Low currents? Then you are obviously not to apply signals of any kind? I've
an instrument, that is (theoretically) capable of measuring DC femtoamperes
(1 fAmpere = approx 4,000 electrons/second). I use a 1 cm thick Teflon cable
with many braids. However, after I've calibrated the instrument by
discharging a 1pF capacitor charged to 1 Volt, the problems begin, I have
the test socket in a Faraday cage, but it doesn't help: if someone opens the
door to the lab or if a person walks in the room, the instrument goes mad
and no stable measurement is possible. Humidity variations in the room have
an impact, so have the neon lights in the room. So, what level of currents
do you speak about? And what bandwidth(s) does your instrument use? And what
source are you measuring? Is the source a single component or a network of
RLC? And what accuracy do you need? If you have to do measurements like the
one I describe above, you need help by a specialist. There are so many traps
and you will absolutely go into one or another. And you can't get help from
cable 'noise' declare. Again, you need a hand from an experienced engineer.
I think I've a paper from Keithly how to measure low currents, can send it
to you (if I find it), if you are interested. But first read the below
little article which demonstrates basics:
http://www.tmworld.com/article/CA214124.html
My two fA
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Joe Russeau
Skickat: den 28 november 2007 14:51
Till: [log in to unmask]
Ämne: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
Hello Inge, Brian and Tom,
Thank you for your replies. They were very helpful and will help me to
formulate additional questions to ask the manufacturers of said cables. I
am working on some testing applications to measure low currents and was
interested in having a better definition of what to look for from a
materials standpoint. I should have been more clear. Things like impedance
resistance (typically, 50 or 75 Ohms, but is one better than the other or
does it matter), aluminum, copper or steel shielding, braided verses foil
shielding, the outer coating (PVC vs. Teflon, leaning towards Teflon), etc.
I have found many different manufacturers that claim to have "low-noise"
cable that are made with different materials and impedance resistances as
defined above. I was simply wanting to determine which materials were best
for low noise cable and was hoping there was a document that specifically
defined the pros and cons of those materials. Perhaps I'm asking too much.
Thanks again.
Best Regards,
Joe Russeau
----- Original Message -----
From: "Hernefjord Ingemar" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, November 28, 2007 7:27 AM
Subject: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
right, Brian, we can go on and on with noise. We even have cosmic noise
caused by sunwind, noise from high velocity particles from space, gamma
noise and more noise. But I think the original question was on a more '
daily' level.
/Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: den 28 november 2007 10:29
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
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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