Instrument Interconnection Cables - Page 1
Forum section: electronics / beginners

author: DJPhil
author_rank: Frequent Contributor
timestamp: June 06, 2010, 07:31:04 pm
content: Where do you guys get your BNC cables, tees, and terminators from?50ohm RG58 seems appropriate for most low frequency (<50MHz) analog work where rise time isn't much of an issue. Is this correct?I've shopped around a bit and it seems that 50 ohm RG58 is not often available in lengths shorter than three feet or so. Ideally I'd like to pick up a half dozen or so short lengths of cable for connecting scope, various signal generators, frequency counters, and the like. To do all that with three foot cables would make an awful mess.I won't often need to make my own cables, so buying a crimper and raw materials seems like it'd cost more than it's worth.What do you guys think?Edit: Changed RC to RG, brainfart from typing before coffee.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: June 06, 2010, 08:16:33 pm
content: I am aways making my own cables , BNC, Type N, PL .. what ever .And actually I am confused, what is the question ?Get 50 meters of cable , and build what ever you like .. !!

author: Unknown
author_rank: Unknown
timestamp: June 06, 2010, 08:31:48 pm
content: I assume you mean RG-58. RG-58 is basically the standard/cheapest common 50 ohm coax cable, and perfectly fine up to at least 1GHz or so (probably more), especially for short runs. For low frequency (although possibly not 50MHz), you probably don't even have to pay attention to transmission lines or termination, and could just use any shielded cable. Pomona makes short cables (part no. 2249-C-<length in inch>, eg. 2249-C-12). They are quite expensive (same price as their longer cables), sold by most distributors like Digikey and Mouser. RG-174 is another popular type of coax, it's thinner and more flexible than RG-58, but with more loss. Probably perfectly fine for most low frequency work, but usually more expensive and more fragile.For higher frequencies or longer cables (if the cable is longer than about one tenth of the wavelength), you have to match the impedance of the source, cable and load. Most sources in the lab are 50 ohms, so that means you should use 50 ohm cables. Most loads are either 50ohm or high impedance (eg. low bandwidth scopes). For high impedance loads without internal 50 ohm termination, it's best to use a feed-through terminator, since this minimizes the parallel capacitance, but for low frequencies, a tee plus regular terminator works, too. The main exceptions are audio (600ohm) and video (75ohm), but in both cases the frequencies are usually low enough that you can safely ignore this.I get my terminators from ebay, since I often get better quality (eg. AMP, King) for the same price. Not a lot can go wrong except being destroyed by too much power. Tees are cheap enough at the regular distributors, so I don't bother with trying to find alternative sources.

author: tecman
author_rank: Frequent Contributor
timestamp: June 06, 2010, 08:54:52 pm
content: Go to Jamecowww.jameco.comGood selection, low prices and good servicePaul

author: charliex
author_rank: Frequent Contributor
timestamp: June 06, 2010, 09:20:41 pm
content: www.allelectronics.comhave lots of pulls/cheap which incidently i just came back from their store with a bag of junk

author: DJPhil
author_rank: Frequent Contributor
timestamp: June 07, 2010, 09:03:05 am
content: Thanks for the input guys.I'll look into jameco (cheap power entry modules!), and definitely allelectronics (seems like they've got all sorts of interesting stuff).I appreciate the advice, it helps me confirm that I'm not walking into any big gotchas and that I know the limitations.I might take my chances with some bargain crimpers ($30US or so) and see if it's a similar situation to the RJ-45 crimpers I picked up way back when. They'll work fine for occasional, careful use (which is fine by me), but they aren't built or priced for the professional.