GB654959A - Improvements in high frequency systems particularly for object detection and warning - Google Patents

Abstract

654,959. Automatic frequency control systems. SPERRY GYROSCOPE CO., Inc. Nov. 20, 1944, No. 23085. Convention date, Nov. 24, 1943. [Class 38 (iv)] [Also in Group XL (c)] In an ultra high-frequency radio system for the detection of moving objects, as shown in Fig. 1, a signal is radiated from a directional antenna 23 energized by an electron tube oscillator 11 of the velocity modulated reflex type, the signal frequency being adjustable by variation of the voltage applied to the reflector electrode 16 by the tapped voltage divider 18. An object passing through the signal beam reflects a signal 61 to the directional antenna 24 which also receives a weak direct signal 62 from the transmitting antenna. The reflected signal varies slightly in frequency from the transmitted signal by an amount depending on the velocity of the moving object towards or away from the transmitting antenna, according to the Doppler effect. The reflected and direct, signals combine to feed a signal, weakly amplitude and frequency modulated at the Doppler difference frequency, to the mixer 26 where it is combined with the output of a velocity modulated reflex local oscillator 28 to produce an amplitude modulated intermediate frequency which is fed to the amplifier 35, detector-amplifier 40, and indicator 50. The frequency modulation is negligible. The amplified I.F. signal is also applied to a discriminator tuned to the amplifier frequency, the output of which is combined with a steady voltage from tapped voltage divider 53 and the resultant applied to the reflector electrode 34 of the local oscillator 28, automatically controlling its frequency to maintain the I.F. frequency constant. A switch 49 enables the polarity of the discriminator output to be reversed so that the local oscillator frequency may be controlled with stability at a value either above or below the signal frequency. The local oscillator is adjusted so that when equilibrium is reached, its controlled frequency is such as to produce an I.F. displaced slightly from the frequency to which the amplifier is tuned and at which the discriminator output is zero. The discriminator output accordingly varies with the amplitude modulation of the signal, thus frequency modulating the I.F. signal at the Doppler frequency. As the mean intermediate frequency lies on the sloping part of the I.F. amplifier response curve, the F.M. is converted into strengthened A.M. to reinforce the original A.M. of the received signal, and a regenerative action takes place to considerably increase the percentage A.M. of the signal, which operates the indicator 50. In a modification (Fig. 7, not shown), the output of the I.F. amplifier is rectified to produce a weak signal at the Doppler frequency which is applied through a high-pass filter to frequency modulate the local oscillator. The I.F. output is also applied through a limiter to a discriminator providing a Doppler frequency signal which is amplified to operate the indicator, and also a unidirectional signal which is applied through a low-pass filter to the reflex electrode of the local oscillator to maintain constant the average intermediate signal frequency.