Abstract: A mixer incorporating FET's (4,5) as the switching elements includes a resonant circuit connected as a tank circuit of the local oscillator (6) such that the inherence capacitance of the FET's is automatically compensated for. The harmonic recovery technique is also used by the inclusion of rejection filters at the input and output of the system, which rejection filters have reciprocal impedance characteristics, such as short circuit/open circuit or the like.

Abstract: A dynamic frame length slotted ALOHA communication system has a central station which monitors the traffic congestion and determines the optimum frame length. This is transmitted to the sub-stations in the form of a service information message when there is to be a change in the frame length but not, as is conventional, at the beginning of each frame. The amount of time available for the useful transmission of communication messages is thus optimized.

Abstract: In a dynamic frame length slotted ALOHA system the synchronizing signals are transmitted by a base station at different frequencies (each having a period equivalent to a multiple of the time slot period). The different frequencies denote different frame lengths and the central stations detect these frequencies and select a frame length accordingly.

Abstract: A robust amplifier consists of two delay lines linked by a number of discrete amplifiers. Distortion correction is carried out by distortion feed forward. This is achieved by sampling the output of each amplifier at a coupler and then subtracting this sample from the input signal to later amplifiers along the delay line at directional coupler. As a result each discrete amplifier's distortion is corrected by all of the following discrete amplifiers in the amplifying circuit.

Abstract: A distributed amplifier is provided with an output filter line with the output terminals of a plurality of individual amplifying devices connected to different points along the output filter line. Instead of dimensioning the impedance of the output filter line to provide a perfectly uniform frequency response, it is arranged as a wide band multiple quarter wave impedance transformer. The use of this kind of output filter line provides superior performance when the amplifier is used with mismatched loads and the frequency response can still be very good.

Abstract: A high frequency power amplifier consists of a large number of serially connected transistors. The base connections of the transistors are connected to a potential divider consisting of reactive components, usually capacitors, which form part of a tuned circuit which is resonant at the frequencies at which the amplifier is to be used. The resonant currents flowing in the potential divider ensure that sufficient base current is available for the transistors to provide the required amplification.

Abstract: A frequency generator provides a number of selectable predetermined spot frequencies to a high degree of accuracy in an economical manner. The output from a phase lock loop having a variable frequency divider is divided by a further variable frequency divider. The two dividers are controlled by pairs of divisor values held in a store.