9.1.4 Transmitter Performance Criteria

Transmitter performance is defined by a range of measurable characteristics:

Radiated power. Determines link range and quality and is treated in detail with propagation effects in Chapter 11.
Frequency stability. As noted earlier, the stability of the transmitter’s local oscillator frequency is critical, particularly in AM systems such as SSB where the receiver is required to re-insert the carrier. Frequency stability is normally quoted in parts per million (ppm). For SSB systems the requirement is around 1 ppm, while FM systems, which are not as frequency sensitive, require a carrier stability of approximately 5 ppm.
Spectral purity (spurious output). As we noted in Chapter 2, the modulation process produces not only the desired sum and difference frequencies but also a range of unwanted frequency components. The suppression of unwanted emissions—spurious, harmonic, or intermodulation components—is mandatory for electromagnetic compatibility. In the vernacular, a frequency spectrum that contains only the wanted frequency components is called clean and a spectrum with unwanted components is called dirty. Unwanted components are normally suppressed by filtering, and the output spectrum is normally quoted as the ratio of the unwanted components relative to the carrier level. Transmitters are specified by adjacent-channel leakage ratio (ACLR) and spurious suppression, typically ≥ 60 dB off-tune.
Linearity. For digitally modulated signals, linearity is quantified by the error vector magnitude (EVM) or third-order intercept point (IP3).
Efficiency. Power-added efficiency (PAE) is improved by modern amplifier classes (F, J, Doherty, GaN).
Phase noise and jitter. Affect coherence in multi-carrier and phase-sensitive systems.

The evolution from analogue AM/FM transmitters to digital, software-defined architectures has not altered the fundamental requirements of frequency stability, spectral cleanliness, and efficient power amplification—but the means of achieving them have become far more precise, compact, and programmable. Additionally modern transmitters have powerful capabilities to perform functions such as thermal and telemetry monitoring in which embedded sensors report forward/reflected power, temperature, and bias currents, enabling predictive maintenance and remote management.