Patent GB-1232857-A (GB)
IPC: H03K5/04
Filed: 1968-08-07
Abstract: 1,232,857. Pulse modulation systems; transistor amplifier and sawtooth generator circuits. IAN YOUNG (ELECTRONIC DESIGNS) Ltd. 4 Aug., 1969 [7 Aug., 1968], No. 37634/68. Headings H3T and H4L. In a pulse duration modulation system of the type including means for comparing an input signal with a ramp voltage, means are provided for adjusting the ramp voltage automatically to compensate for unwanted variations in the input signal. General description, Fig. 1.-VI and V2 are data input voltages and V o  and V pk  are reference voltages representing respectively the zero and peak values achieved by the input signals. Field effect transistors 1A to 1D apply these voltages sequentially to a pulse duration modulator including a voltage comparator consisting of a differential amplifier 2 the other input of the comparator being a sawtooth waveform from a ramp generator 3 triggered by a signal on line 9. When the voltage from generator 3 just exceeds the voltage of the input signal the output of the amplifier switches over and a duration modulated pulse is supplied to output 4. When voltages V  0  or V pk  are supplied to the amplifier the output goes to a pulse period comparator 5 and is compared with the duration of a standard pulse t 0  or t pk  derived from a clock source 6 and representing the known amplitude of the corresponding reference signal without the variations arising in transmission. The comparator 5, Fig. 3 (not shown), comprises two bi-stable circuits, which respectively provide a pulse whose duration represents the error when the duration of the input pulse is greater than or less than the locally generated reference pulse, the required reference pulse being selected by gating circuits. The output of comparator 5 representing the zero value is applied to an analogue clamp zero circuit 7 and that representing the peak value is applied to an analogue clamp slope circuit 8, where they are converted into corresponding voltages the appropriate circuit being enabled by a signal A or B. The voltage from circuit 7 adjusts the baseline of the waveform from generator 3 up or down accordingly to compensate for the error and that from circuit 8 increases or decreases the slope of the waveform according to the direction of the error. Ramp generator, Fig. 2a.-Capacitor 18 is charged by current I via transistor 20 and is discharged via transistor 21 which receives a priodic pulse signal at its base, the output being provided at 22. The zero level of the output is controlled by a negative voltage V ZERO  applied to resistor 19 and the slope adjusting voltage is applied to the base of a transistor 23 which sets the level of the current I charging the capacitor 18. Analogue clamp circuit, Fig. 4.-If the output of comparator 5 is a " down " pulse it is supplied via diode 41 to the base of a transistor 42 and if an " up " pulse it is supplied via diode 43 to the base of transistor 44, these pulses only being effective when an enabling pulse is applied to diodes 45, 46. A " down " signal discharges a capacitor 47 via diode 48 and transistor 42 and an " up " signal through transistor 44 causes a transistor 49 to supply current to capacitor 47 via diode 50. The potential on capacitor 47 is fed via a filter 51, 52 to an amplifier including a field effect transistor 53 whose source is connected to the amplifier output and drain is connected to the base of a transistor 54 the collector of which feeds the bases of complementary transistors 55, 56, this arrangement providing a low output impedance. Duration-to-amplitude conversion.-A width modulated signal t, Fig. 1, is gated into the pulse period comparator 5 and compared with a pulse received from the voltage comparator 2 as the result of the application thereto via a field effect transistor IX of a voltage from analogue clamp 10, Fig. 4, receiving the output of comparator 5, gating signals X being applied to transistor IX and analogue clamp 10 for this purpose. The resulting analogue output signal may be applied to a conventional servo amplifier feeding a motor which through a gear-box adjusts a wiper of a potentiometer providing the balancing signal for the servo amplifier. Any fluctuation of voltage V across the potentiometer would affect the position of the servo motor and to avoid this the voltage V is supplied as a calibrating voltage to the duration-to-amplitude converter.