In a digital radio telephone a pulsed transmitter is generally used, the radio frequency (RF) signal being generated in a phase-locked loop the reference frequency of which is obtained from a high-accuracy crystal oscillator, for example a voltage-controlled temperature-compensated crystal oscillator (VCTCXO). In this case the voltage-controlled oscillator of the transmitter cannot operate at the same frequency as the RF output signal, because the pulsing of the transmitter would cause excessive variation in the output frequency. Likewise, a phase-locked loop (PLL) is too slow to be able to correct the frequency error of the transmitter to the desired level at the beginning of the transmission time slot. Thus, digital modulation, for example Quadrature Pulse Shift Keying (QPSK) modulation, is performed more easily at a carrier frequency lower than the RF output frequency. For this reason, a system has been used in which modulation is performed at a lower intermediate frequency (VHF), which is then mixed with an unmodulated UHF frequency to produce the RF output signal.
Such a system requires the frequency of both the unmodulated (UHF) signal and the intermediate frequency (VHF) signal, to have a high degree of accuracy. In practice, the intermediate frequency signal and the UHF signal are each generated in a synthesizer or a VCTCXO, in which case the circuitry will be expensive, since, for example, the VCTCXO is an expensive special component.