suitable bnc connectors for an oscilloscope

[ghostman11]

i have been looking on ebay for some BNC connectors so i can make some general test leads up for an oscilloscope, i have found loads that are advertised as being 75ohm for CCTV, are theese ok for making scope cables up with or do i need a specific type of BNC plug? also what is the best coax to use?
thanks chaps

 

[Reloadron]

Hi Ya Ghostman

When selecting coaxial cable the impedance is chosen for the intended use. For example RG 58 is 50 ohm and RG 59 is 75 ohm and there are many other impedances with RG 58 and RG 59 only being two of the more popular for what your purposes would be.

The BNC connectors are chosen based on the cable. For example if I use RG 58 (most common for me) is this particular 50 Ohm cable then I would likely buy these particular BNC connectors.

Some consideration must be given the cable for example is the center conductor stranded or solid and the center conductor diameter. Some center connector pins are solder while some are crimp.

When buying crimp style connectors you need the correct crimping tool designed for the connector.

IN conclusion there is no "best" coax but typically RG58 50 Ohm is quite common for bench patch cords etc.

Ron

 

[ghostman11]

cheers for the info Ron, i just knew it wasnt going to be straight forward lol, yeah the cables or mainly just patch cables for both 100mhz and 250mhz scopes, and also for conecting scope and LA trigers together through the external conector

 

[Reloadron]

The trick is to make sure you look at the cable specifications (dimensions) and get the right connectors. I hate that sinking feeling when there is sudden realization that what I bought won't fit.

Nothing is ever as simple as we figure it should be. However, it is best to have the tools to make your own even though BNC patch cables can be bought.

Ron

 

[saturation]

I am skeptical about many eBay 50 ohms BNC connectors or accessories. Since its nearly identical to 75 ohms in appearance without very close inspection of construction and the specs, most novices won't notice the difference; its often sold as 50 ohms when its truly 75 ohms, or worse. 50 ohms BNC connectors have a slightly higher premium cost as its primarily used in electronics work whereas 75 ohms are mass produced for the consumer and commercial use video industry.

On a whim, I bought a few splitters, including from a non-authorized distributor then tested them against a real Amphenol connector, which is rated to at least 1 GHz, and maximum at ~ 4 GHz. At 1 MHz square wave, the eBay and non-authorized BNC connector had a small amount of ringing to suggest it was the wrong impedance, at 100 MHz it was 2x in size. Some did not fit properly, and were loose. Splitters cost about $3 each, some as low as $1, whereas Amphenol is about $3-6 each from an authorized source such as Jameco.com.

 

[ghostman11]

thanks for the info saturation, i dont need many so more than likely i will order from farnel now. i didnt realise the implications of the cheaper ebay ones, for me a BNC was a BNC was a BNC! i had assumed all were equal and any old coax would fit
but that is why i am smart enough to ask the pro's
cheers chaps its been a great help

 

[saturation]

You're welcome. Farnell ergo element14 is pricey but authorized. For such an item that you'll likely have for life, it will help not to save a few dollars then get stumped by strange waveforms that are due to transmission line issues introduced by the probes/connectors. FWIW here is what you get with a brand BNC connector:

http://www.amphenolconnex.com/product-specifications/bnc

Specs you can rely on. I use this because its common here and low cost, Pomona is another good brand but more costly, but feel free to check similar known BNC items from other EU and Asian makers, it won't matter if its made in China or not, so long as the maker stands up to its spec.

 

[squishy36]

it won't matter if its made in China or not, so long as the maker stands up to its spec.

Maybe, maybe not. Lemme explain. I worked at HP for 25 years and back in the 80's, lab engineers were allowed to take stuff from lab stock home for "G jobs". Now, HP's lab stock was a dream, especially if you had worked at other companies where you had to learn to rat hole crud because the bean counters had screwed everything up. Anyway, I wound up taking 5 or 6 BNC patch cord cables home and these were HP-branded cables, made in the US, etc. And they are the best cables I've ever used, bar none -- I'm still using them nearly 30 years later and they still work perfectly. I also have some Pomona BNC patch cord cables and these are also good, just not quite as good as the HP ones made in the early 80's. Finally, a few years ago I did some consulting work for a well-known US company and took out some of my time in trade for some BNC patch cables made by a wholly-owned subsidiary of theirs. These are cables assembled in the US, but made with parts imported from China (I've visited their factory). These cables work, but mechanically they are second-rate, as they sometimes don't mate smoothly with the female BNC connectors on my test equipment.

Now, I'm just a hobbyist who occasionally messes around with test equipment. But when working on something, I can make hundreds of connections with BNC patch cords (I standardized on RG-58 patch cords for my hobbyist lab). Over decades, this means tens of thousands of connections -- and you will rue the day you bought those inferior cables.

That's why I like the old adage "Buy the best and only cry once".

 

[ghostman11]

i realy like HP stuff i have 2 of there logic analyzers wich are brilliant, i origanaly picked ebay as there is no minimum order etc, but i guess there is no saveing in buying something cheap that dosnt do the job!

 

[saturation]

Ah, I couldn't agree with you more, what quality that was. But, nearly everything you buy today is probably made in China, although there is a strong movement for most recently made stuff to be made in EU and USA again. Amphenol's commercial BNC is widely made in various parts of the world as its low tech technology, but BNC made to MILSPEC or aerospace specification is made in more tech savvy countries like Japan, Taiwan, EU or USA.

Here is a Pomona BNC F-F adaptor, All-spec has the country of origin of most of its parts, and good quality connectors often engrave the brand and a product number on them. Fairly easy to copy but low quality one's don't bother due to cost, although they could easily counterfeit them, so best to buy through authorized supply chains for the good stuff.



**broken link removed**

Pomona BNC MM adapter of recent vintage now made in USA.

Maybe, maybe not. Lemme explain. I worked at HP for 25 years and back in the 80's, lab engineers were allowed to take stuff from lab stock home for "G jobs". Now, HP's lab stock was a dream, especially if you had worked at other companies where you had to learn to rat hole crud because the bean counters had screwed everything up...

That's why I like the old adage "Buy the best and only cry once".

 

[JimB]

For good quality coax cable and connectors, in the UK I recommend these guys:

http://www.whwestlake.co.uk/

JimB

 

[ghostman11]

thanks jim i will drop them a mail. the tight arse part of me wants to save cash but the small nagging sensiable part thinks its a bit pointless spending money good quality test gear then buying cheap crap to connect it