Patent GB-718128-A (GB)
Title: Improvements in or relating to electronic circuits for deriving a voltage proportional to the logarithm of the magnitude of a variable quantity
IPC: G06G7/24
Filed: 1952-02-04
Abstract: 718,128. Radar. GENERAL ELECTRIC CO., Ltd., and HAMBLIN, C. L. Feb. 4, 1953 [Feb. 4, 1952], No. 2955/52. Class 40 (7). [Also in Group XXXVI]  A range tracking device for a pulse radar system comprises a circuit responsive to incoming synchronizing pulses generating an index pulse occurring at an adjustable time interval after the transmission of a radar pulse and for bringing the index pulse into initial approximate time coincidence with a received echo pulse occurring with the time interval, a pair of integrating circuits each varying the charge on a separate capacitance associated therewith by an amount proportional to the time integral of that part of the echo pulse envelope occurring respectively before or after the index at each pulse repetition, a separate circuit associated with each capacitance for normally holding the charge thereon at a datum value and for returning it to datum after any variation according to the same time exponentional law,a circuit for limiting the capacitance voltages variations within a range corresponding to a predetermined minimum deviation of charge from datum and for differencing the two voltages, and a smoothing circuit deriving a continuous voltage proportional to the time integral of the difference output and to the logarithm of the difference of the parts of the echo pulse occurring before and after the index, which is applied to control the occurrence time of the index pulse so as to zeroize .the difference output and accurately align the index on the centre of the echo pulse.  .Fig. 3 shows a system wherein a transmitter 31 radiates pulses of radio energy from antenna 33 and produces synchronizing pulses to trigger, a flip flop circuit 35 which generates positive going rectangular pulses whose durations extend from the instant of transmission for a time corresponding to the maximum range'of the system. Such pulses trigger a sawtooth generator 37 to produce linearly decreasing voltage strokes lasting for the duration of each applied pulse, which are applied to a comparator 38 together with the output of an integrating circuit 39 referred to hereafter to generate a short pulse which coincides with equality between the sawtooth voltage and the integrated voltage and is thus, displaced over the time interval representing the range of the system in correspondence with the value of the integrated voltage. The short pulses from the comparator circuit trigger a strobe generator 40 to produce on terminals 41, 42, a pair of spaced strobe pulses, wherein 'the trailing edge of the earlier coincides with the leading edge of the latter, which pulses are respectively applied to terminals 12a, 12b of two identical computing circuits 30 (Fig. 1) supplied on their terminals 1a, 1b with radar range pulses received by antenna 34 and receiver 32 in correspondence with the pulses emitted by the transmitter 31. In each computing circuit, the incoming pulses are capacitance coupled across a biased D.C. restoring germanium diode 5 and through a series resistor 3 to the control grid of a Miller integrator pentode 4, which is also connected to the anode of a diode 8 whose cathode is potentiometer biased, e.g. 10v negative to earth and capacitance coupled to the positive going strain of "earlier " strobe pulses on terminal 12. Each pulse cuts off diode 8 and allows pentode 4 to conduct (or may alternatively be applied to the biased suppressor grid of the pentode) so that the integrator circuit operates for the pulse duration, and on arrival of a target pulse overlapping the strobe pulse the pentode anode voltage rapidly falls by a voltage proportional to the time integral of that portion of the target pulse envelope coinciding with the particular strobe pulse, whereby capacitor 15 is correspondingly discharged, and after cessation of the strobe pulse recharges exponentially from the H.T. supply over resistor 7. The resultant pulses across the capacitor are capacitance coupled to the control grid of a cathode biased triode amplifier 17, which is also connected to the H.T. supply through a resistor 18 and earthed through D.C. restoring diode 19. The pulses are limited at a constant voltage level set by the cathode resistor 20 and it is shown that the area of each limited pulse is proportional to the logarithm of the corresponding voltage excursion at discharge of capacitor 15, and thus to the logarithm of the area of the corresponding target pulse occurring within the duration of the leading strobe pulse. The anode output of triode 17 is capacitor coupled to a germanium D.C: restoration rectifier 23 and the control grid of a triode 21 connected in a series subtracting circuit with a triode 22 whose control grid is energized by a train of limited pulses from a circuit similar to the above wherein the area of each pulse is proportional to the logarithm of the area of the corresponding target pulse occurring within the duration of its trailing strobe pulse and wherein the anode circuits are earthed while the cathodes are returned to a negative H.T. source. Alternatively the anodes of the second computing circuit may be supplied from the positive H.T. source and the cathodes returned to earth while a phase inverter circuit is inserted between the output and the control grid of triode 22. The anode of triode 22 is connected directly to the control grid of a second Miller integrator pentode 26 at whose anode 28 a voltage appears which is proportional to the time integrated difference of the logarithms of the areas of each target pulse occurring within the durations of its associated leading and trailing strobe pulses. The further integrating circuit 39 produces a smooth control voltage which is reduced to zero when the coincident edges of the successive strobe pulses are aligned with the centres of the target pulses, and is applied to the comparator circuit 38 to control the occurrence of the strobe pulses in time according to the range of the target producing the echo pulses, and thus reduce the control voltage to zero. The pulse output of the comparator circuit may operate an auxiliary strobe generator to brighten the target pulse being tracked on a C.R.T. display. In a modification for computing using the circuit of Fig. 1 a continuous voltage analogue wave may be applied to terminal 1 and regularly recurring pulses to terminal 12 to discharge capacitor 15 at each pulse by an amount proportional to the time integral of the input wave during each pulse, or alternatively the pulses at terminal 12 may be of short duration and the circuit of pentode 4 re-arranged so that at each pulse the capacitor 15 is discharged proportionately to the value of the input voltage during each pulse. The output voltage will then be proportional to the logarithm of the time integral of the input voltage and alternatively to the logarithm of the input voltage, coincidentally with the control pulses.