Patent GB-601136-A (GB)
Title: Improvements in or relating to pulse lengthening circuits and radar systems embodying such circuits
IPC: G01S13/42
Filed: 1945-08-10
Abstract: 601,136. Pulse demodulators. OXFORD, A. J. H. Aug. 10, 1945, No. 20645. [Classes 40 (v), 40 (vi) and 40 (vii)] A pulse lengthening circuit for demodulating amplitude-modulated pulses comprises a capacitor charged by each pulse at a time constant of the order of the pulse length and discharged by a valve after a predetermined time less than the pulse interval, to produce a stepped waveform interrupted at the pulse recurrence frequency and conforming to the modulation envelope. In a radar system employing conical scan with automatic servo control, a strobing pulse controls a gating circuit to allow only echoes from a selected target to pass to a demodulating circuit according to the invention, which forms part of the control circuit for the servo arrangements. The conical scan has a periodicity of 25 c/s. so that a misalignment of the antenna produces amplitude modulation at 25 c/s. on the pulse train from the selected target, the amplitude and phase of the modulation envelope depending upon the direction and amount of the misalignment. The pulse train passes exclusively to the demodulator which produces a continuous sinusoidal voltage representing the cyclic displacement of the lobe axis from the reflecting target. The voltage is divided into azimuth and elevation error voltages by a sector switch rotating in synchronism with the antenna. Each sector is connected to earth through a resistor and capacitor in series, and the capacitor is charged to the average potential of the error wave during the time of make for the sector. The difference in potential of opposite sectors is proportional to the amount off-target in azimuth or elevation, and the pairs of voltages are applied to direct coupled differential amplifiers driving split-field azimuth and elevation motors which operate to cancel the error voltages and keep the antenna aligned on the target. Alternatively, the error voltages from the switch may be displayed on misalignment indicators as described in Specifications 600,512 and 601,127. Time-base circuit.-In the Figure, capacitor C1 is normally charged through resistors R1, R2 and valve V2 conducts. A positive locking pulse at terminal 15 reacts as a negative pulse through diode V1, charging capacitor C1 negatively to render valve V2 non-conducting. Tuned anode circuit L1, C2, C3 is shock-excited and after a fraction of a cycle, capacitor C1 has re-charged sufficiently to restore conduction. in valve V2 and damp out further oscillation. The resultant saw-tooth waveform is applied to the X-plates of indicator 8 through capacitor C4 and also to the grid of strobing valve V3. Strobing circuit.-When the saw-tooth voltage neutralizes, the negative bias from the strobe position control VR2, valve V3 conducts and the resulting negative waveform at the anode is differentiated by circuit C6, R4 and inverted in valve V4. The pulse developed in resistor R6 is applied to the grid of the indicator 8 as a strobe spot which can be superimposed on the selected target by manual control VR2. Demodulating circuit.-The positive strobe pulse from valve V4 is fed to the suppressor grid of gating valve V5 and video signals are injected at terminal 16. Valve V5 conducts only at a coincidence of strobe and video pulses and charges capacitor C11 negatively, the subsequent discharge through R11, C12 producing a preliminary pulse lengthening. Valve V6 always conducts and at the incidence of a selected pulse charges capacitor C13 to a level proportional to the pulse amplitude through diode V7, which prevents discharge until valve V8 is rendered conducting by a positive locking pulse at terminal 15a. The lengthened pulses retain the 25 c/s. amplitude modulation of the selected pulses, and are smoothed by stray capacities in cathodefollower valve V9, to produce a sine waveform across resistor R15 for amplification in valve V11 and application to the sector switch through terminal 18. Automatic gain control.-The 25 c/s. error voltage is applied to the Miller valve V10, which smooths out the 25 c/s. variation and produces at the grid a voltage proportional to the average signal amplitude over several cycles. The output at terminal 17 is applied to the grids of the intermediate frequency amplifiers in the receiver, control VR4 determining the maximum screen voltage and VR3 the threshold control for A.G.C. Specification 601,124 also is referred to.