is rg58 cable affordable for oscilloscope probes? - Page 1
Forum section: electronics / beginners

author: 001
author_rank: Super Contributor
timestamp: November 10, 2017, 08:56:27 pm
content: I`m homebrewer.My projects below 10mhzCan I connect my gear (oscilloscope, voltmeter, generators) with selfmade bnc cables (via rg58)?

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2017, 09:25:08 pm
content: Yes, certainly, RG58/U is the general purpose coax for most standard leads - assuming that you need 50 Ohms RF impedance. They are also used for other impedances at low frequency, eg. 600 Ohm outputs of Audio frequency generators. These use them as simple screened cables at low frequency rather than 50 Ohm RF transmission lines.Self made cables are economical as long as you can obtain the BNC connectors at reasonable price - Crimp connectors are cheaper than solder type but you need the specific crimp tool.Note that if you are connecting directly to a scope at RF frequencies then you will normally want to set the input impedance to 50 ohms, either internally selected or using a through-terminator) to maintain signal integrity. Be careful not to overload 50 ohm terminators.You can purchase ready made cables reasonably cheaply too, it's worth making a comparison. Note that quite a lot of ebay cables are 75 Ohm (RG59) for video applications, so do check.BNC Crimp plug fitting:Quote

author: 001
author_rank: Super Contributor
timestamp: November 10, 2017, 09:37:02 pm
content: Thanx!I found this at local store for $0.2Is it will be ok for me?Is it best way to solder shield?Is rg213 really much better than rg58 for me?

author: 001
author_rank: Super Contributor
timestamp: November 10, 2017, 09:43:42 pm
content: I find also some big variantit soldering points assymetrical and looks identical to china audio connectorsis it ok?

author: Gyro
author_rank: Super Contributor
timestamp: November 10, 2017, 09:53:48 pm
content: The first type you showed (7920) ismuchbetter! It looks a good price. The second one is only really suitable for audio.No, it isn't normal to solder the outer braid. The shield braid of the cable is spread out during assembly and then compressed as the BNC connector is screwed together. This makes very secure electrical contact with the silver plated connector body.You assembly it as follows:http://www.vaqtec.com/repository/uploads/files/pages/bnc-plug-assembly.pdfEdit: No, I don't think you will see any advantage from RG213 in your application.

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2017, 10:16:23 pm
content: RG-58 is fine electrically, but somewhat unwieldly. If you spend a bit more, there are softer and more flexible 50 ohm cables available.

author: 001
author_rank: Super Contributor
timestamp: November 11, 2017, 02:40:31 pm
content: Quote from: Benta on November 10, 2017, 10:16:23 pmSanx!Sorry for stupid question butcan I use simple guitar cable since my projects are far from hundreds of MHz?(Guitar cables always extra flexible and have braided cooper and static carbon shields. Typical Capacity is about 100pF/m )PS - microphone cable sucks anyway since it is not "true coaxial" and capacity so big

author: cdev
author_rank: Super Contributor
timestamp: November 11, 2017, 03:06:30 pm
content: There is a lot of variation between different brands of RG58U coax. Some variants I've used are quite flexible and easy to manage. I don't know how those variants are for RF loss though, their loss may be higher, but the loss for just a few meters (patch cables) should be small well up into the GHz range if you use the right impedance connectors with the coax you pick and attach them properly. Different connectors vary a lot in quality. Also, some kinds of BNCs are 50 ohm, some 75 ohm.SMA is better than BNC at UHF, and N connectors are superior to SMA.The worst are the badly named "UHF" connectors. (PL259/SO238) They make lousy connectors at UHF.The connectors are important. Attaching them properly is important. Badly attached cables cause a significant amount of loss at higher frequencies.RG58 is pretty inexpensive. I have a lot of BNC patch cables made with RG58. They perform well and are reliable.Many patch cables also use RG316 which is thinner but has a higher durability jacket, but is still flexible enough to work with. However, its thin enough to not be a great size match for many BNC connectors.Its a better match to SMA. Also its much more expensive than you would think for its size. But its really good for short patch cables, its really the best choice for many RF applications.Its loss is higher than Rg58 at higher frequencies, but not that high if the length is kept short.RG174 is the cheapest and way too thin for the BNC connectors you pictured.Its kind of fragile compared to the others. Its what you see in the cheap scope probes, etc. It can be less reliable because the wire in it is so thin.

author: cdev
author_rank: Super Contributor
timestamp: November 11, 2017, 03:41:13 pm
content: The loss of a cable is determined in part by the material in the insulation in the sleeve between the two cables. Cables designed for RF use and interchangeable should use a specific low loss material. (most of the cable names originated in MIL-specs) You cant just use any old insulated wire and expect it to work. (although sometimes it may in a pinch, its unlikely to be optimal or even close.)Different plastics vary a lot in this area.That said many people have successfully used certain kinds of plastics as RF transmission lines at HF. Many people swear by clear radio shack speaker wire for dipole antennas on low HF bands for Field Day. Its also popular for use in baluns.The insulation used in the twisted pairs in Cat 6 Ethernet cable also likely has low loss. As does the insulation used in high speed ribbon cable used in high speed PATA cables and SATA cables.But lots of other plastic materials are likely to be lossy. Plastics that don't heat up in a microwave oven (when stripped off the wire thats in them, not left on) might have low lossbut testing them would not be wise.

author: macboy
author_rank: Super Contributor
timestamp: November 23, 2017, 07:11:29 pm
content: Quote from: cdev on November 11, 2017, 03:06:30 pmScope probe wire is very special and is not RG174. Scope probe wire has an extremely fine center conductor, almost invisible except under magnification. The resistance of this would be specified in some tens of Ohms per foot!. Typical scope probes have about 100 ohms resistance from connector to probe body, and this is not a single resistor placed somewhere, it is distributed along the special lossy coax cable. This is necessary to dampen the signal to prevent effects of reflection at the scope input, as it is not a impedance-matched transmission line system. The very small surface area also reduces the cable's capacitance per foot to help reduce the probe tip's overall capacitive load.