GB986633A - Improvements in or relating to swept frequency oscillators - Google Patents

Abstract

986,633. Automatic voltage control systems. H. M. POSTMASTER GENERAL. June 1,1961 [June 9, 1960], No. 20355/60. Heading G3R. [Also in Divisions H3 and H4] An oscillator is both swept and controlled in frequency by applying its output to a frequency detector to derive a reference signal bearing a linear, logarithmic inversely logarithmic or other relation to the frequency which reference signal is then additively combined with a sawtooth or other variable amplitude external signal so that the external signal frequency sweeps the oscillator and yet the combined signal is maintained substantially constant. The oscillator may be a modified Wien bridge transistor R-C oscillator VT1- VT3 with identical networks N 1 -N 2 corresponding to the capacitances; the input to R 1 , MR3, R 2 controlling the oscillator frequency by varying the resistance of series and shunt rectifiers MR1, MR2; VT4 being a voltage clamping circuit setting a lower limit to the oscillator frequency. The oscillator output is amplified at VT5-VT7 and passed to an amplitude limiter and slicer VT9,yT10 producing a rectangular wave and to emitter follower VT10 preceding a diode pump frequency meter circuit MRS, VT12 with output applied through VT11, VT13 to a comparator circuit VT14, R7, R5 with other input, e.g. sawtooth, applied at A. The thus added inputs to VT14 have subtracted from them a constant bias voltage applied to the emitter of VT14 and the difference amplified by VT14 passed through a smoothing circuit R9, C4 to the input circuit R1 MR3, R2 controlling the oscillator frequency sweep. A loop comprising amplifier VT17-VT20 and detectors MR6,MR7 produces an output dependent on the difference between the amplified oscillator amplitude and a standard voltage set by potentiometer VR1 and Zener diode MZ1, and is applied to vary the resistance of a diode MR4 shunted across the oscillation feedback loop N3 of the R-C oscillator, to maintain its output amplitude constant. In a modification the frequency detector is a conventional diode pump circuit fed by an emitter follower whose input is shunted by a ladder network of series resistors and shunt capacitors, so that the frequency detector output is inversely proportional to the logarithm of the frequency.