Abstract: An M-ary FSK receiver in which a frequency down-converted signal is treated as an M-ary DPSK signal with symbol repetition coding. The output of a frequency demodulator (20) is integrated (22) over an optimally chosen time interval which is a fraction of the symbol interval to obtain N phase change estimates per symbol. These N sub-symbols are manipulated (24, 26) to obtain corresponding soft decisions which are then summed (32, 34) over a symbol period (or a substantial fraction thereof) to provide an overall soft decision for each bit comprising an M-ary symbol. These outputs may be used directly as soft decisions for forward error correction or applied to a threshold circuit to obtain hard decisions.

Abstract: A method of, and apparatus for, symbol timing recovery, comprising asynchronously sampling a signal burst at a predetermined plurality of times per symbol period, combining corresponding samples occurring in successive sampling cycle periods, making a soft decision on each combination of samples, and selecting for demodulation a timing offset(i) giving the largest soft decision.

Abstract: A method of, and receiver for, receiving and demodulating M-ary FSK symbols, where M equals two or four, comprising over sampling a received signal to obtain sub-symbols which are treated as repeated DBPSK .pi./4DQPSK symbols, respectively. The sub-symbols are applied to a delay and multiply demodulator in which the duration of the delay has been optimised to give M equidistant points on a unit circle. The output from the demodulator comprises log likelihood ratios which are then integrated in an integrating filter to give a maximum likelihood estimate of the bits comprising the symbols transmitted.

Abstract: A digital modulator in which symbols representing bits are encoded and assigned to respective points of constellation of points representing the modulation scheme. The quadrature components of a vector representing each of the constellation points are contained in a truth table (14) in which the columns comprise the respective constellation points and the rows comprise the relative in-phase and quadrature component values. In response to the assignment of a symbol to a constellation point the appropriate column in the truth table is read out in parallel as a binary word and the bits of the word are applied to respective first stages (S10) of a plurality of shift registers (SR1 to SR8) which store the histories of the respective component values for the previous (N-1) symbols.

Abstract: A frequency multiplying circuit comprises a differential amplifier formed by two transistors whose emitters are connected by a current source to a low voltage supply rail, collector resistors are connected to a higher voltage rail as is also done with the base electrode of one of the transistors, the base electrode of the other of the transistors has an oscillator connected thereto. An LC resonant circuit coupled between the collectors of the two transistors and is tuned to select either the basic frequency of the oscillator or an odd harmonic thereof.

Abstract: A voltage controlled oscillator having an electrical means of frequency control which is able to operate from a supply voltage which is lower than is desirable to use with a "VARICAP" diode. The oscillator comprises a pair of transistors whose base and collector electrodes are connected together, the base electrodes being connected to a supply rail and the collector electrodes are connected to a tunable parallel LC resonant circuit. A frequency determining device, e.g. a crystal, is connected between a tapping of the resonant circuit and the emitter of one of the transistors to form a first feedback path. The crystal together with an integratable small value capacitor connected between the emitters of the two transistors form a second feedback path. The phase shifts due to the first and second feedback paths are summed vectorially at the collectors of the two transistors. By varying the gain of each of the two transistors then the resultant phase shift, and thereby the frequency of oscillation, are varied.