Before explaining Adjacent channel power, it is necessary to make mention of Electrical Noise. This subject will be covered more deeply later, but for now understand simply that noise, as it relates to communications systems, is defined as an unwanted random fluctuation in an electrical signal.
As we have seen previously, the Tx signal is produced by a locked oscillator within the Synthesizer. However, the output produced by such oscillators is not totally “clean.” That is to say that it does not just contain energy at the wanted frequency (the ideal case), it also contains some lower amounts of energy (i.e: noise) at unwanted frequencies too (the real case).
The Adjacent Channel Power of a Transmitter then, is simply a ratio of the power transmitted in the wanted channel to the power un-intentionally transmitted in the neighbouring channel, i.e:
Transmitter Adjacent Channel Power is measured in dBc. All this means is the ratio of the power in the adjacent channel relative to the wanted carrier.
In reality, there are other sources of noise apart from the oscillators. For example, the power supplies for the oscillators and Pre-Driver, Driver & Final stages ay contain noise that is superimposed directly on the output signal. The higher power Transmitter stages like the Final may also produce noticeable noise outputs of their own too.
Note: That for the ACP parameter, the carrier is not modulated so the power in the adjacent channel largely consists of noise. A related parameter exists, known as Sideband Spectrum, which measures the power in the adjacent channel relative to the wanted in the presence of modulation. In that instance, the power in the adjacent channel consists not just of noise but of modulation artifacts too.