even a small fart...
/Inge
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Brian Ellis
Sent: den 4 december 2007 11:27
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
Not practical. To put 100 hermetic boxes into a climatic chamber, working at high temps and humidity, and guarantee no leakage currents over a 3-month test is a very tall order. You would still need twice or more times as many PTFE wires connecting them to the outside world, because you would have to switch between bias and measuring voltages (implying each box carrying some complex switching devices working at
1E15 ohms). With 100 such boxes, individually screened from each other, you may even have enough space in the chamber for your test pieces, if you are lucky. Please realise that SIR testing is practically state-of-the-art and a big sigh 20 m away can affect the readings (well, almost!).
Brian
John Burke wrote:
> Understood, but could not the high impedance measuring circuit be in
> the probe?
>
>
>
> John Burke
>
> (408) 515 4992
> -----Original Message-----
> From: Brian Ellis [mailto:[log in to unmask]]
> Sent: Sunday, December 02, 2007 11:51 PM
> To: TechNet E-Mail Forum; John Burke
> Subject: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>
> Yep, this is impractical! Firstly there is no signal in the way you
> are talking about. Effectively, there is a resistance, maybe as high
> as
> 1E12-1E14 ohms. This is in an environment of up to 85°C, in some
> cases, and 95% RH. Under those conditions, you could not risk your "mast-head"
> ampli being of lower resistance than what you are trying to measure:
> in fact, you need it to be at least one order of magnitude higher.
> Furthermore, you want your expensive instrument to maintain its
> integrity over a number of years.
>
> The traditional way of doing this is via a law promulgated by a Mr Ohm.
> You apply a known DC voltage (usually between 5 and 100) and you
> measure the current. Applying the voltage is not difficult. Measuring
> minute currents is. You minimise leakage by using the best insulation
> on the connecting wires, PTFE, but that damn difficult fA meter is
> tricky to drive and could not be reliable at the "mast-head". As it
> is, everything has to be done in a Faraday cage to minimise outside influences.
>
> As I mentioned before, the instrument I designed measured the high
> resistances dynamically but with extremely low-impedance circuitry. It
> didn't even need a Faraday cage or coaxial cables, just plain ol'
> Teflon insulated wire at <dollar/m. It was Columbus' egg.
>
> Brian
>
> John Burke wrote:
>> Classically, you should re-address the issue.
>>
>> It is common practice to measure the signal and boost the result as
>> close
> as
>> possible to the receptor site for low amplitude signals or where the
> signal
>> to noise ration will be a distorting factor..
>>
>> A couple of examples would be a mast head amplifier where the signal
>> is boosted at point of reception. Another would be a hard drive where
>> the signal from the spinning disc is amplified at the point of pickup
>> on the spinning disc.
>>
>> So maybe I am not understanding this issue, but what is wrong with
> boosting
>> the signal or measurement at the probe, by incorporating the
>> electronics front end within it before it goes into a
>> cable?..............or is this totally impractical for this application?
>>
>> John
>>
>>
>> John Burke
>>
>> (408) 515 4992
>> -----Original Message-----
>> From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Naisbitt
>> Sent: Sunday, December 02, 2007 2:00 PM
>> To: [log in to unmask]
>> Subject: Re: [TN] SV: [TN] SV: [TN] Low Noise Coaxial Cable Question
>>
>> Joe and Techies
>>
>> We are carrying out an extensive study on cabling for SIR and should
>> have some info that we might be able to publish in due course.
>>
>> Kindest regards,
>>
>> Graham
>>
>> [log in to unmask]
>>
>>
>>
>> On 30 Nov 2007, at 19:22, Joe Russeau wrote:
>>
>>> Hello Seth and Inge,
>>>
>>> Seth thank you for that very educational response.
>>>
>>> Basically, I am researching all of the different SIR test equipment
>>> available. I'm looking into purchasing a newer system (By-the way,
>>> this is not an invite to SIR equipment manufacturers to bombard me
>>> with product literature). Anyway, one of the groups I have been
>>> speaking with recommended using their low noise coaxial cables. I
>>> asked how they classified the cable as low-noise. They responded
>>> that they pull the cable taught, drop a weight on it and measure the
>>> resulting charge. If no charge, then it is considered low noise.
>>> What I had hoped for with asking my question and the question to
>>> TechNet, was to find out what materials were best for low-noise
>>> coaxial cables. I have been contacting different cable
>>> manufacturers, who claim to have low-noise cable, to try and
>>> determine if the materials sets are consistent from manufacturer to
>>> the next. I figured that would allow me to find the cabling and
>>> perhaps make the cables myself. What I have found is that each
>>> cable manufacturer uses different materials for their low-noise
>>> cable. So, now I'm back to square one. So perhaps I should ask the
>>> question this way. If you were in the process of measuring low
>>> currents, as is done in an SIR test, and you were looking into
>>> coaxial cables as the transfer vehicle from the DUT to the
>>> measurement equipment, what materials would you prefer in the cables
>>> to give the best data integrity?
>>>
>>> Best Regards,
>>>
>>> Joe Russeau
>>>
>>>
>>> ----- Original Message ----- From: "Hfjord"
>>> <[log in to unmask]>
>>> To: <[log in to unmask]>
>>> Sent: Friday, November 30, 2007 1:27 PM
>>> 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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