The design of a modular microwave frequency synthesiser: synthesisers for an 8 GHz and 23 GHz digital radio

This paper describes the design and measurement results for an adaptable synthesiser module which is used in the transceiver units of microwave frequency digital radios. High capacity digital communications systems require high-quality, low-cost frequency sources. The synthesisers described are used as the local oscillators in microwave transceivers: the synthesiser´s output is used for up- and down-conversion of phase-modulated data. A considered approach to the design of the microwave synthesiser is required because digital radio links are sensitive to phase noise which is detected along with the desired modulation signal: the effect is a degradation of the system´s bit error rate performance. The effect of the synthesiser´s phase noise on the system performance of the digital radio is discussed and theory used to determine the required synthesiser phase noise performance goals is then introduced. The synthesiser uses a wide-tuning PLL topology based on YIG oscillator technology. The design uses a mix of technologies: low frequency circuitry (<7 GHz) is implemented on a hybrid multilayer PCB; the microwave modules include passive circuits (filters and power splitters on microstrip, frequency-doublers) and active circuits (MMICs). Depending on the output frequency requirement, some of the modules in the 23 GHz radio synthesiser can be replaced or discarded to form an 8 GHz radio synthesiser. The synthesiser topology described is capable of generating frequencies at C, X, Ku or K band (G, H, I, J, K band: new frequency band allocations)