TABLE OF CONTENTS The deciBel (dB) represents a logarithmic ratio between two quantities. Of itself it is unitless. If the ratio is referred to a specific quantity (P 2, V 2 or I 2 below) this is indicated by a suffix, e.g. dB V is referred to 1 V, dBm is referred to 1mW.
Common suffixes
| suffix | refers to |
|---|---|
| dBV | 1 volt |
| dBmV | 1 millivolt |
| dB V | 1 microvolt |
| dBV/m | 1 volt per metre |
| dB V/m | 1 microvolt per metre |
| dBA | 1 amp |
| dB A | 1 microamp |
| dB A/m | 1 microamp per metre |
| dBW | 1 watt |
| dBm | 1 milliwatt |
| dB W | 1 microwatt |
Originally the dB was conceived as a power ratio, hence it is given by:
Power is proportional to voltage squared, hence the ratio of voltages or currents across a constant impedance is given by:
Conversion between voltage in dB V and power in dBm for a given impedance Z ohms is:
dB V versus dBm for Z = 50 ?
| dB V | V | dBm | pW | dB V | mV | dBm | nW |
|---|---|---|---|---|---|---|---|
| 20 | 0.1 | 127 | 0.0002 | 30 | 0.03162 | 77 | 0.02 |
| 10 | 0.316 | 117 | 0.002 | 40 | 0.10 | 67 | 0.2 |
| 50 | 0.3162 | 57 | 2.0 | ||||
| 0 | 1.0 | 107 | 0.02 | 60 |
