Abstract: A radio transmitter includes a bank of pre-distortion circuits each of which is selectively connectable between the mixing stages of the transmitter and the final linear power amplifier stage of the transmitter. The pre-distortion circuit to be connected is determined by the RF frequency of the signal to be transmitted. A method of analyzing the distortion produced by a linear power amplifier is also described. In this method two tones are applied to the amplifier and the resulting down-mixed signal is subjected to a FET in order to derive the spectrum and plot the amplitude and phase distortion against the two tone amplitude. Analysis of the results of such testing allows the pre-distortion circuits to be designed to compensate for distortion produced by the amplifier over the complete operating frequency range of the transmitter.

Abstract: An interferometer type DF system uses an array of five antennas (A,B,C,D,E) arranged at the apices of a regular pentagon to define five wide apertures along the sides of the pentagon and a further five apertures along the diagonals. The phases of the signals received by each antenna, are measured modulo 2.pi. and processed to give a unique bearing of the radio source to the accuracy of the widest aperture defined by the array. One method of processing the phases is to calculate from them the Fourier coefficients of the Fourier series representing the spatial phase distribution. By comparing the difference between each calculated coefficient and a corresponding order coefficient of a set of imaginary antenna phases expressed as integral multiples of 2.pi., the complete 2.pi. phase differences between the measured phases modulo 2.pi. can be found.

Abstract: A direction finding system has two antennas with response patterns at 90.degree.. A sense antenna 14 is mounted to produce a sense output dependent on the received radio signal but independent of its bearing. A commutating and combining arrangement receives the antenna outputs and produces a sequence of control signals each of which represents a respective one of the following: the North-South output, the East-West output, the sum of the NS and EW outputs, the difference between the NS and EW outputs, the sum of the NS and sense outputs, and the sum of the EW and sense outputs. A receiver produces an output having a magnitude dependent on any amplitude modulation on the received radio signal but independent of its bearing.

Abstract: A circuit for rapidly assessing communications activity on a particular radio channel being jammed has a filter tuned to the receiver IF and to the input of which the incoming signal (at the receiver IF) is switched. The waveform of the resultant filter output for the signal on the particular channel--that is, falling within the filter passband--is produced in conjunction with interfering signals in adjacent channels. It is shown that the nulls in the waveform of the interfering signals occur at time instants dependent on the passband of the filter, not on the frequency of the interfering signals. The circuit therefore is timed to measure the level of the filter output at the instant of one or more of the nulls of the interfering signals, thus avoiding interference by the latter.

Abstract: The invention relates to phase locked loops in which a variable-frequency voltage-controlled oscillator (VCO) feeds a phase comparator via an adjustable divider having a division factor N. The comparator compares the phases of the divided frequency (Fd) and a reference frequency (Fr), and adjusts the VCO to produce phase equality. To enable the VCO frequency (FO) to be adjusted in smaller steps than Fr and yet maintain a loop bandwidth greater than the step size with good spurious performance (thus providing a "fractional N synthesizer"), Fo is made slightly more than N.Fr. The phase detector thus produces a phase error signal. At periodic instants, determined by the period of the frequency difference between Fo and N.Fr, a control unit temporarily increases N by unity so as to bring Fd and Fr into phase.