I will chime in on Joe's behalf. This actually started with a
conversation between he and I.
We do SIR testing. As such it is not unusual to be measuring resistances
on the order of 1E11 or 1E12 ohms. In general, a voltage is applied to an
unknown resistance and a resulting current is induced. We are both doing
testing at DC, not AC domains. A very good text on low level measurements
comes from Keithley Instruments, originally authored by John Yeager. In
John's experience, the measurement world changes a great deal between 1E11
and higher decades of resistance measurement. Above 1E12 ohms, you have
to start watching other sources of current generation that can erroneously
add to your desired signal. This includes things like EMI coupling and
triboelectric currents from moving cables. EMI coupling and guarding is
why co-ax and other shielded cables are used.
Keithley, in their very low current measuring instruments and switching
cards, makes use of "low noise cables", which essentially have graphite
coating the outer surface of the inner dielectric, to reduce triboelectric
charges from the braided shield whenever the coax is flexed. Of course,
such specialty coax is pricey and led to Joe's more generic questions
regarding specifications or measurements of "low noise" and how low a
noise is "low noise".
That Joe is a demon when it comes to trying to exactly understand
measurement signals. That's a good thing.
Doug Pauls
Hfjord <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/30/2007 12:27 PM
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[TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
Exellent! However, Joe has still not told us WHAT he wants to do with the
cable. I doubt he is on a level, that dielectric intrinsic noise will play
a
role. Few test engineers work with such problems. If he isn't a Nobel
Prize
aspirant of some kind. I suggest someone finds remedy against Tinnitus.
He/she will get the prize, I'm sure. And the inventor will create a lot of
noise...he-he
/Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Seth Goodman
Skickat: den 30 november 2007 18:55
Till: [log in to unmask]
Ämne: Re: [TN] SV: [TN] Low Noise Coaxial Cable Question
Joe,
Some of what has been mentioned is really part of other cable
specifications and has nothing to do with noise created in the cable
itself. For instance, penetration of electromagnetic fields inside a
cable shield is often described by the manufacturer's specification for
shielding effectiveness. To the extent that the cable has in imperfect
coaxial shield, it can convert ambient electric or magnetic fields into
a loop voltage or flowing current. If external fields are the source of
your problem, you want cable with high shielding effectiveness as
opposed to low-noise cable. For electric fields, improving the shield
means thicker braid, finer wire in the braid, better conductivity of
braid wire and plating, more than one braid and/or a foil shield. For
magnetic fields, a shielded twisted pair may do better.
The traditional electronic noise sources, which are Johnson (thermal)
noise, shot noise and a group of unrelated mechanisms that produce 1/f
noise, do not produce appreciable noise in cables. When cable
manufacturers list a cable as low-noise, they usually mean triboelectric
noise. If the connected circuit puts a dc potential on the cable, then
mechanical flexing can change the capacitance between conductors, which
will cause current to flow in the external circuit. The triboelectric
and capacitance change with motion can together be called microphonics,
as they are both motion-related. Another source or noise in
high-voltage cables is leakage.
Regards,
Seth Goodman
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