011, 09:14:30 pm
content: Quote from: Mechatrommer on March 03, 2011, 05:19:18 pmYou're procedures are confusing to me, mecha. Some general comments.efgh.jpg: I presume you mean the images are from left to right, up then down.efghAll quoted images seem to have your vertical setting at 1V/div.e = that's what the output looks like if unterminated, ~ +3.5V. You cannot get a stable sync with that signal, but you can with f, ~ +1V.g = that's ~ 7VACpp.h = as expected, at ~ half of g, but it looks like almost 4 V, it suggests something is wrong; maybe your resistors of 2x "100" ohm in parallel is <> 50 ohms.Your reply below sounds like you've forgotten how to read graticules.Image b.jpg is ~7VACpp.Image c.jpg is correct, that is 3.5VACpp.SeriesTermination means parallel, as in the image I left for reference.If you use 100 Hz to avoid ringing, you wont' see the effects of correct impedance matching, like it was demo'd with image e and f. Its clearest when your cable is 1/10th the wavelength of the test frequency, if you use a 1 meter cable, the test frequency should be >= 35MHz. Using the square wave from the sync output port at 10 MHz, the harmonics are above that, so you can see the effect more strikingly.Enjoy.Quote from: Mechatrommer on March 03, 2011, 05:10:49 pm

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2011, 10:43:10 pm
content: Where do you live?In the UK we have UHF PAL TV which are uses 50 Ohm co-axial cable and is fine for most purposes.If I remember rightly, 75 Ohm is more common for VHF FM radio receivers.

author: deephaven
author_rank: Frequent Contributor
timestamp: March 03, 2011, 10:50:21 pm
content: Quote from: Hero999 on March 03, 2011, 10:43:10 pmIt's normally 75 ohms for UHF TV too.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 04, 2011, 12:09:23 am
content: Quote from: saturation on March 03, 2011, 09:14:30 pmyou are right on left right up down. i was annoyed when uploaded the pictures individually, vertical arrangement make me hard to trace. you are right, i tried to make it consistent at 1V/div. sorry for making your brain worked very hard.Quote from: saturation on March 03, 2011, 09:14:30 pmyou are right, maybe something wrong with my resistors setup, i was doing some dirty tinkering last night (combined the series and parallel resistors together, 4 resistor overall, 2 series 2 parallel, and short the series to get parallel termination type, its quick and dirty, the same to my lab condition right nowthanx for the stability advice.Quote from: saturation on March 03, 2011, 09:14:30 pmi dont know what happened to me again, i tend to stick to positive side (of the voltage), i've not thinking clearly, urgh!you are right again, now i have re-arrange my mind 3.5Vpp is 1.75V positive max, and -1.75V negative min. i failed at double checking stuffsQuote from: saturation on March 03, 2011, 09:14:30 pm7.jpg is my definition. after the Vpp definition correction, now it start to make sense. the parallel is the way to go (#2 in 7.jpg)Quote from: saturation on March 03, 2011, 09:14:30 pmquoted for my reference later@Hero. me in Malaysia, we use PAL format.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2011, 06:58:28 pm
content: Quote from: deephaven on March 03, 2011, 10:50:21 pmYes I was talking nonsense. TVs are 75 Ohm in the UK too.

author: Unknown
author_rank: Unknown
timestamp: March 05, 2011, 02:44:22 pm
content: Quote from: saturation on March 02, 2011, 05:04:10 pmBe careful with assumptions. I used a cable similar to the left one once, and shielded the grabber leads because noise pickup was a big deal (I think I was dealing with an amplifier with a gain of 10k, so the input signals were very low level). Bandwidth was not an issue, signals were only 1MHz or so. I found out that touching the 'coaxial' part with my hand had a significant effect on the signal: it was not shielded at all, probably just two parallel wires (didn't cut it open to verify). In the end I used a standard RG-58 BNC-BNC cable and soldered some short wires on a female BNC connector.Quote from: saturation on March 02, 2011, 05:04:10 pmI don't think 75MHz is fast enough to see minor impedance mismatches (of course you'll notice horrible things like screw terminals). The wavelength is about 3m, so you're only just below the limit for transmission line theory with a standard 1-2m cable.You would need a VNA or TDR to accurately measure them, or at least a fast step (fast compared to the size of your mismatch), otherwise it's just some lumped capacitance/inductance that will have a minor influence on impedance.Quote from: saturation on March 02, 2011, 05:04:10 pmBNC 4-11GHz? 4GHz is quite high, 11GHz is completely unrealistic IMO. At 4GHz, I would definitely use brand name connectors.Quote from: saturation on March 02, 2011, 05:04:10 pmNow you're contradicting yourself.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 05, 2011, 03:28:08 pm
content: Quote from: alm on March 05, 2011, 02:44:22 pmi believe he meant the whole setup, ie cable and connector. where the cable only capable of the said BW (4-11? GHz). however, looking back at the wiki by Wim_LQuote from:http://en.wikipedia.org/wiki/Coaxial_cableandQuote from:http://en.wikipedia.org/wiki/Coaxial_cable

author: saturation
author_rank: Super Contributor
timestamp: March 05, 2011, 04:31:27 pm
content: Hi, yes mecha@ you read me right. I agree with alm too, but we have different ways of thinking.Its more the manufacturers comments regarding the limits of BNC than mine, a lot depends on the quality of components in the cable build, to reach the upper limits of its capability. This is where you can separate the cheap connectors & cables from the better ones.See the links I put for Amphenol, it was they who suggest it can reach 11 GHz. Here's more:http://www.mwrf.com/Article/ArticleID/8993/8993.htmlLikewise, for cables some can be straight through unshielded wire, or not, or even terminate in 75 ohm BNC connectors! Caveat emptor. Again, depends on who makes them.http://www.amazon.com/Pomona-Male-Alligator-Test-Clip/dp/B0000WU364As for transmission lines, generally you start including such notions when the cable length is ~ 1/10 the wavelength, so 75 MHz ~ 1.3 foot. shooting a 10 MHz square wave puts the harmonics in this park. But it can happen sooner for more reasons related to impedance mismatch, as the Hantek sync signal clearly shows, QED.

author: ArtemisGoldfish
author_rank: Regular Contributor
timestamp: March 11, 2011, 06:42:49 pm
content: Quote from: Mechatrommer on March 03, 2011, 06:20:08 pmMaybe a silly question, but you have compensated your probes, right?

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 13, 2011, 03:33:50 pm
content: looking for coaxial cable, found in chinese ebayhttp://cgi.ebay.com.my/RF-Coaxial-cable-M17-119-RG174-50-feet-/300426266059?pt=UK_Computing_Networking_SM&hash=item45f2cd05cbthere is RG400 which is N/A in wiki, from the spec given by the seller, this RG400 is good to 12.4GHz 2x braid outer diameter 4.95mm. seems a good choice for my 3G antenna cable and for measurement with my BNC connector, any opinion if i should buy this instead of RG174 or RG58? is it flexible instead of semi rigid?ps: i locally bought 75ohm satellite cable at very cheap price for experimentatin later, but would like to get the best for the money and performance cable. so thinking between rg174, rg58 and rg400. there is higher freq cable, but that is semi rigid and i think it will not very comfortable to work with if longer length cable is needed.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 13, 2011, 04:00:32 pm
content: and i read about TNC connector, and RP TNC. is TNC can be used to connect to Rigol scope? (BNC compatible?). and why there is RP (reverse polarity) TNC? i was looking for my usb broadband antenna connector, there is no TNC-CRC9, just RP-TNC-CRC9http://cgi.ebay.com.my/RP-TNC-Plug-CRC9-pigtail-antenna-3G-huawei-PCMCIA-/290543854094?pt=Camera_Cables_Cords&hash=item43a5c3620e

author: jahonen
author_rank: Super Contributor
timestamp: March 13, 2011, 04:29:58 pm
content: I made an experiment how well plain wire connection will work under impedance matched conditionsAs you can see, the result is all over the place. What that measurement does not show, is the sensitivity of this bogus connection to even slightest variations of geometry. That means if you move the wires, the frequencies of those deep notches will shift frequency.I calibrated the setup using a SMA-SMA-extension as a reference. Result is shown in last picture.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 13, 2011, 04:46:30 pm
content: thanx janne for the precious effort. even though i dont have a clear clue on how to do the precise setup, but from the graph, it proved the inconsistency of loose wires. i'm getting a better cable. maybe i misunderstood the RP-TNC version is actually somekind of female (in male) connection. instead of normal male connector. i'm also eyeing on SMA connectors, as my CRC9 only has "to SMA" adapter. i was playing with rf antenna which i was not aware that its actually using sma connector, i didnt know what i was doing. still, can i assume that TNC "plug" is compatible with BNC "jack"? with better performance? TNC is up to 11GHz where BNC is only 3GHz according to wiki.

author: jahonen
author_rank: Super Contributor
timestamp: March 13, 2011, 05:36:44 pm
content: The setup was quite simple, tracking generator output of a spectrum analyzer using a semi-rigid coax to "DUT", and then from "DUT" to the RF input of the same spectrum analyzer using second coaxial cable. Spectrum analyzer and tracking generator can be calibrated to null out any irregularity of test setup, which I did, using a short SMA female-female extension.As for your question about TNC compatibility, TNC is only compatible with TNC connectors, although one can always get adapters.Regards,Janne

author: Unknown
author_rank: Unknown
timestamp: March 13, 2011, 05:39:15 pm
content: Quote from: Mechatrommer on March 13, 2011, 04:00:32 pmOnly with an appropriate adapter, just like any other connector like SMA and N. It's not compatible with BNC, although they're related.Quote from: Mechatrommer on March 13, 2011, 04:00:32 pmThey're required to use a 'non-standard' connector so the consumer can't mess with the antenna, reverse polarity connectors are not compatible with standard connectors. The result of the widespread use of RP connectors for wifi is that some distributors have more RP-SMA than SMA connectors.Quote from: jahonen on March 13, 2011, 04:29:58 pmLooks horrible, as expected. Thanks for doing the actual test. Performance is fairly consistent with the expected lambda / 10 rule-of-thumb (looks OK-ish to 100MHz or so).

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 13, 2011, 07:33:07 pm
content: ok thanx guys, i will not into trouble with tnc.i made the 75ohm cable, this is the best i got:a.jpg = the cable+connector, soldered 50ohm terminator and male-maleb.jpg = measurement of Hantek 3x25 10MHz synch out (top=original cable, bottom=my 75ohm cable, unterminated and terminated).c.jpg = while play around, noise/harmonics/distortion is picked up in rigol FFT on 5MHz sine function (the same with my and hantek cable), OT.from b.jpg, maybe the capacitance of my newly made cable is higher than original cable as the higher frequency is rejected as in (left side, unterminated) picture, but i'm not 100% sure.

author: saturation
author_rank: Super Contributor
timestamp: March 14, 2011, 05:25:45 pm
content: This looks great. Your 75 ohm cable empirically has a better response than the factory 50 ohm cable. But as you can see the termination gets the best frequency response, given the rising edges of your square wave is still better when terminated. Your cable construction looks great! I am much more sloppy, sorry to say.I noticed those harmonics on your 5 MHz, the top of the sine wave seems clipped for some reason. I noticed sometimes the Hantek needs a warm up period, believe it or not. If you turn it on, and just type in a MHz frequency, sometimes it has more distortion. But if you start with the default, 1kHz, let it output, then slowly ramp up after each waveform stabilizes before the next increase: 10kHz, 100kHz, 1MHz, then 5 MHz, it produces the best response. What I do is just type a zero and enter to move up the frequency, then 5 to overwrite the 1 to make 5 MHz. Not sure why it does this, but it only happens when its started from a cold start, if you let it warm up for at least 10 minutes, you can do as you please!Quote from: Mechatrommer on March 13, 2011, 07:33:07 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 14, 2011, 06:02:45 pm
content: better response? hmm, if you say so. i'm not capable of proving that, but thanx. yes, not much difference in terminated cable (which is the way when measuring or real application i think). the trick in sloppiness is in the black rubber, and i'm not intending to show whats insidethanx for the tip for stable output saturation. Cheers

author: saturation
author_rank: Super Contributor
timestamp: March 14, 2011, 06:09:55 pm
content: NICE! Thanks for taking time to do this. You have top quality connectors, they look gold plated! One could stabilize the response by twisting the wire to emulated twisted pair, this should put a more repeatable fixed inductance and capacitance between conductors. Would be interesting to see what kind of response you get. After getting one set of readings say with 5 twists for that cable length, twist the wires more and double the twists/inch, this should improve the response even more.Quote from: jahonen on March 13, 2011, 04:29:58 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 14, 2011, 06:29:47 pm
content: i guess thats the ideaMr Oliver Heavisidecame up with 1.3 centuries ago.

author: jahonen
author_rank: Super Contributor
timestamp: March 14, 2011, 07:38:55 pm
content: Quote from: saturation on March 14, 2011, 06:09:55 pmThat was also suggested elsewhere, maybe I'll try that in near future. Also, I thought I'll also try similar setup but with RG-174, that should be considerably better. BTW, I used a SMA torque wrench (0.50 Nm) to tighten the connectors, important thing to get consistent readingsAt GHz frequencies, even slightest sloppiness at connectors or cables, will show up as a measurement error.Mechatrommer: Your 75 ohm coax is very short for any reflections to show up for those rise times, try something like 5-10 meters, to amplify the effect. Cable capacitance has nothing to do with frequency response, only thing it affects is the signal velocity in the cable. It is sometimes difficult for people to refrain from substituting a lumped LC-filter in place of the cable, this of course leads to completely wrong results, a transmission line is not a low-pass filter! In real cable (or in any transmission line), L's and C's are distributed evenly along the whole length, so if one wants to replace the cable with L's and C's, it must be done with large number of small L-C-sections.Oh, I forgot to mention that one can use a TDR technique to determine the cable impedance, as mentioned by others. If you have reasonable length of open-ended coax (or whatever transmission line structure) and feed a fast rise time step of U0 to that from impedance Zs, then what you'll see right at the feed end of the coax is a step function, which has initial height of U=Z0/(Z0+Zs)*U0, i.e. if source impedance is equal to the coax impedance, the initial step will be at half of the final height (it is left as an exercise to reader to solve the expression for Z0). Cable is basically resistance equal to the nominal impedance from source point of view. The voltage will eventually rise to its full height, after the reflected wave from the open end travels back to the source. Of course, this measurement technique requires that you have relatively non-intrusive way to measure the signal at source without significant loading.Regards,Janne

author: saturation
author_rank: Super Contributor
timestamp: March 15, 2011, 12:13:46 am
content: Hi jahonen,I hope you do decide to test with a twistYou can just take both ends of the cable with the connectors mounted and simply twist the whole assembly; the geometry will be more stable that way and it could make a difference. As you tighten the twists, the geometry will be even more 'compacted' thus stabilizing the conductors position together and thus, the capacitance, at the very least.Here's an actual measured test of RG-174 vs RG58, seems like its feasible for short cable lengths to make more ergonomic lab cables, given the measured values of these folks. I think their actual data is close to spec sheet values.http://www.dxing.info/equipment/rg_174_coax_bryant.dxAs for the torque wrench, I wouldn't doubt it, how did you know to torque it to 0.5Nm? I found a good supply of Amphenol Connex BNC splitters for the same price of no-name Chinese BNC splitters, and oh my, even at MHz speeds, I can see the difference, albeit its small, its clear approaching GHz speeds, the Amphenol craftmanship has the better chance of getting the proper result. It would seem for best results too, you'd need to confirm the cables Zo unless its a good brand premade cable from Pomona, Tek, Agilent or such.The Zo technique is nifty, its familiar but seems buried in my memory somewhere, I'll give measuring Zo a whirl. See the link above, they measured it for each frequency too, but a step function puts all those sines together for a good square dance

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 15, 2011, 01:29:39 am
content: thanx for the link, will get back to it later.

author: jahonen
author_rank: Super Contributor
timestamp: March 15, 2011, 12:34:09 pm
content: I tested the twisting, no dramatic effect on the response. It seems that the geometry is still simply too irregular for >700 MHz (quite obviously). I think that even below that, the impedance is probably all over the place.The torque wrench I used is actually Huber&Suhner 74_Z_0-0-79, 0.45 Nm (remembered wrong), which is "SMA economic, standard style" according to H&S. I'm not sure if the actual tightening torque is absolutely critical as long it is consistent. It seems to vary quite a bit from manufacturer to another (even H&S makes three kinds of SMA torque wrenches, 0.45, 1.00 and 1.95 Nm), and also which kind the SMA connector is. Greatest advantage is IMO that it is easy to tighten the connectors enough without having a fear about damaging the threads with regular spanner (0.45 Nm is really low torque).Regards,Janne

author: saturation
author_rank: Super Contributor
timestamp: March 15, 2011, 02:21:35 pm
content: Nice, thanks a bunch for giving a whirl, jahonen. The change in geometry did improve the response, when looking at the curves side by side, certainly not the level of a proper cable.Its a great demo of the effect of the types of cabling, wires and construction, on GHz transmission.Will keep a look out for those wrenches.Quote from: jahonen on March 15, 2011, 12:34:09 pm

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 15, 2011, 03:49:43 pm
content: so how about the correct coaxial cable? or even the 75ohm one? saturation's link only show test up to 20MHz. maybe there are lot of info on the net, but i prefer the one that is done with the same setup same person to better comparebut i wont ask too much. i thankful for what i already got.btw. how actually this test is conducted? i'm assuming the device will give a signal at certain frequency and then check the attenuation on the output, and then increase the frequency bit by bit, check the attentuation and so on until we get the nice graph? is that what is called "frequency sweep"?

author: jahonen
author_rank: Super Contributor
timestamp: March 15, 2011, 06:25:49 pm
content: Next, I'll try these. It was quite difficult to spread the braid to four separate pigtails, but I think that the result was acceptable. Length is about the same than in one with loose wires.Second one is just a SMA-SMA cable (RG-174) which I made a while ago. I expect that to be reasonably good.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 15, 2011, 06:57:53 pm
content: great! cant wait for that!ps: wondering... whats the difference it will make if that SMA-SMA cable is tested winded like in the picture, compare to if its run straight... hmmm

author: jahonen
author_rank: Super Contributor
timestamp: March 16, 2011, 08:07:38 am
content: And here are the results, first one is with RG-174 + SMA PCB connectors, and second is for RG-174 coax + proper coax connectors. FYI, the cable length is about 1 meter.It seems that PCB connectors connected this way, can be used to about 2 GHz before loss happens, maybe more. I'm not still sure about reflections, the VNA is not currently around so I can't verify that.And for your question about how the coax is installed, it does not matter if the cable is coiled or straight, as long as the geometry inside the cable is not distorted. This is because there is no field outside the cable.Regards,Janne

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 16, 2011, 10:07:19 am
content: thanx alot janne. its worth a thousand words. you basically concluded the original thread topic, empirically. thanx alot and i mean it.

author: saturation
author_rank: Super Contributor
timestamp: March 16, 2011, 11:59:48 pm
content: I second mecha's thanks, jahonen. The series of photos just nails how good the proper cabling can be.Reading on SMA connectors, wikipedia says they have a rated life of about 500 actuations, do you think this is true and connect-disconnect cycles wears the connectors enough to be practically noticeable? [ not so much mechanically, but enough to affect its GHz response].

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 17, 2011, 03:40:42 am
content: looking aroundhttp://www.amphenolconnex.com/SearchResults.asp?ProductID=189ordering is a bit confusing. and they seem ship to US and Canada only. so i'm not in the list

author: saturation
author_rank: Super Contributor
timestamp: March 17, 2011, 02:07:33 pm
content: Yes, mecha, its only good in North America. Its probably just a distributor relationship. Search google for Amphenol or a similar well known competitor for connectors, Asia or Malaysia.http://www.interfacebus.com/connectors_coax.htmlWhen I'm not sure about components, I just buy one or two pieces and test it myself. When I'm satisfied with the quality then I buy again. That's why its good to have a lab with a capability to push your working tools to its highest limits. At work, we buy only through distributor channels and all the parts are premium price, but 99% they work very well.Quote from: Mechatrommer on March 17, 2011, 03:40:42 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 06, 2011, 03:30:48 pm
content: so i got my lego set from chinarf ebay seller. among most important item are crc9 connector for my broadband usb modem, bnc plug, rg174 cable, and 50ohm termination (which is sux... below) and other needed adapters + sma's. early conclusion i think that the chinarf is know shit about rf, or dont know what he's selling, either way he knows shit about what he did to me.annoying thing... i ordered 2x 50ohm termination. what i got is one 75ohm and one is 0ohm termination (yes zero ohm!), as arrowed in the 1st pic. other stuff seem fine. except i'm start to question the spec reliability since the terminator is not a good news. if you spot anything peculiar in the pictures pls warn me.second thing is, i thought the bnc plug crimp is with detached centre pin to be crimped to center wire, but what i got is the center pin is sticked to the plug with only with protruding copper (conn.jpg) not sure whether to crimp the cable to it or solder. that was not so obvious in the ebay listing picture, but i guess my mistake for not checking carefully. and rechecking, he inded doesnt sell any such detached center pin for rg174 cable, only for rg58.last picture is dds signal with diy 50ohm termination with original dds 50ohm cable (ch1) (3.5Vpp from dds signal) and chinarf's 75 ohm (75.74 to be Uni-T's exact) with diy 75ohm cable ch2 (synch out). look jaggy, not sure why, maybe dds instability. any explanation?ok most important question, pls help:1) having read how to connect cable and connector together from earlier link, i think the idea is to shield centre wire as much as possible with metal shield (ferrule?). and for stronger connection, can i solder the arrowed in conn.jpg? can i solder the centre wire to the centre pin?2) any possibility repairing the 0ohm terminator? seem hard to pull out, it is a solid metal.i hope to hear some advice before i put these legos together.here again saturation's link for my reference...http://www.dxing.info/equipment/rg_174_coax_bryant.dx

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 06, 2011, 04:17:51 pm
content: so this is how it looks like inside the RG174 cable. the centre wire is many small copper wires twisted together, not one single solid copper like my earlier 75ohm white cable. and its diameter is a bit smaller compared to rigol probe wire (2.77mm vs 3.01mm)

author: nyo
author_rank: Contributor
timestamp: April 06, 2011, 08:28:09 pm
content: The bnc with 2 screw terminal you got is a balum, it has an internal resistance already.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 07, 2011, 02:51:58 am
content: Quote from: nyo on April 06, 2011, 08:28:09 pmi measured it has no resistance. just a plain BNC to "2 wires" adapter.here is another pictures the chinarf asked for verification. she seem not to understand the problem. "she" stated she will forward it to "her" store dept.

author: joelby
author_rank: Frequent Contributor
timestamp: April 07, 2011, 04:17:42 am
content: The 0R terminator might not be a broken 50/75R terminator - sometimes you do actually want a short circuit (e.g. for calibrating a network analyser).It does seem like they've mixed up their parts thoughCan you buy them locally and test them in store? Jaycar sell 50R terminators here - they're not too uncommon.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 07, 2011, 06:53:40 am
content: Quote from: joelby on April 07, 2011, 04:17:42 amthanx. good to know there is 0R stuff, but still, that is not what i want. i dont want 0, i dont want 75, i want 50 ohms. i dont have vna, so 0 ohm terminator is useless to meQuote from: joelby on April 07, 2011, 04:17:42 amour local store only sell what most people need here, ie 75ohm cable for tv. even the bnc connector they dont have. list all the 50ohm coax cable in wiki, not a single one they have. do they care a single hobbiest like me? i think notthe seller replied back saying to resend the terminator with normal shipping, they cannot afford express like i paid before, so hope it gets here, and another two weeks to wait, sux! they have excel and fast comm/support though and i deal directly to them, not through ebay since they need immediate payment once "buy it now" button is clicked, which is also sux, so i comm direct to them, seems they are honest seller. but as you said, maybe they got mixed up, so i forwarded any good advice here to them, ie separating 50 & 75ohm parts. hope they listen, for their own good.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 07, 2011, 06:41:19 pm
content: completed my 1st BNC-BNC 50ohm coax. let me go back to previous test... hantek cable (top), diy 75ohm (bottom) ...and compare with the same 10MHz signal, latest test for comparison pictured below.