GB1335680A - Rate discrimination circuit - Google Patents

Abstract

1335680 Frequency detectors; transistor oscillators AMERICAN OPTICAL CORP 15 Dec 1970 [15 Dec 1969] 1933/73 Divided out of 1335679 Headings H3A and H3T [Also in Division A5] The disclosure is substantially identical to that in Specification 1,335,679, but the claims relate to a rate discrimination circuit comprising means for detecting the occurrence of each of a plurality of events, a capacitor 49, a charging resistor connected in series with said capacitor, means for applying a current pulse to said capacitor in response to each operation of the detecting means, a discharging resistance connected to said capacitor for discharging the capacitor and an output circuit operative in response to a rise in potential across said capacitor greater than a predetermined value, said charging and discharging resistances including a common resistance path 38, 42 having a first impedance for current charging said capacitor and a second impedance lower than the first impedance for discharging current from said capacitor. Received positive or negative pulses are amplified by transistors T1, T2 operating in class A and are fed to switching circuit T3, T4, T5. Transistor T4 conducts on receiving negative pulses, and transistor T3 and phase inverter transistor T5 conduct when transistor T3 receives positive pulses. When the charging pulses for capacitor 49 are spaced apart sufficiently the voltage swing across capacitor 49 between charging and discharging through diode 42 (effectively shorting resistor 38) and resistor 47 is sufficient to cause transistor T6<SP>1</SP> to conduct and also transistor T6 to conduct if switch S is closed, thereby discharging capacitors 57<SP>1</SP>, 57. The circuit components are chosen so that transistors T6, T6<SP>1</SP> conduct in response only to signals at terminals E1, E2 having a repetition rate about that of natural heart beats and frequency components above about 20 Hz but not much above 30 Hz. Resistors 45 and 38 are chosen so that the charging time constant for capacitor 49 is long enough to avoid causing transistors T6, T6<SP>1</SP> to conduct if a single narrow R.F. signal is picked up, and to avoid causing transistor T6 to conduct in response to a signal on terminals E1, E2 caused by a pulse at terminals E3, E4. In the absence of inhibiting signals at the base of transistor T6, or if switch S is open, transistors T7, T8 and capacitor C57 form a free running relaxation oscillator and capacitor 57 charges through resistors 35, 37 until transistors T7, T8 and T9 conduct to discharge capacitor 65 via terminals E1, E2 to produce an output pulse. Resistor 37 controls the pulse width, and resistor 35 the repetition rate.