I would like to know how to find rationales if i have around 20 cables that need to be tested for EMC by using Measurement Techniques
Until now i was choosing the most longest available cable because it can act as an antenna resonant for frequencies but i want to know if i can find the rationales by using any Measurement Technique and use this cable for all the EMC Tests

 

Obviously, cables are passive parts, so why would you EMC test a cable? So I presume it is connected to something that generates some kind of signal. Without knowing what the cable is connected to and the nature of the device its like saying how long is a piece of string.
You may think the longest may be the worse case but it could also be the short ones, depends on the signals fed into the cables and how it is terminated.

 

Those are analog cables and connected to a device

 

Obviously, cables are passive parts, so why would you EMC test a cable? So I presume it is connected to something that generates some kind of signal. Without knowing what the cable is connected to and the nature of the device its like saying how long is a piece of string.
You may think the longest may be the worse case but it could also be the short ones, depends on the signals fed into the cables and how it is terminated.

Analog cables connected to device and both the device as well as cables are considered to be DUT

 

EMC will depend on frequencies involved. The length of the cable may enhance or retard any resonant frequencies in the signal you are transmitting over it. The length may also enhance or retard interfering external signals.

What is the range of cable lengths? If each cable varies by a small amount, it's not likely to bother you unless you are working with very high frequencies.

You are correct, in order to test the cables they'll need to be in situ.

Are you trying to minimize test time by testing only one cable? Is this finished product testing?

 

Cables all by themselves are susceptible to pickup of external signals. So while they are passive devices, they can certainly receive EMI that bothers what they are connected to. So it is very reasonable to test them. There are some specifications, and certainly the application will determine how much interference entering the cable is acceptable. And usually the specification specifies the length of cable exposed.

 

EMC will depend on frequencies involved. The length of the cable may enhance or retard any resonant frequencies in the signal you are transmitting over it. The length may also enhance or retard interfering external signals.

What is the range of cable lengths? If each cable varies by a small amount, it's not likely to bother you unless you are working with very high frequencies.

You are correct, in order to test the cables they'll need to be in situ.

Are you trying to minimize test time by testing only one cable? Is this finished product testing?

Typically the range of the cables are around 2 to 3m i am currently working from 30MHz to 1GHz for Radiated Emission and upto 2.4GHz for Radiated Immunity
Yes i am trying to minimize the setups with the Worst case cable and the product is still at the end of Development phase.
To minimize the time and cost i would like to have a measurement technique to measure all the cables and choose the worst cable which is emitting more radiation and the same apples to Immunity

 

To really be thorough, you'd want to test all of the cables.

To minimize time in the test lab it appears that you want to test only a few cables. I'd suggest a minimum of 3, your longest, your shortest and the cable closest to your mean cable length. If you see a worse trend in the shorter cable, you may want to test one of the less short cables. Same for the long cable, if things are worse with your longest cable then test one that's slightly shorter.

It would help to know what type of device you are testing as well. Devices with low frequency clocks, e.g. under 10 MHz or so generally don't have circuits that have GHz edge rates. For something like that you aren't likely to see any EMC issues. If you are working with RF devices, especially GHz range transceivers (wifi or cellphone), then you should absolutely test thoroughly.

I've taken some products through EMC testing that presented interesting results. One product had a TV tuner to allow the customer to input cable TV to it to have picture-in-picture capabilities. For that we needed to ground the incoming RG-6 shield to the chassis of the device. On one product we had a monochrome LCD. The default background of what we displayed was grey which was created by having alternate pixels turned on and off. Doing this created a huge spike at the pixel clock frequency. To fix this, we simply tested the device with a white background. Problem solved. In general, worst case fixes typically were copper coating the inside of the device case or other attempts at shielding.