GB1270528A - Improvements in or relating to modulated oscillators - Google Patents

Abstract

1,270,528. Pulse modulation circuits. E.M.I. Ltd. 14 July, 1969 [12 July, 1968], No. 33256/68. Heading H4L. In an oscillator providing output pulses whose repetition frequency varies linearly in accordance with an input control signal, the input signal V 1 is applied to the base of transistor VT1 which develops equal and opposite voltages across resistors R1, R2 which are applied to the bases of complementary transistors VT3, VT2 respectively. Transistors VT2, VT3 with emitter feedback resistors R3, R4 act as equal constant current sources of opposite polarity, one or other of which is directed by a diode switch D1 to D4 to an integrating capacitor C, the diode switch being operated by the output of a differential amplifier A having positive feedback provided via resistors R5, R6 to its non-inverting input terminal. The junction between the capacitor C and the diode switch is connected to the inverting input terminal of amplifier A. The amplifier A changes its output alternately between two limiting states as the difference in magnitude of its two input signals tends to change sign and at other times rests in one or other of its steady states, Fig. 3. When the output voltage V 0 is at a positive limiting value this voltage is supplied to the non-inverting input of the amplifier A and also to the junction of diodes D1, D4. Assuming there is only a small voltage across C, diodes D1 and D3 will be biased off, D4 will conduct and the collector current from transistor VT2 will charge the capacitor C via diode D2, the voltage across C rising linearly to produce the rising portion of the triangular waveform, Fig. 2. When the difference between the rising voltage V applied to the inverting terminal and the voltage applied to the non-inverting terminal of the amplifier is reduced sufficiently, the gain of the amplifier will increase and a negative transient will be fed back via the feedback path of the amplifier to rapidly reverse the polarity of the output V 0 so that a negative bias appears at the junction of diodes D1, D4. Under these conditions D1 and D3 will conduct D2 and D4 will be biased off and the capacitor C will discharge through D3 and VT3 so that the voltage V c will fall linearly until the difference between it and the negative bias applied to the non-inverting input is sufficient to cause the amplifier gain to rise and the resulting positive feedback to rapidly reverse the polarity of the output V 0 and the cycle is repeated. The control voltage V 1 controls the collector currents of transistors VT2, VT3 and hence the slopes of the triangular waveform VC and the frequency of the output voltage V 0 .