Questions about BNC Stuff! - Page 1
Forum section: electronics / beginners

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 21, 2013, 01:12:26 am
content: Hello everyone!I've been hunting around on ebay for things I've needed for a while and I'm going to buy them after they are approved by you guys!I mainly need to know if this stuff will work nicely to hook up a frequency generator to an oscilloscope and any other combinations of stuff that I want.http://www.ebay.com/itm/180430746150?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649http://www.ebay.com/itm/121147936959?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649http://www.ebay.com/itm/271178318906?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649http://www.ebay.com/itm/270634170197?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649http://www.ebay.com/itm/K1BO-BNC-Male-to-Dual-Binding-Posts-Banana-Connector-Plug-Test-Adapter-/251279907115?pt=US_Audio_Cables_Adapters&hash=item3a8173192bI will be purchasing a few of each most likely.Thanks for all of your help.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 01:21:46 am
content: Those T connectors are shit. I ordered four from that same seller, and received one working and three with broken internal connections. The working one also no longer works. Get yourself a few ofthese. (No, the whole body is not gold-plated, it's a photo effect.) They cost less than double, which still works out less when you consider your chances of the shitty ones working.RG-59 cable is 75 ohm, you need 50. Also, there are a lot of crappy coax cables floating around eBay... be careful.The probes are decent.Those cheap terminators work. Don't expect them to work up to Holy Shit GHz... but they work.I find BNC/banana adapters much more useful in the opposite gender - male banana to female BNC.

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 01:26:59 am
content: Quote from: c4757p on August 21, 2013, 01:21:46 amReally? I know the build quality is crap and I wouldn't trust them beyond moderate frequencies, but I never suspected their technology was actually sufficiently advanced so they wouldn't even work at DC.Quote from: c4757p on August 21, 2013, 01:21:46 amDepends on what's on the other side. The male banana to female BNC is nice if you want to connect say a signal source to a DMM, although not all DMMs obey the standard spacing. The other kind is useful if you want to connect alligator clips to your function gen, for example. Only for low frequency work, obviously.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 01:31:14 am
content: Quote from: alm on August 21, 2013, 01:26:59 amYep..... completely open circuit between the male and the two females. (I think the females were still joined). In fact.... they actually worked better athigherfrequencies due to the capacitive coupling of the presumably tiny break in the conductor.QuoteThat reminds me - you can find somewhat crappy BNC-alligator cables all over. I find these very useful for low frequency work.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 21, 2013, 01:38:45 am
content: Quote from: c4757p on August 21, 2013, 01:21:46 amAh, good to know they are junk. Perhaps I'll get some on mouser like you linked if shipping is cheap, otherwise I'll just find others on eBay for a bit more $.Good thing I asked about the cable before buying it I guess.I actually ended up getting probes for only $8 on a bid but they are almost the same as the ones I listed so they should also be okay.Good, I only have a 100MHz scope and can't imagine even fully using that.I'll probably use those adapters for the binding post function to just stick some wires on more than anything, otherwise I still find it more useful to have the female on it because I have a few cables I made go from male banana jacks to alligator clips.Thanks for your help!

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 01:42:59 am
content: I've got some junk 75 ohm coax lying around that I stuck a couple spare connectors on (a couple BNC-BNC and a couple BNC-banana). Itisuseful for low-ish frequency (I'd say under 10 MHzhighest interesting frequency- including harmonics of square waves) and any time you wouldn't bother terminating it anyway.I have a handful of those cheap terminators and haven't been able to exceed their abilities on a 150 MHz scope, that required the 400 MHz one

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 01:49:32 am
content: Quote from: c4757p on August 21, 2013, 01:31:14 amThat also works. An alternative that I like is female BNC connectors with short pig tails with banana plugs, alligator clips or grabber hooks. Pomona sells those (eg.Pomona 3788). The advantage is that you can stick them on the end of any BNC cable, making a modular system. This is much more convenient than having several long cables of BNC to other stuff, in my opinion. Easier to store and less likely to get tangled.Quote from: c4757p on August 21, 2013, 01:42:59 amA good BNC connector is supposed to be good to 3 GHz or so, so 150 MHz should be close to DC for those terminators.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 01:53:35 am
content: Quote from: alm on August 21, 2013, 01:49:32 amYou underestimate the crappiness of cheap eBay crap.

author: EpicIntelGamer
author_rank: Regular Contributor
timestamp: August 21, 2013, 01:55:54 am
content: Quote from: c4757p on August 21, 2013, 01:53:35 amI agree, it's far too easy to underestimate how horrible quality stuff you can run into sometimes.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 01:59:44 am
content: After some of the shit I've seen, I would not expect a cheap terminator to perform better than a bottom-of-the-barrel 47 ohm 1/4W axial resistor shoved up the ass of a BNC, until proved otherwise.Hell.... I wouldn't be really all that surprised to see precisely that if I cut one open...

author: edpalmer42
author_rank: Super Contributor
timestamp: August 21, 2013, 02:14:30 am
content: Quote from: EpicIntelGamer on August 21, 2013, 01:55:54 amIt's worth remembering that the biggest use for 50 ohm terminators was for 10Base2 (Thinnet) Ethernet cables at the blazing speed of 10 Mbps. I suspect that all the cheap terminators sold now are designed to similar specs. If you want better quality, you'll have to step up to the name brands.I bought a bunch of cheap terminators and made some rather crude measurements of return loss at different frequencies. At 100 MHz, they were all > 30db. At 1 GHz they varied from maybe 8db to 18db.Ed

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 02:28:19 am
content: Quote from: c4757p on August 21, 2013, 01:53:35 amI don't expect GHz, or even 100 MHz, performance from the eBay BNC hardware, but I don't consider decent performance at 150 MHz particularly impressive either.Quote from: c4757p on August 21, 2013, 01:53:35 amNeither would I. But I'm not sure if the VSWR of a terminator with a 47 Ohm carbon film resistor inside a male BNC connector at 150 MHz would actually be that horrible.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 02:30:38 am
content: True...

author: edpalmer42
author_rank: Super Contributor
timestamp: August 21, 2013, 03:37:54 am
content: Quote from: c4757p on August 21, 2013, 01:59:44 amActually, that would be just about perfect - if the resistor was carbon composition. Patent #2891223 talks about making precision attenuators by grinding off carbon composition resistors to raise their value to get perfect values of attenuation and VSWR. It might also work for terminations. I got the patent number off of a Texscan attenuator that's rated for 1 GHz.Ed

author: G7PSK
author_rank: Super Contributor
timestamp: August 21, 2013, 07:32:20 am
content: Those Tee connectors are 75 ohms. The giveaway is the for video in the description they are aimed at the CCTV market.What I do for cables is buy BNC's and cable separately and make up my own cables to the required length (very often tuned length) I also made my own 50 ohm terminators as well, that way you can have small neat ones using SMD resistors inside the BNC and nice big ones for dummy loads etc al at a fraction of the price of commercial units.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2013, 07:51:57 am
content: Quote from: alm on August 21, 2013, 02:28:19 amI've pulled a 75 OhmTektronixthrough termination apart & found two parallel 150 Ohm axial resistors!

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 09:46:41 am
content: Quote from: edpalmer42 on August 21, 2013, 03:37:54 amThat may have been a good idea in 1954 when your options were wire wound or carbon comp, but these days film resistors provide superior characteristics.Quote from: G7PSK on August 21, 2013, 07:32:20 amLook at the side pictures. There is insulation around the conductors, looks like 50 Ohm to me. Or at least an attempt to get close. Compare to the pictures onthis page. And CCTV is sufficiently low frequency that almost any connector that works at DC is going to work.Quote from: vk6zgo on August 21, 2013, 07:51:57 amThey probably figured that the TV/video people don't care about impedance matching anyhow, based on the great connectors they use (Belling-Lee, RCA) :p. I believe 75 Ohm BNC connectors are rated for significantly lower frequencies (somewhere under 2 GHz?) than 50 Ohm connectors, even with good hardware.If I remember, at least for 50 Ohm, Tek had three different through terminators: a normal one good to 1 GHz, a 'high' power one good to 100 MHz or so, and a very accurate one designed for DC and audio frequencies.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 12:29:19 pm
content: Quote from: alm on August 21, 2013, 09:46:41 amEven up to 1 GHz? Do you have to use special ones without the spiral track? (Do those even exist?) Seems to me that inductance might be significant at 1 GHz.I might actually try that - I've got a bunch of BNC connectors and probably have some carbon comps lying around...

author: madires
author_rank: Super Contributor
timestamp: August 21, 2013, 12:47:07 pm
content: Quote from: c4757p on August 21, 2013, 12:29:19 pmAFAIK there are some with a bifilar track to reduce inductance.

author: c4757p
author_rank: Super Contributor
timestamp: August 21, 2013, 12:54:56 pm
content: Ah yes... of course

author: Unknown
author_rank: Unknown
timestamp: August 21, 2013, 01:13:41 pm
content: See the book RF circuit design by Christopher Bowick. A quote from page 2:QuoteThe book then goes on to discuss how wire wound resistors are inductive at moderate frequencies (doesn't discuss Ayrton-Perry winding), metal film is much better than both, having significantly reduced parasitic inductance and capacitance, and how thin film chip resistors are a major improvement over metal film. If you look at the plots, carbon composition becomes capacitive at about 5 MHz, a 5 Ohm metal film resistor becomes inductive at about 200 MHz, a 100 Ohm metal film resistor is resistive up to about 500 MHz (they didn't measure at higher frequencies), 1 kOhm becomes capacitive at 200 MHz and 1 MOhm becomes capacitive at about 10 MHz. No idea how much effect the construction of the carbon comp has on this, but I certainly wouldn't assume any carbon comp to give good results.The bookAnalog SEEKrets by Leslie Green(local EEVblog mirror of freely available PDF copy) also discusses resistors in chapter 4. He describes carbon composition as having poor behavior above a few tens of megahertz due to skin effect. He mentions that low-value (< 10 Ohm) wirewounds are inductive above 30 kHz but that above 1 kOhm shunt capacitance (which is increased by Ayrton-Perry winding) becomes a problem. He considers film resistors far superior, although describes how MELF (and obviously also normal leaded resistors) have some inductance due to the groove, so he describes an alternate cut that is supposed to perform well up to 10 GHz. Chip resistors can also perform well into the tens of GHz.

author: G7PSK
author_rank: Super Contributor
timestamp: August 21, 2013, 03:30:44 pm
content: I have some of the cheap BNC from ebay and although the plastic goes to the edge it is thinner than a proper 50ohm BNC. There appears to be BNC manufactured for the CCTV industry that are rated for 75ohms but have a similar appearance to 50ohm ones The one I have will only take RG 59 cable with the thin core wire. The problem for me is I have over a 100 of them that I got from ebay some time ago.

author: Marco
author_rank: Super Contributor
timestamp: August 21, 2013, 03:35:52 pm
content: Don't the really fancy terminators use circular resistors?

author: vk6zgo
author_rank: Super Contributor
timestamp: August 22, 2013, 12:48:39 pm
content: Quote from: alm on August 21, 2013, 09:46:41 amIn older establishments,where terminations are of uncertain age or specs,the terminations I saw may have been of an age with the Tek 545B where they would need to be OK to around 36MHz.Tek through terminations do have a problem of becoming unscrewed with age,& I have seen several matching my description,but never chased up the part number & hence specs.