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Cable parameters?
2

Cable parameters?

Cable parameters?

(OP)
Sirs, colleagues,
 the CSI-2400 spectraanalyzer cable, for accelerometers with the built-in amplifiers has broken down, - it is necessary to replace. Are there any pitfalls besides that cable mast be shielded (15 ì is required)?

With the best regards,
Dmitry Semenov

RE: Cable parameters?

2
If your accelerometer is a Piezo-Electric type with built in amplifier and not a charge-coupled accelerometer then you will be OK with just about any type of cable.  However, to avoid interference I would strongly suggest that the cable be screened twisted pair.  If you have multiple pairs (for multiple accelerometers) then each pair should be individually screened.  You can use coaxial cable but I would only use that as a last resort and only for portable data collector applications.

ron.frend@predicon.net

RE: Cable parameters?

(OP)
Greetings ronfrend!
     Doing cable repair, I have found out, to my surprise, that its screen is not connected to anything. So it cannot be grounded. Is it normal?
     I would like to learn if cable entrance resistance is of importance. How to choose entrance resistance of a cable?

Regards
D. Semenov

RE: Cable parameters?

It has become common practice to ground the screen at one end only - this is mainly for fixed accelerometer systems.  This is because if you ground at both ends and there is a difference in ground potential you will get a ground loop.  With very small voltages you can very large loop currents flowing which is very undesirable.  You should find that your cable is grounded at the collector/analyser end only - NOT at the accelerometer.

By cable entrance resisance I assume you mean nominal impedance?  If so don't worry too much about it.  This is only really critical if you are using charge coupled accelerometers (fairly rare nowadays) or if you are going to very high frequencies over a long cable length.  

ron.frend@predicon.net

RE: Cable parameters?

(OP)
Hi ronfrend!

Can you say how high mast be frequencies to be affected by nominal impedance of 10 m cabel? The accelerometer is a Piezo-Electric type with built in amplifier.


Regards
D. Semenov

RE: Cable parameters?

Fmax     =       10^9/(2*3.1402*C*V/(Ic-1))


 where, Fmax =  maximum frequency
    C    =  cable capacitance (picoFarads)
    V    = maximum output from sensor (volts) assumed @ 5V
    Ic    = constant current from signal conditioner (mA) - 2.4 mA
    109 = scaling factor to equate units

This is for cable length in feet and cable capacitance in microfarads per foot.

If you don't want to do the calc I have attached a table to give the max transmittable frequencies for cable length and cable capacitance.  Note that this is frequency transmitted along the cable - NOT Fmax.  This should equate to your sampling frequency or about double your Fmax.  I got this info from IMI in New York.

    Cable Capacitance (picoFarads/foot)
Cable Length (feet)    16    26    30    42
              10       142084   87436   75778   54127
              20        71042   43718   37889   27064
              30        47361   29145   25259   18042
              40        35521   21859   18945   13532
              50        28417   17487   15156   10825
              75        18945   11658   10104    7217
             100        14208    8744    7578    5413
             150         9472    5829    5052    3608
             200         7104    4372    3789    2706
             300         4736    2915    2526    1804
             400         3552    2186    1894    1353
             500         2842    1749    1516    1083
            1000         1421     874     758     541
            2000          710     437     379     271
            3000          474     291     253     180
            4000          355     219     189     135
            5000          284     175     152     108
           10000          142      87      76      54

RE: Cable parameters?

Speaking of Vibration cables, we are currently using a High Temp Low Noise cable, which as a layer of Kapton, but not as the primary insulator or outer cover.  The Electrical guy's are having fits about this cable being utilized in an aircraft because of the controverse over Kapton as it relates to aircraft fires.  Any help with this one?

RE: Cable parameters?

Kapton is being looked at with a lot of scruitiny by aircraft manufacturers. There has been plenty of focus on it by the FAA as well. I'd suggest a PTFE insulation such as MIL W-22759/8 as an alternative.

RE: Cable parameters?

Hello Dmitry, can one determine the cable parameter from the a dc step test? That is by applying a step dc input voltage and monitoring the output. Thank you
Lotten, South Africa

RE: Cable parameters?

(OP)
Hi, Lotten!
    I think you are right. Applying sine signal and wiring its frequency also can do it. But to do test??? – It never came to my mind because some one should know the answer with appropriate accuracy.

Regards,
D.Semenov

RE: Cable parameters?

Speaking of noise, does anyone know of a testing laboratory that can help us develop a test specification for cable with low triboelectric noise?  We are using ISA RP 37.2 which applies to accelerometers but has a paragraph devoted to cable.  Our sensors have high impedances (up to a couple of gigohms) in common with accelerometers, but the ISA standard does not adequately screen out noisy cable.  Our current spec is 5mV peak to peak.

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