9.1.3 LO

A transmitter must radiate a stable carrier frequency to avoid adjacent-channel interference and to ensure correct demodulation—especially in SSB or ISB systems where the receiver must re-insert a suppressed carrier. Maximum stability is achieved with a crystal oscillator; however, fixed-frequency crystals limit the number of available channels. Early commercial radios typically provided 6–18 crystal-controlled channels, and frequency changes required physical crystal replacement.

To overcome these limitations, most modern transmitters employ frequency synthesis, which derives multiple carrier frequencies from a single, high-stability reference oscillator.

9.1.3.1 Frequency Synthesis

A frequency synthesizer can generate any carrier frequency within a radio’s band from one reference source, preserving its accuracy and stability (Figure 9.7). Two main approaches exist:

Direct method. The desired carrier frequency is produced by a combination of mixing, addition/subtraction, and frequency multiplication or division. Although precise, this technique creates numerous spurious components and requires extensive filtering.
Indirect method. The modern standard employs a voltage-controlled oscillator (VCO) locked to a reference frequency by a phase-locked loop (PLL). Programmable dividers set the output frequency digitally. Fractional-N PLLs allow fine frequency resolution while maintaining loop bandwidth. In many SDRs, direct digital synthesis (DDS) generates the carrier entirely in the digital domain, converted to RF by a wideband DAC.

Figure 9.7. Block diagram of the concept of frequency synthesis.

Temperature-compensated (TCXO) or oven-controlled crystal oscillators (OCXO) provide frequency stabilities better than 0.01 ppm, and precision transmitters may be GPS-disciplined for long-term accuracy.