Brian, it looks like you've been involved in myriades of projects! Anyway,
your principle is used by Keithley. They make an instrument that can measure
attoamperes! 1 attoampere = 6 electrons per second!! They use a
Voltage/Current source same way as you did, i.e. a kind of capacitor that is
discharged through TeraOhm resistors. The discharge current is plotted vs.
time and a number of such events are integrated and computerized to give a
readout. Dynamic principal with other words. I have their instrument P/N
somewhere, if you are interested, or you will find it yourself. I believe
it's a 100K dollar instrument. Instruments have undergone a remarkable
evolution. My private and cheap and simple handhold Fluke is capable of
measuring microamperes with a resolution of 0.1 uA. Price :  350 USD.
Are you still mixing with electronics at home?
Can't help admiring you, oldie!
/Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Brian Ellis
Skickat: den 1 december 2007 11:03
Till: [log in to unmask]
Ämne: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question

AAAAAH! Now I understand the question. In the dim and distant past, I 
designed and built a prototype Insulohmeter IRMA-2, a basic improvement 
on IRMA-1. Unfortunately, for political reasons imposed by the company I 
was working with, it was never put into production. Believe it or not, 
it could measure up to 1E14 ohms ± 30% at 5-100 V or 1E12 ohms ± 2.5% at 
5-100 V, without screening/shielding of the PTFE cables. How could this 
be? Because it did the measurement dynamically, not statically. A PS 
capacitor was charged to the required volts and allowed to partially 
discharge through whatever was required to be measured. The voltage 
across the capacitor was monitored with respect to time and the SIR was 
calculated from the discharge curve. It worked beautifully. Why was it 
so noise free? The capacitor effectively short-circuited the measurement 
circuit and its reactance, even down to <50 Hz, was several orders of 
magnitude less than the resistance being measured, so any induced 
voltages were negligible and did not affect the measurement.

Unfortunately, this is academic, because IRMA-2 never got beyond a damn 
good working prototype.

Brian

Douglas O. Pauls wrote:
> 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
> Please respond to
> TechNet E-Mail Forum <[log in to unmask]>; Please respond to
> Hfjord <[log in to unmask]>
> 
> 
> To
> [log in to unmask]
> cc
> 
> Subject
> [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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