RG58 CU Coaxial cable maximum voltage

Thread Starter

Dritech

Joined Sep 21, 2011
901
Hi all,

I was looking at the datasheet of an RG58 CU Coaxial cable and noticed that the maximum rated voltage is listed as 2V. Am I interpreting something incorrect? Also, is this cable good to be used for oscilloscope test leads?

Datasheet attached.

Thanks.
 

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Thread Starter

Dritech

Joined Sep 21, 2011
901
Ohh, it's the usual use of a dot instead of a comma :( Seeing that this is an Italian company, I wasn't expecting this. Is this cable good to be used for oscilloscope probes please?
 

MrChips

Joined Oct 2, 2009
30,795
Ohh, it's the usual use of a dot instead of a comma :( Seeing that this is an Italian company, I wasn't expecting this. Is this cable good to be used for oscilloscope probes please?
No.
What is the make and model of your oscilloscope?
What kind of probe are you trying to fabricate instead of buying ready made probes?
You should be using a probe that has 10x attenuation.
 

DickCappels

Joined Aug 21, 2008
10,169
Two problems come to mind immediately: reflections from an impedance mismatch and very high capacitance which will load down the circuit you are looking at. It is amazing how inexpensive you can find decent scope probes if you look around a little. I have found that the lease expensive one's I've found developed problems without a few years.
 

MrChips

Joined Oct 2, 2009
30,795
Here are reasons why RG-58 is not a good substitute for oscilloscope probes.

1) RG-58 is thicker and not as flexible. RG-174 would be a better choice. In anycase, read on.

2) Assuming that your oscilloscope input is a BNC jack, you will need special crimping tools and a BNC male connector to terminate the coax cable.

3) RG-58 and RG-174 have characteristic impedance of 50 ohms whereas oscilloscope inputs are usually 1M ohms. This is a severe mismatch of impedances and will result in signal reflections.

4) Proper oscilloscope probes should have x10 attenuation and impedance that matches the input impedance of the oscilloscope. You would be well advised to purchase proper oscilloscope probes.
 

Jon Chandler

Joined Jun 12, 2008
1,048
RG-174 is not good for leads subject to constant flexing and bending. The center conductor is copper-plated steel – under flexing conditions, the stiff center conductor can punch through the insulation and short to the shield.


Trust me. Troubleshooting this condition is next to impossible.
 

MisterBill2

Joined Jan 23, 2018
18,463
I see different voltage ratings for RG58, and probably some are more the military qualified ratings over the military temperature range.
As for using either RG58 or RG174 as scope leads, it depends very much on the application. If I am checking the output of an audio amplifier, 8 ohms impedance, no problems, but if it is a high impedance source then there can be a mismatch error source.

None of the RG174 that I have used had copper plated steel strands, that would be the severe-duty stuff. A small magnet can do a quick check and verify yes or no on a sample.

AND I never ever use crimp-on BNC connectors. The solder-on ones are so very much better. It does take a bit of skill to install them, though. AND one must be able to solder properly. Crimp connectors can seldom be re-used and a replacement is never available when needed.
 

MisterBill2

Joined Jan 23, 2018
18,463
That may be fine at 2 kHz, but how many measurements does one take at 2 kHz.

My advice is: Try it, but keep an eye out for artifacts caused by the use of a coaxial cable.
For the lower audio frequencies the effect of cable capacitance is less, no question. especially from the low impedance output of a function generator. At 21 Megahertz, examining the gate voltage of an oscillator there will be a bigger difference. Years ago, in my AC circuits lab in school, one lab instructor had a very nice scope lead made from some silicone, high voltage insulated, shielded cable. Probing a test point with about a 5000 ohm source impedance, the response on the scope was down to half at 8 Khz.

Bottom Line: Not all shielded coaxial cables are the same. They are not al suitable for a applications !
 

Jon Chandler

Joined Jun 12, 2008
1,048
None of the RG174 that I have used had copper plated steel strands, that would be the severe-duty stuff. A small magnet can do a quick check and verify yes or no on a sample.
I looked at FIVE data sheets for RG174 cable (the first 5 I found), which included

Belden
Alpha Wire and Cable
L-Com
Pasternack
Fairview Microwave

and ALL of them specify a copper-clad steel core. Digging a little deeper at Digikey, one company, Huber+Suhner, Inc., supplies an RG174 cable with a copper core. It appears to be the rarity, well down on Digikey's list.

My friendly advice about shield shorts is based on the voice of experience in the hopes it might prevent somebody else the frustrating experience of trying to troubleshoot a problem that changes every time a cable is moved. Unless you specifically search out and buy RG174 with a copper core, it's likely you have RG174 with a steel core and the potential for problems in high flex applications.

And did you really check the RG174 you used in the past with a magnet just in case?
 

MisterBill2

Joined Jan 23, 2018
18,463
Steel core wire is rather obvious when it is bent. The wires were used for a headset and all of the strands bent the same, like soft copper. Normally I do not use the stuff because it has too much loss for RF.
 
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