searching for Active Probe coaxial Cable - Page 1
Forum section: electronics / projects

author: bartver
author_rank: Contributor
timestamp: August 29, 2018, 10:57:23 am
content: Hello,I want to make some active probes for GHz+ frequencies. Found some design already up to 1GHz on the net.Problem, i need a coaxial cable that combines with 2 extra leads for the powersupply of the amplifier in the probe-head.Searching the web finds me triax and CCTV siamese cable, but that has 75ohms and is way to thick.Was hoping on finding flexible cable RG174 style size, that has 2 thin wires running through the shielding or so.I can add 2 wires externally and for testing i will, but the finished product should have some nice cable.Anyone any idea where to find such cable, or how its called. ?tnx, Bart

author: 6PTsocket
author_rank: Regular Contributor
timestamp: August 29, 2018, 10:36:30 pm
content: Quote from: bartver on August 29, 2018, 10:57:23 amIsn't there a way to duplex the DC on the coax, with the signal? Antenna amps do it that way with the amp mounted at the antenna.Sent from my SM-G900V using Tapatalk

author: jbb
author_rank: Super Contributor
timestamp: August 30, 2018, 12:15:24 am
content: If you’re getting a few, this sounds like a job for a custom cable. You can probably get a coax line and a (shielded) multicore all overmoulded together.Also, the cable manufacturer might be able to do the connectors or overmoded strain relief for you - I.e. a complete cable subassembly. Overmould tools do cost money (some hundred $$) but that’s much less than injection moulds.

author: bartver
author_rank: Contributor
timestamp: August 30, 2018, 05:14:07 am
content: Hello,first, thank yo for replying !!The idea is to make a DC-to-GHz probe with the ability to measure both positive and negative signals.Therefore I need a dual-rail powersupply to the probe-head.And i only need the cable, there's no need (and space) for special connectors etc.I have a lecroy with probus interface, it uses modules that click to the scope which can hold a lot of electronics, i want to 3d-print something similar :http://teledynelecroy.com/doc/ca10-adapter-with-current-transformer-appnoteThe cable should be as flexible as possible, i could shrinkwrap external wires or the cable from a usb cable (has shielding and is flexible) for power and control.But, commercial probes use this cable, i have an old philips FET probe that works this way, so it should be available somewhere !the search continues.....73'sBart

author: DaJMasta
author_rank: Super Contributor
timestamp: August 30, 2018, 05:45:10 am
content: I know you can get dual-coax RG6, but that's 75 ohm stuff that may not be great for the high frequencies either. Why not just a bunch of wires as needed and use sleeving to bundle them together? At least for prototyping it should keep them together enough, and with some filtering on the probe end and maybe some shielding on the cable in between, you should be able to keep noise on those DC lines down just fine, so long as your signal wire is well shielded.You could also just look for a dead active probe to cut the cable out of for a one-off.

author: Marco
author_rank: Super Contributor
timestamp: August 30, 2018, 09:13:39 am
content: Quote from: 6PTsocket on August 29, 2018, 10:36:30 pmYou could do phantom power. You would need to add some circuitry to add a DC voltage to the input signal, and subtract it at the scope end (irrelevant to bandwidth, it's just a capacitor at high frequency, would require a tiny bit of bias current into the input).

author: ConKbot
author_rank: Super Contributor
timestamp: August 30, 2018, 11:58:01 am
content: For a form factor prototype, definitely just make your own with coax, wire, shield braid, and some heatshrink tubing. (You can get 4 foot pieces, or long rolls)For production, you can get a cable company to take something like RG316-DS, twist however many power wires/filler strands around it, tape it, braid a shield over that, and extrude the whole thing with a jacket of your choice. Work with them on flexibility being the goal, and they can help with the spec. With the right use of filler material, slick tape under the braid, and right jacket, the cable will remain flexible.

author: coppercone2
author_rank: Super Contributor
timestamp: August 31, 2018, 08:42:03 pm
content: Heat shink tubing kinda sucks for flexible stuff.Bundle your wires together with the coaxial cable, bind it on regular intervals with short peices of heat shrink, I mean like 1cm long, so you are not trying to deform a lot of it.Then wrap it in shield and finally with a nylon cable cover, then heat shrink that to the cable. Maybe a dash of silicone under the heat shrink bits to prevent the wires from moving independantly of eachother, but only in a few points. Should be like a piece of spaghetti. The heat shrink adds a ton of resistance to bending if you use one big piece.The only problem is that your nylon braid is only attached at the two ends with heat shrink, so you have the 'chinese finger trap' effect, so don't jerk the cable off. But it looks and performs fine so long you literary don't try to jerk it off.