Patent GB-1493808-A (GB)
Title: Doppler radar system
IPC: G01S7/292
Filed: 1974-11-29
Abstract: 1493808 Radar MULLARD Ltd 29 Nov 1974 51762/74 Heading H4D [Also in Division G4] As described, a Doppler radar system of the kind in which two beat signals having a relative phase difference are produced from received echo signals, comprises a circulator 13, Fig. 2, one port 14 of which is connected to a transmitter (e.g., a Gunn oscillator) and other ports 15, 16 of which are respectively connected to an aerial and to first ports 19, 20 of two hybrid rings 17, 18. The connection to the port 20 is longer than that to the port 19, so that there is a phase lag, e.g. of 90 degrees, between the echo signals fed to the hybrid ring 18 and those fed to the hybrid ring 17. A portion of the microwave power from the transmitter is fed via a coupler 21 to second ports of the hybrid rings. Sum signals are fed from third ports of the hybrid rings 17, 18 to respective mixer diodes 22, 23, the outputs of which are applied to respective capacitors 24, 25. There are thus produced two identical beat (or Doppler) signals of audio frequency, which are nominally  90 degree phase spaced, the sign depending upon whether a target is approaching or receding. The capacitors are connected to the inputs of amplifiers 6, 7, Fig. 5, having gains sufficiently high to produce, in the absence of beat signals, outputs which are due to noise in the system. It is convenient to clip the output signals from the amplifiers 6, 7 (Figs. 3, 4, not shown). Said output signals are applied to a phase detector 8 which is, preferably, a D-type edge-triggered flip-flop having a clock input to which one signal is applied, a D input to which the other of the signals is applied, and complementary Q, Q outputs (as described, only the Q output is used). The phase detector is so arranged that each time the amplifier output signal at its clock input attains a triggering level it produces an output signal having either a first level or a second level according as the amplifier output signal at its D input has an upper or a lower level, and maintains that output signal level until the amplifier output signal at its clock input next attains the triggering level, when the output signal level changes only if there is then a change in the level of the amplifier output signal at said D input. With a noise signal input alone the phase detector output signal has an average level which is between said first and second levels. If the level of the phase detector output signal changes from this average value so that either the first or the second level predominates, it is an indication that related fixed phase beat signals are present as well as noise. As shown in Fig. 5, the level of said output signal is developed across a capacitor 29, which is fed through a resistor 28. The time constant of this RC combination is selected having regard to the expected noise-to-noise correlation time in the system. A threshold element 27 comprises two transistors 30, 31 connected as a differential pair between voltage supply lines  V via a common emitter resistor 32 and respective collector resistors 33, 34. The base of the transistor 30 is connected to the capacitor 29 and that of the transistor 31 to a reference voltage V R . Normally, with the average level appearing at the capacitor 29, the transistor 31 is non-conductive. If the level at the capacitor 29 decreases to the threshold value the transistor 31 becomes conductive, and a change of voltage at a collector terminal 36 signifies the presence of an approaching target. If an indication of a receding target is required, similar circuitry is provided which is connected to the complementary Q terminal of the phase detector 8. The frequency of either of the beat signals can be measured to determine the velocity of the target. In another embodiment, shown in Fig. 6, a capacitor 42, which is connected to base of a transistor 43 is chargeable through a resistor 41, and is dischargeable through a resistor 40 and a transistor 39. When the output voltage level from the phase detector 8 falls, due to an approaching target, the transistor 39 is rendered non-conductive, and the voltage across the capacitor 42 rises. This causes the transistor 43, the emitter of which is clamped to about half the supply voltage by means of Zener diodes 45, 46, to become non-conductive. The consequent change of voltage at a collector terminal 47 signifies the approaching target. If desired, the output signals from the amplifying means may be passed through non-linear circuit means which causes their mean or average level to be different from the level which is midway between their most positive and most negative swings. The system may be used for traffic signal control or for the detection of intruders. Specification 1,357,457 is mentioned.