Electronics > Beginners

What's wrong with this RG58 cable?

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gf:
When I feed a square wave signal from an AWG into my scope, using different RG58 BNC cables of 1m length, I notice a rise time of ~7ns with some of the cables, while others do increase the rise time to 9ns. Since rise times add up geometrically, this would mean that the cable alone accounts for 5.5ns.

So I did a TDR of these cables (shorted at the end), applying a ~2.7MHz square wave signal, and got the attached results. The TRD of the "good" cable looks basically as I did expect (for a shorted end), but the signal of the "bad" one does not return to zero between the pulses.

I'm curious which deficiency of the cable does exactly lead to this behaviour?
Is this still within the specs of an RG58 cable?

Thanks,
gf

edavid:
The simplest explanation is that it's mismarked 75 ohm cable.

tggzzz:
What is the risetime of the signal?
What is the risetime/bandwidth of the scope?
How are the scope, signal and cable connected together?
What's the scope input impedance?

I would find it easier to interpret if the pictures showed the signal, then the signal applied to the cable with the cable unterminated/open and terminated in 50ohms.

jmelson:

--- Quote from: gf on March 11, 2019, 10:49:46 pm ---
Is this still within the specs of an RG58 cable?



--- End quote ---
There are DOZENS of variants of "RG58", such as RG-58A/U, RG-58C/U and so on, that have SLIGHTLY different characteristic impedances.  They vary between 49 and 53 Ohms or so.  But, what you showed might indeed be mismarked 75 Ohm cable.
One way to tell is to put various resistors on the far end instead of a short.  A resistor that exactly matches the cable's impedance will give no reflection.

Jon

gf:
Thanks for the responses so far. I'm still wondering whether it is just an impedance issue. Impedance mismatch certainly leads to reflections, but is it also suppused to "blur" the pulse edges? I'll follow-up with more information later.

gf

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