Patent GB-750133-A (GB)
Title: Improvements in or relating to automatic tuning systems
Inventor: MILLER WILLIAM FRANCIS
IPC: H03J7/16
Filed: 1953-06-10
Abstract: 750,133. Automatic frequency-control systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 28, 1954 [June 10, 1953], No. 16024/53. Class 38 (4). The receiver local oscillator LO of a radar transmitterreceiver is swept over a frequency range by a search motor M1 until the difference frequency between the transmitter pulses and local oscillator is within a predetermined frequency difference from the desired value, whereupon (1) the search motor M1 is stopped; (2) a second motor M2 responsive to the frequency difference between the local oscillator frequency or a harmonic or sub-harmonic and the resonant frequency of a cavity resonator CP is thereupon energized to tune the resonator CP till the said frequency difference is within a predetermined value from zero; (3) electronic tuning means V2 then adjusts the local oscillator frequency to accord with the cavity resonator acting as a standard. As shown, the search motor M1 is responsive to the output of a discriminator AFCD energized from a mixer AFCM to which the transmitter frequency and local oscillator frequency are applied through attenuators A4, A1. When the discriminator response range is reached the discriminator output energizes through valve trigger circuits one or other of relays RL1B, RL2B (Fig. 3) controlling a polarized relay RL4 so that the contacts RL4A, RL4B change over and the energization of the search motor M1, e.g. a shaded pole type, is reversed. At the same time a narrow bandpass signal presence detector becomes energized within the discriminator control range and operates a relay to open contact RL3A in the common supply lead to stop the search motor M1. The local oscillator frequency is fed through an attenuator A3 and rectifier MR1 to a loop L1 coupled to the cavity resonator CP and having adjacent it a loop L2 which screens the loop L1 alternately every half cycle of a modulating frequency, e.g. 60 kc/s., applied to the loop L2 from a modulating source MO through a rectifier MR2. The phase sense and amplitude of the modulated voltage reflected back from the resonator CP to the loop L1 depends on the sense and degree of mistuning of the resonator CP (due to its sidebands lying asymmetrically on the flanks of the resonance curve of resonator CP), and after demodulation by the rectifier MR1 and amplifying at MFA is passed to a phase conscious amplitude detector PHD also receiving a reference demodulating wave from the source MO. The demodulated D.C. output is then passed, together with a voltage derived from a loop L3 and presence detector PD, responsive only when the cavity resonance frequency is near the desired value, to an amplitude threshold device AD responsive to the difference in outputs of PHD and PD and similar to the relay system (Fig. 3) used in controlling the motor M1. The motor M2 thus reversibly controls the tuning piston TP and is finally stopped by the presence detector PD becoming energized when the cavity resonator and local oscillator frequencies nearly correspond. Finally, stoppage of both motors M1, M2 allows centrifugally operated switches SW2A, SW2B to close an electronic control path between the detector PHD and a valve V2 finely controlling the fequency of the oscillator valve VI. During control, all of contacts RL1A, RL2A, RL3A (Fig. 3) will be open but any subsequent frequency drift between the transmitter and receiver will cause contact RL1A (or RL2A), Fig. 3, to close, and operate motor M1. Preferably, to prevent hunting, there is a delay in opening switch SW2B and so stopping simultaneous electronic control. The resonator CP is then automatically retuned to accord with the local oscillator LO. The switches SW2A, SW2B may alternatively be controlled by the motor controlling relays. In a modification the demodulated voltage applied to the phase-conscious rectifier PHD through an amplifier MFA is derived from a rectifier in series with the loop L3. Rectifier MR1 (Fig. 1) is then omitted.