Patent GB-682239-A (GB)
Title: Improvements in or relating to radar systems
Inventor: BRANDON PERCY SAMUEL
IPC: H03D9/06
Filed: 1949-06-24
Abstract: 682,239. Radiolocation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Feb. 16, 1950 [June 24, 1949], No. 16875/49. Classes 40 (v) and 40 (vii). [Also in Group XL (b)] In an F.M. radar system, radio-frequency energy from the transmitter is modulated by energy from a local oscillator to provide a signal comprising a component of the transmitter frequency and two components of sideband frequencies, this signal being mixed with the received echo signals in a non-linear device, the output of which includes components corresponding to the side-bands and carrier which would be produced by modulating the output of the local oscillator by the difference frequency between contemporaneous transmitted and received signals, so that, by rectification of the output of the non-linear device, a signal of the difference frequency (which is proportional to the range of the corresponding echo-producing object) may be obtained. A signal of frequency ft is fed from the transmitter in the area H3 of a " Magic-T " circuit, Fig. 1. The arm H1 of the circuit is terminated by a crystal X and the arm H2 is terminated by an impedance (which may be another crystal) approximately equal to that of the unexcited crystal. A signal of frequency fi, derived from a local oscillator, is applied to the crystal X, whereby the impedance of the crystal is varied and the circuit behaves substantially as a balanced modulator, side-band components of frequency (ft +fi) and (ft - fi) being delivered by the arm E. Also, owing to a slight initial unbalance between arms H1 and H2, a small component of the carrier frequency ft is also delivered. If the arm H2 is also terminated by a crystal, this is excited in antophase with respect to that in the arm H1. The output from the arm E is applied to the silicon crystal X2, Fig. 2, together with the received echo signal of frequency (ft+fb), fb being the difference frequency due to the time delay between the transmitted and received signals. The crystal output includes components of frequencies fi, (fi+fb), (fi - - fb) which are applied to an I.F amplifier IFA tuned to the local-oscillator frequency fi. The output from amplifier IFA is rectified in detector D to derive an output of the difference frequency fb which is used to measure the range. In this system, the output of the crystal X2 is of higher frequency than would be achieved by direct mixing of the transmitted and received signals, whereby the Specification states the signal-to-noise ratio is improved. It is stated that, as a modification of the invention, the signal of local-oscillator frequency may be added in quadrature to the side-bands (fi+fb) and (fi-fb) to produce effectively frequency-modulation of the local oscillator frequency by the beat frequency. The Specification also describes a system for increasing the signal-to-noise ratio which does not fall within the scope of the invention. In this system, the transmitted and received signals are initially heterodyned with the output from a local oscillator by means using a silicon crystal, to frequency-change this signal to a comparatively low frequency at which they are heterodyned by means not using silicon crystals to derive the difference frequency. The local oscillator may be provided with automatic frequency control so that it follows the transmitter in variation of frequency and substantially constant-frequency signals are derived from the frequency changing. As an alternative, the local oscillation may be derived by heterodyning the transmitted signal with a constant-frequency local oscillator, whereby the local oscillation would maintain a constant frequency difference with the transmitted signal.