Understanding dBmV and the dB
The dBmV is a voltage value relative to a reference – in this case 0 dBmV.
dB(1 mVRMS) – voltage relative to 1 millivolt across 75 Ω. Widely used in cable television and off air antenna signals, where the nominal strength of a single TV signal at the receiver terminals is about 0 dBmV. Cable and antenna systems use 75 Ω coaxial cable, so 0 dBmV corresponds to −78.75 dBW (−48.75 dBm).
The reference level was defined in the late 1950s and references the minimum voltage required by a television tuner to produce an excellent television picture. In the early days of television, voltages generated by antennas were measured in microvolts (uV). It was decided that 1000 microvolts at the input of the television tuner was the minimum required to produce excellent video image quality. Of course other factors also affected the tuner’s performance but it was generally accepted that 1000 microvolts was the target value. In an effort to simplify the large numbers associated with using the microvolt scale, the decibel-milivolt scale was created and the 1000 microvolt requirement was represented as 0 dBmV.
Voltage values above and below the reference of 0 dBmV are incremented in decibels or “dB.” Therefore, the difference between 6 dBmV and 2 dBmV is 4 dB. Often the short-hand way of speaking about the dBmV scale confuses people. For example, the output of an amplifier may be 44 dBmV but is incorrectly stated as “44 decibels” when in fact it should be said as “44 decibel-millivolts.
Digital television tuners operate with a lower minimum requited voltage or about -12 dBmV. You will note this is 12 dB below the reference level of 0 dBmV for analog TV. Also, digital signal measurements made by newer signal level meters are the result of a sequential set of small measurements, combined and averaged to produce an “average digital power” measurement. Despite the measurement process, the result is reported as a dBmV value.
Confused? Don’t be. Just remember that for the new digital television tuners, you need a signal level between -12 dBmV and 16 dBmV.
Steve Zahn, Doc. #113005C
© 2011 Used By Permission