Patent GB-1375060-A (GB)
IPC: G01R13/20
Filed: 1972-11-01
Abstract: 1375060 Voltage measuring COULTER ELECTRONICS Ltd 1 Nov 1972 [4 Nov 1971] 50383/72 Heading G1U [Also in Division H4] After each signal of a sequence of time-varying signals whose maximum amplitudes are expected to increase progressively along the sequence, the gain of an amplifier is adjusted in such a way as to attempt to bring the maximum value of each signal to a common reference level. Three amplifying arrangements are described for this purpose in each of which a fixed gain amplifier 74 is preceded by an attenuator which is adjusted after each signal; in both the first Fig.3 and the second Fig. 5 (not shown) the attenuator is constituted by a potential divider comprising a fixed resistor 92, Fig.3, and a variable resistor, FET 96, taken down to a reference potential on a capacitor 132, the potential divider being adjusted after each signal has been amplified by applying the maximum value of the signal to the divider, comparing 108 the output of the amplifier 74 with a pre-set reference level 114 and appropriately varying the potential stored on a capacitor 120, and hence the effective impedance of the FET, until equality obtains; and in the third, Fig.6, the attenuator is constituted by a ladder attenuator 334 controlled by a counter 322 which is stepped in accordance with the result of the comparison 108 of the amplified maximum signal and the reference level 114. The particular application described relates to a particle size analyzing system, in which the sizes of a sequence of particles are determined 10, Fig.1, and for each particle one of a plurality of counters or other devices, 12, in accordance with the range in which the particle size lies, has its contents augmented by unity. Periodically the contents of the counters are read sequentially 16, 38, to produce a sequence of dots representing a frequency distribution curve on a c.r.t. or graphical plotter. Since particle sizes are continually being determined, all counts are continually increasing, so that in the absence of the invention a sequence of read-outs from the counter would probably look as shown at 61, 62, 63, 64, Fig.2. In addition to the basic range counters, the system also includes a further set of counters or integrators 20, each of which corresponds to a particular particle size and is augmented by unity when a particle less than that size is encountered. These counters are also periodically sequentially read to produce a cumulative frequency distribution curve of the form shown at 60, Fig.2. Clearly, the count, in the 'top' counter (i.e. the extreme top right-hand point of curve (60) corresponds to the total number of particles analyzed, and it is this count which is used by the invention to control the input attenuator to amplifier 74, Figs.3, 6, so that the maximum point on the cumulative frequency distribution curve 60, Fig.3, always occurs at a reference level labelled 100% on the c.r.t. The system has an operating cycle consisting of four places. In phase I, Fig.4, the basic range counters are read out. Their signals A138, Fig.4, are passed on lead 24, Fig.3, via a closed switch 86 to the potential divider 92, 98, and the attenuated signal B144, Fig.4 is amplified 74 to produce an output signal at a point 32, C146 Fig.4, which is displayed on the c.r.t. Phase II is similar except that the cumulative frequency counters are read A140, attenuated B148 and amplified to give signal C146. In phase III the input to amplifier 74 is earthed by the switch 86 and the output, via a closed switch 130, is compared with ground 80 so as to provide a zero off-set voltage which is stored on capacitor 132. In phase IV the attenuated output B160, Fig.2 from the 'top' cumulative frequency counter is reapplied to the amplifier and the output C162 is compared with the reference level 114 to adjust the effective impedance of FET 96. This process repeats and, as can be seen, even though successive read-out signals A, Fig. 4, progressively become larger, the maximum amplitude produced by the amplifier 74, Cl Fig.4, remains substantially constant.