EMC Question of the Week: March 28, 2022

Screen of a Spectrum Analyzer showing harmonics of a signal with a fundamental frequency of 1 MHz

Which specification of an EMI test receiver is used to determine the noise floor?

  1. 1-dB gain compression
  2. Nominal Dynamic Range (NDR)
  3. Displayed Average Noise Level (DANL) 
  4. Level Measurement Uncertainty (LMU)

Answer

The best answer is “c.” Displayed Average Noise Level (DANL) is a direct measure of the noise floor. This can either be with a specific resolution bandwidth (e.g. −120 dBm @1 kHz RBW) or normalized to 1 Hz (e.g. −150 dBm/Hz). The DANL is generally frequency dependent and usually increases with frequency. It is typically measured with the input terminated in 50 Ω (or the nominal impedance of the analyzer).   

1-dB gain compression is a specification that applies to amplifiers. It is the input power level that causes the amplifier gain to drop 1 dB relative to the linear gain.

The nominal dynamic range of an EMI test receiver is generally defined as the ratio of the largest to the smallest signal levels that can be measured without external attenuation or amplification. It is not a measure of the noise floor, although the noise floor generally determines the smallest signal levels that can be measured.

Level measurement uncertainty is not strictly defined and is a function of many parameters. Nevertheless, it's generally a measure of the accuracy and repeatability with which the test receiver is capable of measuring a narrow band peak well above the noise floor.

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