EMC Question of the Week: March 10, 2025

One way to reduce the thermal noise floor of an EMI test receiver measurement by 20 dB is to reduce the  

1. room temperature from 20 ^oC to 2 ^oC.
2. reference level from 20 dBm to 0 dBm
3. resolution bandwidth from 100 kHz to 1 kHz
4. frequency span from 500 MHz to 5 MHz

Answer

The best answer is “c.” The thermal noise power is directly proportional to the bandwidth. Reducing the resolution bandwidth by a factor of 100 reduces the noise floor by 20 dB.

The thermal noise power is also directly proportional to the temperature, but that's the temperature in kelvin. 20 ^oC is 293 K. 2 ^oC is 275 K. A temperature reduction from 20 ^oC to 2 ^oC is equal to a reduction from 293 K to 275 K, which corresponds to a power reduction of less than 0.3 dB. 

Changing the reference level makes the noise floor move up or down on the screen, but it doesn't affect the amplitude of the noise floor at all. Similarly, changing the frequency span has no effect on the noise floor as long as the resolution bandwidth doesn't change.   

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