Best coax test leads? - Page 1
Forum section: electronics / testgear

author: TohKah
author_rank: Newbie
timestamp: February 18, 2016, 04:38:44 pm
content: Hello all,I'm a bit unclear of what would be best for my situation. I need a 3' male BNC to male BNC, a 3' male BNC to banana plugs, and a 3' male BNC to alligator clips. Not to hard to find BUT I need the best in keeping out external noise as these will be hooked to multiple highly sensitive to noise test equipment pieces (one being a pico-ammeter) in a relatively electrically noisy environment in a research physics lab. Picoammeter had a BNC to alligator clip test lead that if you twisted it would generate a current and cause the meter to go haywire up into the mA range amazingly enough. I found an old RG59 cable and soldered clips on it and appears to stay relatively steady in the nA range which is acceptable. So whats out there for sensitive stuff?I then just need some basic male BNC to male BNC for classroom use by students - hooking basic oscilloscopes to function generators etc. Those I figure basic ol RG59 cable and ends would work (monoprice has 3' for $1.60 each which is great because i need 24 of em!) I also need 12 basic 4' male BNC to bananas for the same environment.I've seen the stuff on ebay but unsure of the quality for the classroom stuff - doesn't look all that sturdy like they just soldered basic 2 wire conductor cable to a BNC plug - no shielding.Any suggestions?Thanks in advance...

author: tooki
author_rank: Super Contributor
timestamp: February 18, 2016, 04:59:28 pm
content: Pomona makes lovely BNC-coax-minigrabber test leads. I got a bunch from this vendor, who sells new old stock for super cheap.http://stores.ebay.com/resourcesunltd4u/

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: February 18, 2016, 06:25:51 pm
content: Buy Belden RG58 cables - these are about the only coax cables that can be used as test leads because they are soft. You can always cut off BNC connector on one end and add banana lead.

author: uncle_bob
author_rank: Unknown
timestamp: February 18, 2016, 06:36:32 pm
content: HiOk, here's the gotcha:Coax is designed for RF shielding. It is not the ideal thing for low frequencies. It gets used there, but things like twisted pair are a better way to move around low noise signals at low frequencies. As you try to make coax work below a few hundred KHz, you do weird stuff that is not at all compatible with a test lead.So ... that said, go with something with a Teflon dielectric and a small cable diameter. If that's a bit expensive, drop back to RG-174. For a test lead at low frequencies, RF loss does not matter. A nice flexible lead is always a good thing.Bob

author: TohKah
author_rank: Newbie
timestamp: February 18, 2016, 06:44:15 pm
content: hmm interesting, i may have to do some digging and create some twisted pair trial cables to see how they do

author: edavid
author_rank: Super Contributor
timestamp: February 18, 2016, 06:48:28 pm
content: Quote from: TohKah on February 18, 2016, 04:38:44 pmFirst of all, youdon'twant RG59 for test equipment... it's 75 ohm, and most of what you find is not very well shielded or constructed, since it's just intended for video use.For ordinary lab use, Pomona 50ohm coax cables are good, but they just have ordinary shielding. (Of course if you are using banana plugs or alligator clips, the cable shielding is not really an issue.)For critical picoammeter measurements, you need low triboelectric cable, which is expensive. Keithley is one source.For your student lab, you could get the RG58 (not RG59!) cables from Monoprice, but they are quite a bit stiffer than Pomona, and the BNCs don't attach as easily.Not sure about the BNC to bananas... most people use BNC-BNC with a Pomona BNC-banana adapter.

author: MSO
author_rank: Contributor
timestamp: February 19, 2016, 02:54:15 am
content: National Instruments says very nice things about Belden 83317 shielded twisted pair for use with their LCR meters. It runs about US$4.00 per foot, but I was unable to locate a seller who sell less than 100 foot minimum.It might be a bit stiff (I couldn't justify buying 100 feet) for use as test leads. It has minimum bend radius of 1.25 inches.National Instruments:http://www.ni.com/tutorial/3078/en/#toc2Belden 83317 (PDF):http://www.belden.com/techdatas/english/83317E.pdf

author: HAL-42b
author_rank: Frequent Contributor
timestamp: February 19, 2016, 03:20:40 am
content: SATA cables are shielded differential pair 100 ohms. I wonder how they would fare as test leads. Opinions?

author: Performa01
author_rank: Super Contributor
timestamp: February 19, 2016, 08:40:22 am
content: Quote from: HAL-42b on February 19, 2016, 03:20:40 amFor lab gear we need 50 ohms unbalanced with BNC connectors (N or SMA for higher frequencies). For most shielded differential pair cables, the characteristic impedance between inner wire(s) and shield is not well defined and nowhere near 50 ohms (25...40 ohms for SATA). Even if it were, it might be difficult to fit a BNC(m) plug on such a cable.

author: TohKah
author_rank: Newbie
timestamp: February 19, 2016, 02:13:38 pm
content: Quote from: edavid on February 18, 2016, 06:48:28 pmalright, yea for what the students are doing/learning about the rg58 sounds perfectly fine. durability is key as they will get handled quite a bit by inexperienced hands.i found these for a great price and appear to be goodhttps://www.cablewholesale.com/products/coaxial-cable/rg58-coax-cables/product-10x1-01103.phponly $40 for 24 of them which isnt bad at all since were on a bit of a budget.

author: TiN
author_rank: Super Contributor
timestamp: February 19, 2016, 02:43:33 pm
content: If you into picoampere zone, you need low triboelectric triax cable. There are affordable options for cable, even on ebay, but connectors and adapters cost significant money.

author: TohKah
author_rank: Newbie
timestamp: February 19, 2016, 02:46:55 pm
content: Quote from: TiN on February 19, 2016, 02:43:33 pmYea mostly measure nA but might as well get the best you can. This is for a Keithly meter.

author: alanambrose
author_rank: Frequent Contributor
timestamp: February 19, 2016, 04:45:09 pm
content: For 1mV-100V / 1uA-1A / DC-1MHz anything will do - the tougher / cheaper the better in a student lab - loads will go missing etc.For low noise / low V / low A cables you need something a bit better: either the kosher ones specified for the meter (if the budget runs to it), or maybe silver screened + twisted pair audio might work at low frequencies with some low thermal emf connectors. There's a thread on low emf cables here:https://www.eevblog.com/forum/buysellwanted/diy-low-emf-cable-and-connectors/msg246248/#msg246248The students would learn a lot from considering / adjusting for the non-negligible effects of cables on low level measurements.As a ballpark, I just got some quotes from jswilley for low thermal cable / terminations:+ low thermal forks and rings $2.99+ SPSCTCS-4 $7.75 per foot+ STC-4 $5.50 per foot

author: uncle_bob
author_rank: Unknown
timestamp: February 20, 2016, 01:13:24 am
content: Quote from: alanambrose on February 19, 2016, 04:45:09 pmHiDon't give them the low thermal EMF stuff and have them put a thumb on a connection as part of the experiment ....Bob