Patent GB-1427443-A (GB)
Title: Method for converting into a frequency an electrical signal
IPC: H04B1/26
Filed: 1973-12-10
Abstract: 1427443 Pulse modulation circuits COMMISSARIAT A L'ENERGIE ATOMIQUE 10 Dec 1973 [29 Dec 1972] 57026/73 Heading H4L In a known method of amplitude-to-frequency conversion in which the amplitude is converted into a proportional current which is integrated to a maximum value determined by a threshold voltage, which controls the starting and stopping of the integration, the maximum value attained is higher than the threshold value due to the finite switching time. The Specification discloses means to compensate for this error, the overshoot being measured and a corrected threshold voltage formed. As shown in Fig. 3,  the incoming signal is supplied via a voltageto-current converter 2 to a threshold circuit A receiving also the threshold value V 0 . The current is integrated alternately in capacitor C1 or C2 via switch K1 or K2, one capacitor being discharge via its switch K<SP>1</SP>1 or K<SP>1</SP>2 while the other is being charged. The change of state of circuit A controls a monostable 6 and the resulting pulse rate modulated signal is supplied via output S. A logic circuit 4 controls the operation of switches K1, K2, K<SP>1</SP>1, K<SP>1</SP>2 and also of switches K<SP>11</SP>1, K<SP>11</SP>2 which supply the peak amplitude attained by the integration voltage of capacitors C1, C2 alternately to a storage circuit 12. It is arranged that when switch K1 is opened, switch K<SP>11</SP>1 is closed, which is followed by switch K<SP>11</SP>1 being opened and switch K<SP>1</SP>1 closed, the charge and discharge of capacitor C2 being arranged in like manner. The corrected threshold voltage V 0  is derived in circuit 18 from the output V<SP>1</SP> 0  of storage circuit 12, the input signal V e  and a fixed reference voltage V<SP>11</SP> o  in accordance with an expression given in the Specification and graphically illustrated in Fig. 4 (not shown). A capacitor 28 prevents conversion errors in the event of imperfect synchronism of switches K1, K2. In an alternative arrangement, Fig. 5, two voltage-to-current converters 2, 2<SP>1</SP> receive the input voltage, the output of converter 2<SP>1</SP> being supplied to a main integrating circuit including capacitor C via switch K and a threshold circuit A operating a monostable 6 and logic circuit 4 as before. An auxiliary integrating circuit includes a capacitor R having a value P times greater than that of capacitor C, a capacitor C<SP>1</SP> having a value the same as that of C, charge and discharge switches Q, Q<SP>1</SP> controlled by circuit 4 and an amplifier 32 having a gain of -P receiving the voltage at point a and a reference voltage V<SP>11</SP> 0 . The output V 0  of amplifier 32 is supplied as the threshold voltage to circuit A. The sawtooth waveform at a has an amplitude P times smaller than that at b so that at c the waveform at low frequencies has the same amplitude as at b but is reversed. At high frequencies the amplifier 32 no longer follows the signal appearing at a and the output c assumes a lower mean value when the switch K is opened. Capacitor 38 operates in a manner similar to capacitor 28.