Adjacent Channel Power (ACP) is basically the ratio of the energy produced in the wanted and adjacent channels when the Transmitter is in steady state mode. Transient Adjacent Channel Power (TACP) is simply an extension of this, being the ratio of the energy produced in the wanted and adjacent channels when the Transmitter is keyed up or down. Transient ACP is measured in the frequency domain. A related parameter “Transient Behaviour” is measured in the time domain.
When the transmitter is keyed up or down (in other words, the user presses the radios PTT key to talk, or releases the key at the end of the conversation), there are other factors that come in to play that can (and do) momentarily increase the amount of unwanted power produced in the Adjacent Channels. For example, the transmit oscillator may see a change of load as all the following transmitter stages are powered up (they are switched off in Receive mode in the case of Portables and Mobiles). This change of load may have the effect of briefly knocking the oscillator off frequency, thus spreading more power into either the upper or lower adjacent channel. Similarly, when the transmitter stages are powered up, the power supplies see a rapid change in loading and thus the dc voltages may bounce slightly, superimposing noise directly onto the output of the transmitter stages.
It follows then that the Transient ACP performance is always worse than the steady state Adjacent Channel performance.