Custom cable for PXIe-6535

[PragmaTest]

Hi,

I want to build a custom cable for a NI PXIe-6535 card : VHDCI for PXIe-6535 end, 4 subd9 for the other end.

If I take the SHC68-C68-D4 or the C68-C68-D4 as models, I come with 3 questions :

1) Is it necessary to build a 50 Ohms cable ? My signals will be 30kHz max.

2) Is it necessary to have twisted pairs ? If yes, can I build pairs with 1-2 pins, then 3-4, etc...

3) Is it necessary to have a shielded cable ? If yes, how should be the shield ? One shield per twisted pair, or one shield for the whole cable ?

Sébastien

[PragmaTest]

Edit :

30 kHz is not the frequency of my signal but my max acquisition rate.

3 kHz is my commonly used acquisition rate.

[Cdkey]

Hello SebastienM,

I can give you some answers to your questions:

1) The matching impedance is necessary depending of your cable length. If it is smaller than the wavelength (some people say 1/20), you can make your wire without 50 Ohms, but you won't be able to increase the frequency, so it is recommended that you build your wire with the impedance matching.

2) Twisted pairs are used to avoid the noise, they are highly recommended, you can twist a wire with a GND for example.

3) For the shielded cable, it is the same than twisted pairs, it is not an obligation, but it is recommended. And it is one shield for the whole cable.

If you have some other questions, do not hesitate to ask us.

Have a nice day.

Best regards,

Clement Dekeyser

[PragmaTest]

Hi Cdkey,

1) If I consider my signal 3kHz max, with an only 2 meters cable l'm very far away of the 1/20 rule (I find something between 2,5 km and 5 km by calculation). So I guess it's very safe in my case to not use a 50Ohm cable. Are you ok with that ?

2) Ok for the twisted pair. The problem is more about the pairs themselves because of the PXIe-6535 pinout. I can't find a good way to associate a wire with another wire because of the pinout.

3) OK for the shield.

Thanks,

Sébastien