Patent GB-917730-A (GB)
Title: Electrical circuit for transmitting a portion of a signal
IPC: H03H19/00
Filed: 1961-02-06
Abstract: 917,730. Signal transfer circuits. BAUSCH & LOMB Inc. Feb. 6, 1961 [Feb. 18, 1960], No. 4354/61. Class 40 (4). [Also in Groups XXXVI and XL (b)] A signal transfer circuit, capable of behaving as a low-pass filter comprises an input circuit and an output circuit, a transfer capacitor and an auxiliary capacitor, and switching means for alternately connecting the auxiliary capacitor in parallel with the transfer capacitor and discharging the auxiliary capacitor. As shown, Fig. 2, with the switch (which may be mechanical or electronic) at the right-hand position, the auxiliary capacitor 40 and the transfer capacitor 42 are connected in parallel and charged from the input circuit, capacitor 40 being large to give the input circuit a large time constant so as to smooth out variations of input during this period, i.e. to integrate the signal over this period. When the switch moves to the left-hand position capacitor 40 discharges quickly over the small resistance 30 and capacitor 42 discharges slowly over resistance 46 to produce a substantially steady output signal, and when switch 12 moves over to the right-hand side again, the residual charge in condenser 42 is shared with the much larger condenser 40, so that the hang-over is minimized. In a modification, Fig. 3, the invention is used in a circuit responsive to the difference of two (integrated) input signals. With the switches 52, 54 (which operate synchronously) in the right-hand position, condenser 88 charges to the source e i2  and condenser 50 is discharged. When the switches move to their left-hand position condenser 88 shares its charge with the condensers 50, 84, the charges being conveyed with polarity reversed as compared with the input signal e i2  During the ensuing period condensers 50, 88, 84 charge in parallel under the influence of source e i1 , to a value determined by the two voltages e i1 , e i2 , and during the subsequent period with the switches in their righthand position condenser 84 discharges over resistance 86 to provide the output signal e o . In accordance with the dimensions of the various capacitors the signals e i1 , e i2  may be applied over potentiometers to adjust their relative values so that the output e 0  is proportional to the exact difference between the two input signals (each of which is of course integrated by charging the appropriate condensers during the times in which the switches occupy the respective positions). Fig. 4 illustrates a spectrophotometer in which a photo-multiplier tube 111 which produces alternate signal pulses 117 and reference pulses 115 obtained by exposing the photomultiplier to a standard reference condition. The pulses 117 are fed to the upper input circuit (via 140), and the pulses 115 to the lower input circuit (via 124), of a differencing circuit of the type described in Fig. 3 whose output is utilized to operate an indicator 142. Means is provided for varying the H.T. in such a way as to maintain the sensitivity of the photomultiplier constant. To this end the reference pulses 115 and a source of standard pulses 150 are caused to charge a condenser 154 with opposite polarity. Variations of charge of condenser 154 from zero value are applied through a transfer device of the type shown in Fig. 2 to a device 146, which controls accordingly the value of the H.T. applied to the photomultiplier.