Prior to the 1950s, the operating frequency of a military radio was controlled by a crystal oscillator, which necessitated multiple crystals for operation at different frequencies. The 1960s development combined a voltage-controlled oscillator with a programmable digital divider and embedded this within a phase-locked loop (PLL) to enable many operating frequencies to be “synthesized” from a single stable crystal oscillator by changing the division ratio. In the United Kingdom, both Plessey and Racal were pioneers in advancing these indirect frequency synthesiser military radio designs.
The Clansman HF, VHF, and UHF man-pack and vehicle-based combat net radio system was used by the British Army from 1976 to 2010. These military radios, which were constructed by Plessey, Racal, and Philips MEL, introduced single sideband operation and narrowband frequency modulation to forward area combat net radio. In the 1960s, at the early stage of microelectronics development, the programmable dividers and other synthesizer circuits were built at Plessey as custom integrated circuits (ICs) in resistor transistor logic. Radio frequency selection was controlled by manual switches to select the required divider ratio. Also, there was a necessity to introduce a fixed offset to control the different transmitter and receiver frequencies for a given operating frequency. The design of the loop filter was critical to minimize spurious signals and achieve acceptable switching time between channels. As all this was achieved well before the advent of computers, the IC designs had to be created manually on large sheets of graph paper before cutting Rubylith sheets to make the oversize masks required for the IC fabrication. I was a member of the frequency synthesizer group at Plessey from 1966 to 1970, designing the phase comparator circuits, after undertaking six months of IC design training at Plessey Caswell Research Center.
At the same time, Racal’s first HF synthesised manpack radio, Syncal, covered the band 2–7.999 MHz in 1-kHz steps with an intermediate frequency of 10.7 MHz, requiring the receiver synthesizer to operate from 12.7 to 18.699 MHz. In the 1960s, Racal Instruments also produced a 0.1–160-MHz synthesized signal generator, in 10-Hz steps, for automatic testing of the Clansman radios. These PLL synthesizers, which used early Fairchild digital dividers, were pioneered by Keith Thrower, who developed the required theory from scratch. For many years, Keith led Racal Research in Reading, England.
In 1978, Racal expanded this into its Jamming Resistant or Guarded Frequency Hopping Radio (JAGUAR). The JAGUAR V VHF combat net radio, with a 50-km range, hopped the transmission frequency over a band of frequencies making it much more difficult for the enemy to eavesdrop on, or jam, the radio communication. By the late 1970s, much of JAGUAR’s microelectronic circuitry was incorporated into a custom complementary metal–oxide–semiconductor large-scale integration chip, designed by Racal Microelectronics. The equivalent U.S. equipment was the Single-Channel Ground and Airborne Radio System. Slow frequency hopping was a very important development, as it later became an essential requirement of the first and subsequent generations of cellular radio transceivers to overcome transmission loss due to deep fading when located in propagation nulls. The company Vodafone, as spun out of Racal, is currently one of the major cellular operators.
Today, these PLL synthesisers have been superseded by advances in microelectronics that have enabled direct digital synthesis. Here, the signal is generated by fast digital sampling of the waveform, followed by digital to analog conversion and low-pass filtering to directly synthesize the required analog signal frequency.