As discussed in our previous blog post, the Transient Adjacent Channel Power (TACP) is simply an extension of the ratio of the energy produced in the wanted and adjacent channels when the Transmitter is keyed up or down. In reality, the unwanted energy produced by the Transmitter doesn’t just spread into the adjacent channel, it also spreads out far beyond that point. Tx Noise, therefore, is a measure of the unwanted transmitted energy at some specified offset from the wanted frequency.
Eventually, as we go further and further away from the carrier, the Tx Noise will bottom out at a value after which no improvement is seen. This is known as the Tx Noise Floor. This is illustrated below:
Tx Noise is usually quoted as a discrete level in dBm/Hz or a level relative to the carrier level, dBc/Hz.
Generally, the Tx Noise floor beyond the Adjacent Channel is only measured for Base-Station transmitters for reasons that will become apparent later.