FUNCTION GENERATOR WITH PULSE-WIDTH MODULATOR FOR CONTROLLING A GATE IN ACCORDANCE WITH A TIME-VARYING FUNCTION

摘要

1,220,813. Function generators. ELLIOTT BROS. (LONDON) Ltd. 18 Jan., 1968 [20 Jan., 1967], No. 3201/67. Heading G4G. A function generator, Fig. 1 (not shown), comprises a gate circuit switchable by a pulsewidth modulator responsive to an input signal x representing one parameter and producing a train of pulses of duration representing x which switch the gate from non-conducting to conducting for a second repetitively varying input signal y (=f(t)) having the same period as the output of the pulse-width modulator and representing a second parameter, the output from the gate being passed to an averaging or integrating circuit so that if x varies slowly relatively to y, the output Z is given by a further function of x related to the function f. For example, if y = k 1 sin wt. and the gate opens at t 1 = 0 and closes at t 2 = # where # is directly proportional to x, then Z = k 2 (1 - cos ##), Fig. 2 (not shown). Again if t 1 = #/2 and #αx, Z α sin ##. If the input y is a quasi-differentiated squarewave with t 1 coincident with a step of the basic squarewave, and y = exp(- t/T), Zα[1 - exp(- #/T)] for #αx, and if y is derived from a squarewave through a filter of function 1/1 + ST, t 1 occurs at a squarewave step, and (t 2 - t 1 ) α x, then Zα(1 + x - eSPx/SP). Fig. 5 shows a circuit in which input cosine waveform y α cos #t is applied to averaging unit 3 incorporating an operational amplifier with RC feedback giving lag with transfer function 1/1 + ST and earthed over shunt transistor switch 1 controlled by pulse width modulator 2 comprising high-gain amplifier 4 with input summation resistors 5, 6. The cosine waveform is squared at 7 and integrated at 8, 9 to a triangular waveform and this is applied to resistor 5 while input signal x is applied to resistor 6 for comparison, switch 1 being pulsed for a time interval determined by the magnitude of x, so that the output Z is proportional to sin ##, if #αx. For cosine generation, switch 1 is driven through an "exclusive or" gate receiving both the pulse-width modulated output from unit 2 and the squarewave applied to integrator 8, 9 and operating to reverse the pulse-width modulated signal at alternate half cycles. It is shown that: Zα - 2 cos ## where 2#αx. If input y is sinusoidal and if the input to resistor 5 is a quasi-differentiated squarewave, Z is then proportional to sin log e x. More complex functions may be derived utilizing inputs x varying independently of time. In a modification Fig. 6 (not shown), the waveform for the pulse width modulator is derived from a waveform generator which is synchronized from another waveform generator supplying the input y. Alternatively both waveform generators could be synchronized from a common external source.