Patent GB-998900-A (GB)
Title: Analog-to-digital converter
IPC: H03M1/00
Filed: 1961-08-18
Abstract: 998,900. Semi-conductor pulse circuits. WESTERN ELECTRIC CO. Inc. Aug. 18, 1961 [Aug. 22, 1960], No. 29883/61. Heading H3T. [Also in Division G4] In an analogue-to-digital converter of the kind in which the unknown analogue signal is compared sequentially with signals equivalent to the digits in decreasing order of significance there is provided a shift register for supplying switching signals in sequence to the digit stages. Units 111, 112, coupled to master timing source 107, give phase-displaced pulse trains-see Fig. 2- which are clipped at 113, 114. The resultant square waves are fed to the first and third stages and to the second stage respectively of the shift register 105. The pulse train from 112 is also passed to a synchronizer 108. This unit ensures that pulses from 106 pass to register 105 in predetermined time relationship with respect to pulses from 107, as indicated in Fig. 2. Register 105 provides pulses sequentially at its three outputs. Each digit stage uses two tunnel diodes, e.g. 100, 110, which have N-type current-voltage characteristics. A first pulse from source 106 causes source 117 to supply a reset signal which clamps point 153 negative and causes currents - #I to flow in each state, as indicated. Unit 117 responds only to alternate pulses from 106. When a reset pulse arrives from the first stage of the shift register, this pulse in combination with the current - AI causes the diodes in the first stage to switch so that diode 100 is at a relatively low voltage and diode 110 is at a relatively high voltage. Subsequent " reset " pulses from 105 switch the diodes of digit stages 2 and 3 to state 1. The subsequent application from unit 116 of an analogue signal to be digitized reverses current - AI in each stage. When unit 106 gives a second pulse, to which 117 does not respond, register 105 produces a set of three " convert " pulses and a switching cycle is initiated. Upon receipt of the first convert pulse the diodes therefore change so that diode 110 has a low voltage and diode 100 a high voltage-state 2 which causes unit 166 to supply a current through a binary-weighted resistor R to unit 115. If this current is greater than that from the analogue signal, current + AI reverses in stage 1, the diodes revert to state 1 and unit 166 is turned off. This indicates that the most significant digit is "0." However if the current is less, the state 2 is maintained and the most significant figure is a " 1 " The next two " convert " pulses from 105 initiate similar switching in stages 2 and 3 so that signals representing the digitized analogue appear at the output of the converter. The switching operations are described in detail in the Specification. The shift register comprises three stages, each stage including a tunnel diode and resistor, e.g. 501, 502. Each diode is biased for monostable operation when the voltage from its associated clipping circuit 113 or 114, Fig. 1, is relatively high, and for bi-stable operation when the voltage is relatively low. Thus, if a trigger pulse is fed in at 503 when diode 501 is biased for bistable operation the diode shifts to its other stable state, transistor 504 is turned on, and a pulse passes to the first stage of the converter. This pulse is maintained until the voltage from 113 reverts to its high value, whereupon transistor 504 is turned off. While 504 is on, transistor 506 is also on and gives current al. This current keeps diode 521 biased to its bistable state and when the voltage from 114 reaches a low value transistor 524 is turned on. Similarly the third stage is turned on at a later time. The synchronizer, Fig. 6, receives trigger pulses from 106, Fig. 1, and develops pulses so timed that they occur during the time in which the output voltage level of unit 114 is at its low value. The trigger pulses pass via gating transistor 624 to tunnel diode 602 which, being biased to bi-stable operation, switches to its high voltage stable state. Current #I results and sets tunnel diode 612 to bi.stable operation. When the voltage from unit 112 drops to its lowest value diode 612 switches to its high voltage stable point and transistor 617, and hence transistor 621, are turned on. A pulse is given at 623, and the current in resistor 619 causes transistor 624 to be turned off and thus diode 602 to be reset.