Function generator Cable advice needed - Page 1
Forum section: electronics / beginners

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 11:37:43 am
content: I just purchased one of the many FG's on ebay sold without any cables. I didn't realize just how many choices for cables there were until I began shopping for them. It looks like you could easily spend more on cables than I spent for all of my equipment! Okay, that may be a slight exaggeration... though I'm sure there are cables well beyond my price reach. I've never used an FG before and have little to no knowledge of them. I will probably never exceed 20MHz on anything I ever work on. 95% of my work will be in the 100's of kHz range. I'm only a hobbyist, but I don't want to buy "crap" either.The FG is a Wavetek 188. My scope is a Tek 2230. My budget is indicated by the equipment I own. The FG manual says to use RG-58/U, 3' long, 50 ohms. From what I'm reading, RG-58 is obsolete? I see Mouser still sells it. I've also read that copper core and shield are musts. I don't mind buying used, but I'm not very assured that what I would find on eBay would be anything but the cheap "crap." What minimums do you recommend to get started?

author: PA0PBZ
author_rank: Super Contributor
timestamp: April 02, 2014, 11:53:59 am
content: Any more or less decent 50 Ohm coax cable with bnc connectors will do, looks like you are making this more problematic than it is in reality. And by the way, if you are going to connect it directly to your scope you need a 50 Ohm terminator on the input of the scope.

author: GreyWoolfe
author_rank: Unknown
timestamp: April 02, 2014, 11:59:29 am
content: If you are interested in rolling your own, you can go to DXEngineering.com and get RG-58A/U cable for $.29 per foot and get Amphenol connectors and you can build your own quality cables. You can also add test lead wire at 1 end and use things like alligator clips, J hooks and mini grabbers and have a complete custom set tailored to your needs.

author: ConKbot
author_rank: Super Contributor
timestamp: April 02, 2014, 12:26:23 pm
content: RG-58 is only obsolete for networking, but still very much in production for other uses. It used to be used in thin-net token-ring stuff.As stated, RG-58 is cheap, and so are the connectors, though I'd recommend decent crimpers if you're going to be making a bunch. If I was buying a set of crimpers for personal use, I'd gethttp://www.rfparts.com/47-10000.htmlfor the frame andhttp://www.rfparts.com/47-20025.htmlfor the dies for RG-58. $75 for ratcheting crimpers isnt bad at all. The 'wrench-crimp' connectors which dont need crimpers work OK, but they are expensive, and a bit more labor intensive. If you want just a few cables to use and dont want to worry about making your own, then I'd check out the BNC-BNC cable assemblies that L-com stocks (http://www.l-com.com/coaxial-rg58c-u-bnc-plug-coaxial-cable-assemblies-50-ohm-transmission-line-with-bnc-male-connectors) They are high quality, and hold up well in my experience. No the cheapest, but Id rather have a few that hold up than getting a bunch of crap ones off of amazon where the connectors break off quickly, and arent even close to being 50 ohm.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 12:32:48 pm
content: Quote from: GreyWoolfe on April 02, 2014, 11:59:29 amThanks, THAT's what I was looking for and expecting to find at Mouser. Mouser wants $79 bucks for 100 feet of it (cu/cu). I'm guessing the "A" designation is probably aluminum shield? I don't mind paying even a buck a foot, buying by the foot, if I get a better quality (more flexibility, copper core and shield, etc). Are there other cable designations I could use as well? If you know of a good link on cable and/or connector basics, I'd much appreciate that as well. There are just so many!!

author: rstoer
author_rank: Regular Contributor
timestamp: April 02, 2014, 12:56:27 pm
content: I like thesehttp://www.probemaster.com/product_info.php?cPath=5_20&products_id=48from Probe Master. Good quality, gold center conductors, very flexible, made in the USA, available in any length, and reasonably priced. I also have some from Pomona and like the Probe Masters better.BTW: Their 8000/9000 series DMM test leads are also great. 440 strands make them limp as cooked spaghetti.

author: madires
author_rank: Super Contributor
timestamp: April 02, 2014, 01:05:54 pm
content: If you are going to assemble the cables yourself take care about selectling the right connectors for the coax, i.e matching diameter and impedance. For most connectors you can choose between crimp and solder types. For solder types buy only connectors with PTFE/teflon dielectric, cheap plastic ones will simply melt. And don't forget the antikink.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 01:38:04 pm
content: Antikink?

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 03:12:57 pm
content: I would just buy bnc cables with rg58 already terminated. pomona makes good and affordable stuff.if you buy it already terminated in bnc's, it will likely be done right and fully shielded.just be aware that you can find rg58 (50 ohm) and 59 (75 ohm) and you want to avoid the 75 ohm stuff.technically, even bnc's can be 50 or 75ohms, but it probably does not matter too much. if you buy a name brand cable, it will probably have the right bnc's on it. if you buy china 'specials' all bets are all (I would not spend even a penny on a china no-name cable for test gear).

author: Legion
author_rank: Frequent Contributor
timestamp: April 02, 2014, 03:27:38 pm
content: This is the cable that came with my FG.In the middle is a BNC T-adapter and BNC 50R terminator. The 50R terminator is useful for impedance matching. FGs commonly assume their 50R output impedance will be matched so they ouput twice the peak to peak voltage. If the FG is properly terminated you'll get the correct amplitude. Definitely recommend the BNC terminator, otherwise you'll have to run a 50R resistor in parallel with your circuits.

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 03:28:33 pm
content: search for 50ohm 'feed thru' bnc. those are even more convenient.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 03:30:54 pm
content: I appreciate all the suggestions. I purchased the Amphenols and the wire from DXEngineering.I'm still confused about a lot of stuff. Like, why does the FG manual repeatedly express "3 foot" cables? What happens if I use 6" cables? How about 10'? I looked more closely at my scope probes... All of them have the bayonet style connectors (I assume is bnc?) but the probe cable is NOT RG-58, unless I'm reading wrong information (the diameter is ~0.130"). What kind of cable is typically used for scope probes? I'm not planning on making those, but it would still be nice to understand them a little better. They really are quite the mystery for me. In my world, a wire is a wire is a wire... All the other stuff, capacitance, inductance and rf interference is all new to me. If I use the wrong impedance, is it like speakers where I could burn up my test equipment or will I simply get interference in my reading? A little? A lot? Like I said in the OP... I'm not likely to ever do anything above 20 MHz or so.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 03:35:33 pm
content: Quote from: linux-works on April 02, 2014, 03:28:33 pmI didn't spend as much for my FG asoneof those.

author: madires
author_rank: Super Contributor
timestamp: April 02, 2014, 03:41:22 pm
content: Quote from: chipwitch on April 02, 2014, 01:38:04 pmAKA bend protection or stress relief.

author: Legion
author_rank: Frequent Contributor
timestamp: April 02, 2014, 03:56:22 pm
content: Quote from: chipwitch on April 02, 2014, 03:30:54 pmScope probes aren't usually RG-58. They aren't just a simple wire either. They usually have a small amount of circuitry to provide attenuation. Probes easily run into the hundreds of dollars.If the FG isn't impedance matched to the circuit under test then the amplitude of your FG signal will be off. So you may set the FG to output 5 volts peak to peak but when you look on the scope you see 7Vpp or 2Vpp, etc. To understand impedance matching you'll need to learn voltage dividers.

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 03:57:37 pm
content: Quote from: chipwitch on April 02, 2014, 03:35:33 pmI bought some for $20 or so, used, and while its not cheap, you know its done right and can go to high mhz and still be a clean 50 ohms.if you want accuracy and signal integrity, tools cost money, no way around that

author: mrflibble
author_rank: Super Contributor
timestamp: April 02, 2014, 03:59:01 pm
content: Quote from: chipwitch on April 02, 2014, 12:32:48 pmThat's also exactly what I'm currently looking for. Minus the crazy shipping cost from the US to Europe.Any suggestions for those of us on this side of the pond? I'm looking for both affordable RG58 and some solder it yourself connectors with clamp fitting.Quote from: chipwitch on April 02, 2014, 01:38:04 pmI think madires means this (Strain Relief Boots):

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 03:59:33 pm
content: scope probes, at least, have resistance in the wire. by design. they are not just straight 50ohm cables. there's a lot more science to them than it would appear. read up on hp and tek probes; google should find links to explain the concepts in why probes for scopes are not just straight wires (even ignoring the comp boxes; they are still not just straight wires)

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 04:01:21 pm
content: see if this turns up any affordable cables:ebay and search for '10 base 2 ethernet cable'

author: mrflibble
author_rank: Super Contributor
timestamp: April 02, 2014, 04:03:43 pm
content: Quote from: linux-works on April 02, 2014, 04:01:21 pmThe problem with all the cheapo ebay rg58 cables (that I could find) is the highly undefined cable characteristics.

author: linux-works
author_rank: Super Contributor
timestamp: April 02, 2014, 04:08:25 pm
content: 10base2 is the old DEC thin-net standard which required a higher spec cable and is usually not a piece of junk.video cable ('yellow composite rca connectors') is 75ohms and often crap quality, but 50 ohms was never really low-end consumer junk, and so if you find old thin-net cables, they are probably test-equip quality.you can also find 50 ohm term caps from the old ethernet 10base2 stuff, so if you need tee and term along with cable, often the same seller will have all of that.I have old thin-net cables from my days at DEC. I'm sure you can find lots of those still on ebay. the old networking companie's brands are probably quite trustable for quality since they were meant for commercial use.

author: edavid
author_rank: Super Contributor
timestamp: April 02, 2014, 04:11:56 pm
content: Quote from: mrflibble on April 02, 2014, 04:03:43 pmWe're talking about a 4MHz function generator, so any cable should be good enough.OP, the length doesn't matter.Look for a used Pomona cable, they are generally pretty good mechanically.However, you can certainly buy a crappy new Chinese cable... it will be good enough electrically, but annoying to use because of stiff cable and connectors that are hard to attach and detach.

author: chipwitch
author_rank: Frequent Contributor
timestamp: April 02, 2014, 04:38:29 pm
content: @mrfibbles... how much is the shipping? I recently bought some stuff on eBay from a GB seller. Took 3 weeks to get, but I was surprised how cheap shipping was. Shipping was $10 for my order. A little high on the shipping but since I got 30 feet of 19 strand RG-58 for $.29/ft and a dozen BNC connectors for $1.59, I figure I made out. Heck, if I had known, I could have placed my order with yours and sent you your stuff for the cost of shipping. $100 order in the US is free shipping! We both could have saved.