Abstract: Methods of integrating mobile radio systems with paging systems. Paging messages are identified by way of predetermined symbols in a symbol stream which is modulated into an RF signal. The symbols may also enable synchronisation of the receiving station to the signal and/or convey individual status messages. Symbols are preferably derived from existing symbols in the mobile radio protocol. Multiple symbols may be used to represent individual bits in the message and thereby extend the range of transmissions in the system.

Abstract: A control loop (10) for producing an output signal with a stable nominal frequency is provided. The control loop includes inputs for reference (11) and oscillator (25) output signals, a beat frequency generator (12) for producing a signal with a frequency that is the difference between the oscillator and reference signal frequencies, an ADC (14) to convert the beat frequency to a digital beat frequency signal, an estimator (17) for estimating the frequency or phase of the beat signal, an adder (18) for combining an offset and modulation signal and the estimated frequency or phase of the beat signal into an added signal, and a DAC (23) for generating an analogue control signal for controlling the oscillator output frequency.

Abstract: The present invention relates to a double balanced diode mixer in which an output transformer (22) is replaced by an equivalent circuit (50) which approximates the functionality of the output transformer (21). The circuit includes a LCR network (50) which receives a mixed differential input signal from a diode mixer (21) and generates a single ended output signal and further implements a diplexer which terminates the RF input signal.

Abstract: The invention relates to a double balanced diode mixer (10) in which an input transformer (20) is replaced by an equivalent circuit (50) which approximates the functionality of the input transformer (20). The circuit includes an LCR network (50) which filters an input RF message signal and further includes a portion which approximates a functionality of transformer to provide RF differential output signals (61, 62) for transfer to a diode ring (21).

Abstract: A zero intermediate frequency radio receiver that comprises mixing stages, an A to D converter, a digital demodulator, a carrier tracking system and a carrier nulling system is disclosed. The mixing stages receive FM or PM signals and produce analogue in-phase and quadrature baseband signal which are in turn converted to digital baseband signals by the A to D convertor. DC offsets in the baseband signals are removed by high pass filtering prior to demodulation. Tracking enables the baseband signals to be maintained at a centre frequency of substantially zero hertz, while signal strength at this frequency is minimized or mulled before the DC offset filtering. Information which might otherwise be removed by the filtering is thereby retained in the baseband signals, which are relatively less distorted.