estimate cable loss

ebp

Joined Feb 8, 2018
2,332
I would expect any cable like that where there is no published attenuation data to be generally "bad." But that doesn't necessarily mean it is unsuitable for your unspecified application.

It will certainly be much worse than a Cat5e cable, if for no other reason than that the individual conductors are insulated with PVC. PVC causes significantly higher loss than the polyethylene or polypropylene (latter usually used on Cat5). Depending on the frequency (oversimplification), maintenance of characteristic impedance is important to signal integrity. When stranded conductors are used in a pair it is generally much harder to maintain consistent impedance over a length of cable because it is hard to keep the spacing between the conductors consistent - they just don't stay snugly together after twisting. If the cable isn't intended to be constant impedance with tight tolerance, the twisting process may be sloppier, leading to length difference between the conductors in the pair. The higher dielectric constant of PVC means that small "air gaps" between the conductors contribute more to change in impedance than with low dielectric constant materials such as polypropylene.

If you need a low-loss cable, look for one where the manufacturer will at least publicly confess something about loss.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,846
I would expect any cable like that...
Hi ebp,

Thanks for your reply.

Unfortunately that's the cable I need to use, as the cable is ready there. What do you think if send some pre-amplified audio signal over 300m. The audio is voice only, differential signal, about 2V peak to peak. The driver impedance is 220ohm, the receiver is about 1.2K.

What do you think? Yes? No? No enough info??
 

ebp

Joined Feb 8, 2018
2,332
For voice-band audio I think it would likely be fine. I've certainly run audio over similar cable, though not that far.
 

MisterBill2

Joined Jan 23, 2018
6,037
The system should work BUT there are two concerns that may cause problems. The first one is capacitance between the two wires, which will cause the higher frequencies to drop off "some", which means that you may need to adjust the frequency response of the amplifiers. The second concern is induced hum on the signal. In a perfectly balanced system it is exactly canceled out and not a problem, in a real world situation it might be a problem. If hum is a problem there are solutions that often work.
 

Thread Starter

bug13

Joined Feb 13, 2012
1,846
The system should work BUT there are two concerns that may cause problems. The first one is capacitance between the two wires, which will cause the higher frequencies to drop off "some", which means that you may need to adjust the frequency response of the amplifiers. The second concern is induced hum on the signal. In a perfectly balanced system it is exactly canceled out and not a problem, in a real world situation it might be a problem. If hum is a problem there are solutions that often work.
You mean the main hum? how would you get rid of that?
 

MisterBill2

Joined Jan 23, 2018
6,037
If mains hum does get into a system it has been cancelled out by adding a similar voltage out of phase. That works for constant hum sources on most occasions. It is sometimes called "hum bucking" if you want to search for circuits. I used it successfully to cancel phone line hum in an amplifier quite a few years ago.
 

crutschow

Joined Mar 14, 2008
24,946
If the signal is truly differential and the receiver has good common-mode rejection, then hum may not be a problem.
What exactly are the transmitter and receiver circuits?
 

MisterBill2

Joined Jan 23, 2018
6,037
If the signal is truly differential and the receiver has good common-mode rejection, then hum may not be a problem.
What exactly are the transmitter and receiver circuits?
Not all installations turn out to be perfect, so that while a classic differential circuit should be "perfectly balanced", for some reason sometimes that does not happen. In my case it was one unshielded pair in a 200-pair cable several miles long, with my connection being quite a few hundred feet from one end. In a typical microphone installation with decent single-pair shielded cable him would be less likely.
 
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