I'll add 2 little things on top of what Inge has said:
1. If the cable is flexed, it can generate a triboelectric voltage. This
is of little consequence if its characteristic impedance is correctly
terminated at both ends (typically 50 or 75 ohms). If it is used just as
an unmatched screened cable, then it may be of consequence, especially
with high impedance low level signals (analogue or digital).
2. It's not just electron tubes that generate noise; all semiconductors
do, as well. Even passive components, like resistors, do, with two
mechanisms. Ferri-c and -te cored components generate Barkhausen noise
as each magnetic domain "switches" from a +1 to a -1 state at different
times to its neighbours. Capacitors and PC laminates do, as well with
discrete domains in the dielectric switch in an analogical manner. Noise
is a very complex subject. I used to have a book on noise, written by a
guy called Drummer, I think, but I lent it to someone who never returned
it - the bastard!
Brian
Hfjord wrote:
> It's easy. If you are in room with lots of people, and suppose you are
> discussing important things with your boss, e.g. your salary, and he opens
> his mouth to tell you, but a guy near you shouts 'one more beer', you will
> be very disturbed, especially if your boss says 'listen, I tell yo diss
> onnly once'. Likewise, in a signal transmission cable with a wide bandwidth,
> electromagnetic waves from transformers, neon lights, relays and a lot of
> other sources, can leak through the shield of your cable and distort the
> wanted signal performance. In LNAs the background noise from bursts of
> electrons can cause 'noise'. In old electron tube amplifiers, the variations
> of electron emitting caused noise. Same in masers. There are many ways to
> reduce noise, e.g. cool down the source to minus 196 Centigrades, or use
> digital instead of analogs, use narrower bandwidth and so on. So, a low
> noise coaxial cable is a cable that does not permit electromagnetic waves to
> leak through the shield (semirigid copper cables or double shield). Also,
> such a cable has low losses and no discontinuities, which both can generate
> internal noise. Good matching and no reflexions are also important, though
> not belonging strictly to noise. Leakage of signals through the cable
> dielectric when transmitting extremely low level signals at high impedance
> can also be avoided by a good dielectric. With other words, the signal level
> compared to the unwanted noise, should be as high as possible.
> My two S/N
> Inge
>
> -----Ursprungligt meddelande-----
> Från: TechNet [mailto:[log in to unmask]] För Joe Russeau
> Skickat: den 27 november 2007 16:02
> Till: [log in to unmask]
> Ämne: [TN] Low Noise Coaxial Cable Question
>
> Good Morning All,
>
> Can someone point me in a direction to obtain information that explains what
> defines low noise coaxial cable? Thank you.
>
> Best Regards,
>
> Joe Russeau
>
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