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Source and loading impedance for unshield IO cable

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TomS2000

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Hi,

For unshielded IO cable it is recommended to use low pass filter at the exit of cable. The configuration of the filter depends on the source and loading impedance. Different high and low combination of the impedance requires different filters. But what is the source and loading impedance of the filter? Can anybody give some explanation? Thanks a lot!

Tom
 

jiripolivka

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Hi,

For unshielded IO cable it is recommended to use low pass filter at the exit of cable. The configuration of the filter depends on the source and loading impedance. Different high and low combination of the impedance requires different filters. But what is the source and loading impedance of the filter? Can anybody give some explanation? Thanks a lot!

Tom

There are two problems in your question:
1. All transmission lines exhibit the characteristic impedance, a quantity determined by its cross-section geometry. You must find your cable impedance first, then make sure that your sorce as well as load are preferably having the same impedance for the best signal transmission conditions, i.e. a low loss.
2. All filters are designed over an input and output impedance, so they can be tested between a signal source and load having the same impedance. Find any manual of filter design; specifying its impedance is the very first step of the design.

Many cables have e.g. 50 Ohms characteristic impedance, and most test equipment, too. Some cables like CATV and satellite IF have 75 Ohms; symmetrical lines have 600 Ohms, etc. Those impedances are "standardized" and with a standard test equipment, you need no special devices like matching sections to make measurements.

Unshielded cables may be a twisted-pair type; they are cheap but often radiate and also accept radiated emissions from sources around them. To reduce such unwanted interference, the low-pass filter is recommended but its stop-band must be designed to be above the spectrum limits of the desired signal being transmitted by that cable.
 

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