GB1295957A - - Google Patents

Abstract

1295957 Automatic frequency control; monostable circuits WANDEL U GOLTERMANN 30 Nov 1970 [6 Dec 1969] 56688/70 Headings H3A and H3T A variable frequency generator comprises: (a) a counter 110 fed with pulses from an adjustable oscillator 102; (b) a coincidence circuit 111 having a first input from a presettable digital frequency selector 112, and a second input from the counter 110, the circuit emitting an identity signal when the counter output reaches the preset value; (c) a timer 118 for establishing a predetermined reference period; (d) a test circuit 113 receiving from timer 118, a pulse at the end of the reference period, and also receiving the output from coincidence circuit 111, to produce a control signal having a polarity dependent upon the relative order of occurrence of the outputs from timer 118 and circuit 111, which is fed back to control the frequency of oscillator 102; (e) means 108 coupled to the output of timer 118 and to the input to counter 110 for detecting any time lag between the start of the reference period and the first pulse to be counted; and (f) a delay 117 controlled by means 108 to reduce the time lag. The timer 118 is a monostable circuit whose unstable state may be adjusted by potentiometer 120 in accordance with the frequency to which the oscillator is to be set. In a first modification, Fig. 3 (not shown), the delay 117 of Fig. 1 is replaced by another monostable (325) and the timer comprises two monostables (318, 328) both with adjustable periods. This arrangement also has provision for the operating frequency to be given a sawtooth sweep by switching in a sawtooth generator (334). In a further modification, Fig. 5 (not shown), for applying a sweep to a C.R.T. in a frequency sweep test arrangement, the two limits of the frequency sweep are set by additional digital selectors (512A, 512B) which may be switched to the coincidence circuit (511). This circuit also has provision for generating frequency markers, details of the frequency marker generation being given in Fig. 6 (not shown). Monostable circuit, Fig. 4.-With no signal on lead 418<SP>1</SP> and the circuit in its stable state, transistor 438 is conductive and capacitor 433 discharged. When a positive input pulse is applied on 418<SP>1</SP>, bi-stable 436 is switched, but transistor 438 is held on by the connection of lead 418<SP>1</SP> to its base via OR-gate 440. Upon cessation of the input pulse 438 cuts off, and capacitor 433 charges through transistor 441 until the voltage across it equals a reference voltage set by potentiometer 431, the charging rate being set by an applied bias on lead 428b. Comparator 437 then resets bi-stable 436 to restore the circuit to its initial state.