GB1393457A - Circuit for comparing the period of a recurrent signal with a predetermined time duration - Google Patents

Abstract

1393457 Frequency detectors COMPTEURS SCHLUMBERGER 5 June 1972 [7 June 1971] 26042/72 Heading H3A In a frequency-sensitive switch, a unijunction 10 produces an output S when a capacitor C has been charged to the threshold of one of the unijunction control terminals E (A), thereby indicating that the frequency of the input pulse E is too low to trigger the unijunction 10 by way of a second control electrode B2 and discharge C before the threshold is reached. An input pulse E turns off normally-conducting transistors 21, 22 to lower the potential of terminal B2 which is the second base if a normal unijunction is used, or it is the gate if a programmable unijunction is used. If E turns on the unijunction 10 before C reaches the threshold, an output pulse at B1 will be produced but is suppressed by a transistor 23, turned on by the complement E<SP>1</SP> of the input signal E. C is thus always discharged before the unijunction threshold (at E, A), can be reached as long as the frequency of the input signal E is higher than a predetermined value. By discharging C through the unijunction itself, the voltage left on C from which it has to begin charging again is the saturation voltage of the unijunction and as this is related to temperature, automatic compensation of temperature induced variations in the unijunction threshold voltage (at E, A) is assured. To correct for non-linear charging of C through the resistance R a diode D is included, and a parallel potentiometer P adjusts the correction. Alternatively (Fig. 3, not shown), a constant charging current is used.