EMC Question of the Week: September 16, 2019
On an oscilloscope or spectrum analyzer display, dBm is an abbreviated way of representing the unit,
- dB(meters)
- dB(mV)
- dB(μV)
- dB(mW)
Answer
The correct answer is "d". dBm is a unit expressing decibels relative to 1 milliwatt. It is the default display unit on most spectrum analyzers and EMI test receivers. It is equivalent to 10 times the log of the signal power expressed in milliwatts.
On a 50-Ω spectrum analyzer, the signal power in dBm can be easily converted to the signal voltage in dB(μV) by adding 107 dB. In a 75-Ω system, the same conversion is accomplished by adding 108.8 dB.
Decibel notation is generally applied to measurements of signals with a well-defined and quantifiable signal power; so it isn't clear how dB(meters) would even be defined. Nevertheless, even if dB(meters) were a well-defined quantity, it might be abbreviated dB(m), but not dBm.
dB(mV) and dB(μV) are well defined units, but neither has a shorter abbreviated form in common use.
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