This tool calculates the average of any number of signals with power values for each signal expressed in dBm.
Background
dBm stands for deciBel referenced to one milliwatt. It’s a unit that is often used in RF Engineering in the measurement of power.
The formula for converting power in milliwatt to dBm is:
PdBm = 10*log10(PmW)
Some common values are 30 dBm which is equivalent to 1 Watt and 0 dBm which is equivalent to 1 Milliwatt.
Test instrumentation like spectrum analyzers use dBm for displaying power measurements. Sometimes the user might want to compute the average of a number of signals. For instance the average of two tones as shown in the picture below measured on a spectrum analyzer.
Explanation of how to calculate average of dBm
Since dBm is a logarithmic value, dBm values cannot be added directly.
Instead we have to:
- Convert dBm to Watt
- Add Watt values to compute the sum
- Find the average of the total power in Watt – divide the sum by the number of values
- Convert the average in Watt back to dBm
The calculator on this page follows these steps to compute the average.
The running total calculator can be used for steps 2 and 3.
Example Calculation
The average of two tones with values -10 dBm and +20 dBm is +17 dBm.
Note that it would be a mistake to add the two and compute the average as (-10+20)/2 = 5 dBm.
The average of three CW signals with values +3 dBm, +4 dBm and +5 dBm is 4.08 dBm.
If the dBm values are identical, then the average power in dBm is the same as any one of the values. For instance, the average of three CW signals with values +3 dBm, +3 dBm and +3 dBm is 3 dBm.