V & dBm

Signal levels are often quoted in dBm (dB referenced to 1mW). To some radio engineers and technicians this is a meaningless figure, especially when it comes to SINAD measurement (in V for 12dB SINAD). To this end the following graph was created to assist in converting dBm to V at 50ohms (or vice versa).

Although the graph only covers the range -110 to -70dBm (typical of scratchy to solid comms) any voltage can be calculated by applying the '20dB per decimal point' rule. +20dB moves the decimal point 'one to the right' (multiply by 10) and -20dB moves it 'one to the left' (divide by 10). An example would be a signal of 0.3V would be -117.5dB, read by using the value for 3V (-97.5) and adding -20dB to that to move the decimal point one place to the left.

Useful figures.

0dBm = 1mW (regardless of impedance)
0dBm = 0.2236V @ 50ohms
-73.01dBm = 50V @ 50ohms (known as 'S9')
-97.5dBm = 3V @ 50ohms (generally accepted as a 'good' signal)

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