Patent GB-989431-A (GB)
Title: Improvements in or relating to electronic oscillators
Inventor: WOOD JOHN HERRAP
IPC: H03B5/26
Filed: 1960-05-09
Abstract: 989,431. Valve and transistor oscillating circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 2, 1961 [May 9, 1960], No. 16285/60. Headings H3F and H3T. A variable-frequency oscillator comprises an amplifier having a positive-feedback path operative over the frequency range of the oscillator to sustain oscillations therein and a frequency-sensitive negative-feedback path to determine the frequency of oscillation, the negative-feedback path comprising a first circuit producing an output whose phase lags on the input and whose amplitude falls as frequency is increased and a second circuit producing an output whose phase leads on the input and whose amplitude increases with frequency, the inputs of the two circuits being connected in parallel and their outputs being coupled to a common output for the negativefeedback path through a variable proportion device whereby the proportion of negativefeedback signal contributed by each circuit may be varied so as to vary the frequency at which the circuit oscillates. The negative feedback may be effected through a circuit as shown in Fig. 5, in which R 1 , C 1  constitutes the phaselag circuit whose output falls with increase of frequency, R 2 , C 2  the phase-lead circuit whose output increases with frequency and P the variable proportion device. It is shown that if the component values are so chosen that phase shifts of nearly 90 degrees are produced at points band c, a point of zero phase shift may be found on potentiometer P, the output at this point is approximately at its minimum value and although the position of the point varies with frequency, the magnitude of the output at this point is substantially independent thereof. Hence, if an amplifier having just sufficient positive feedback to cause oscillation is combined with a circuit as shown in Fig. 5, conditions for oscillation will only be satisfied, for any setting of the potentiometer P, at the frequency at which the output from the negative-feedback circuit is at its minimumamplitude zero-phase-shift value. A closer approximation to a 90 degrees phase-shift at points band c may be achieved by replacing the circuits R 1 , C 1  and R 2 , C 2  respectively with a Miller integrator and an active differentiator similar to a Miller integrator but with a capacitor in the input arm and a resistor in the feedback arm. Some form of amplitude control is preferably provided in the amplifier; this may be effected by a limiter circuit giving a square-waveform output in the positive-feedback loop, by a lamp or thermistor included in the positive feedback path or by the use of automatic gain control. In an oscillator employing the circuit of Fig. 5 to provide negative feedback, the lower end of the circuit may be earthed and the output of the amplifier connected to the upper end of the circuit. Positive feedback may then be supplied from an antiphase output of the amplifier to the same point as that to which the slider of potentiometer P is connected or it may be supplied from the same output point but applied to an antiphase input point. Alternatively, as shown in Fig. 7, the positive feedback may be applied to the lower end of the negative feedback circuit which is no longer earthed. Fig. 7 shows a circuit employing a three-stage transistor amplifier, the first stage J 1  being an emitter-follower to reduce the loading on the potentiometer P. Negative feedback is applied to one end of network R 1 , C 1 , R 2'  C 2  from the emitter circuit of transistor J 3 , positive feedback being applied to the other end of the circuit from the collector circuit, which includes a thermistor T, of the same transistor T 3 . Further positive feedback is provided within the circuit through resistor R 10  to reduce the size needed for de-coupling capacitor C 6 . Capacitor C 5  is provided to maintain the calibration of the circuit at the upper end of the highest of its three decade ranges which otherwise would be put in error by the circuits R 5 , C 3  and R 9 , C 4  which are provided to prevent spurious oscillations above the maximum working frequency of the oscillator. It is stated that a valve oscillator circuit, based on the same general scheme as Fig. 7, may be constructed in accordance with the invention. However, because of the high input impedance at the grid of a valve amplifier the first stage corresponding to J 1  may be omitted so that only one double triode need be used. Specification 894,346 is referred to.