US2774277A - Systems for spectrochemical analysis - Google Patents

Description

Dec. 18, 1956 R, c. MACHLER 2,774,277

SYSTEMS FOR SPECTROCHEMICAL ANALYSIS Filed Nov. 5, 1952 2 Sheets-Sheet l Ratio Recorder Ampllfler Amplifier 'I no INVENTOR.

RAYMOND C.MACHLER ATTORNEYS Dec. 18, 1956 R. c. MACHLER 2,774,277

SYSTEMS FOR SPECTROCHEMICAL ANALYSIS Filed Nov. 5, 1952 2 Sheets-Shea 2 INVENTOR. RAYMOND C. MACHLER WMW% ATTORNEYS United States Patent 9 Claims. (CI. 88-14) This invention relates to systems, such as those for spectrochemical analysis, which include pulse amplifiers ing current used to drive them. These shifts in phase of the synchronous rectifier have been enough to introduce unwanted signals. More specifically the shifts in phase of the synchronous rectifiers have produced a change in the 5 ratio due solely to said shift of phase, and not to a change in the ratio of intensity of the lines under observation.

It is the principal purpose of the present invention to provide compensation for systems including an alternating current amplifier and a synchronous rectifier wherein the system is compensated for the inherent phase shiftof the synchronous rectifier and more particularly to provide a system which allows asubstantial 'tolerance in the phasing of the synchronous rectifier without introducing signals due to shifts in phase of the synchronous rectifier 'Without regard to the number of spark discharges'within of the alternating current type having in the output circuit a synchronous rectifier and has for an object the provision of systems in which there is avoided variation in the direct current output of the alternating current amplifier and rectifier due to phase shift of the synchronous rectifier.

This invention is particularly applicable to systems utflized in spectrochemical analyses of various compositions where a spectrum will be produced bya spark discharge and where a radiation-responsive device has an output corresponding with the intensity of a selected line or lines of the spectrum produced from the composition. The repetition rate of the spark discharge is controlled in manner such that the energies in successive discharges remain substantially constant within narrow limits. With each spark discharge a pulse is produced by a radiationresponsive device such as a phototube, the intensity of which is measured with reference to the intensity of a selected line or band of the spectrum.

Such measuring systems are preferably of the type disclosed in Williams, Jr. Patent No. 2,522,976. Reference may also be had to the article entitled Electrically Balanced Controller for Flight Testing and Spectroscopy by Albert J. Williams, 'Jr. et al. which appears in AIEE Transactions, volume 65, 1946, pages 205-208, for details of such a ratio-measuring system.

It has been found desirable, as more fully explained in copending application Serial No. 157,541, filed April 22, 1950, for Pulse Amplifier System for Spectrographic Analysis by Albert J. Williams, J'r., issued May 19, 1953 as patent No. 2,638,811, to include preamplifiers in ratiomeasuring systems of said Williams patent No. 2,522,976

and of said publication in order to utilize signals from low intensity lines of the spectrum. However, direct current amplifiers, if used, would increase the dark current signals to the same degree as the signals due to the fainterlines of the spectrum. Hence, the signal-to-noise ratio would not be improved. Alternating current amplifiers improve the signal-to-noise ratio by elimination of 55 the dark current signal, but with alternating current amplifiers it is necessary to rectify the output thereof for appli- Synchronous irectifiers of 'the mechanical type have linear response characteristics, but due to the' factthat such synchronous rectifiers operate with switching contacts, they have been found to be subject to slight phase shifts during operation; that is, like switching operations a given cycle. claimed in the aforementioned copending application have ;been successful in avoiding the introduction of signals due to shifts in phase of the synchronous rectifier when a relatively few number of spark discharges take place during a. cycle, such systems have left something to be desired when many discharges, for example more than six pulsesof radiant energy for each half cycle, are produced. When more than a few pulses of radiant energy are 5 produced in each half cycle, some of the resulting voltage pulses will be reversed by the synchronous rectifier and will subtract from the direct current output signal of the amplifier-rectifier. This is undesirable. Thus, in accordance with the present invention there is provided.

means for applying to the input circuit of the amplifierradiant energyoperable over a range of selected repeti-I rectifier radiant energy pulses during alternate half cycles only of an alternating current supply voltage.

In carrying out the invention in one form thereof, there is provided a spectrometer system having a source of tion rates and aphototube responsive to a selected band of the spectrum of the radiant energy for producing radiant energy pulses at a selected repetition rate. There is further provided an amplifier having an input circuit including said phototube and an output circuit in which there appears an output signal including amplified pulses corresponding to those applied to the input circuit. There is further provided a synchronous rectifier in the output circuit and means for applying to the input circuit radiant energy pulses. during alternate half cycles only of an alternating currentreference voltage, to insure unipolarity 'of the rectified amplified pulses throughout said range of selected repetition rates, and means including the reference voltage for operating the synchronous rectifier to produce switchingoperations at a predetermined time "in the cycle. of said reference voltage. 0

Further in accordance with the invention, thereis provided a system for measuring the relative intensities of radiant energy produced over a range of selected repeti-,

tion rates comprising phototubes respectively viewing the radiant energy, a D. C. ratio recorder for comparing the output of one phototube relative to the other, and an A. ,C. amplifier disposed between each phototube and theratio recorder, each amplifier having an input lcircuit" for application thereto of groupsof pulsesat aselected repetimay occurat different times in the pycle of the alternattion rate and anoutput circuit in which there appears an output signal including amplified pulses corresponding'to those applied 'to the input circuit. The system further, in-' chides a synchronous rectifier means of the mechanical type in each output circuit, means for simultaneously ap-v plying to each input circuit radiant energy during alternate half cycles only of an alternating-current reference voltage .to insure unipolarity of the rectified amplified-pulses r throughout the range of selected repetition rates, fand Patented Dec. 18, 1956 7 While systems of the type disclosed and means for operating' the synchronous rectifier means to 4f "Fora more detailed understanding of the invention :fQr further objectsandadvantagesthereof,"reference is wane had to the following description taken in conjnncf tion witlrthe accornpanyingsdrawings in which: I 7 "Fig. 1 diagrammaticallyillustrates a preferred embodi- 4 scribed.

a in ac ordance w t th n ntion; he mails to! applying to the input circuit-radianteenergy pulses during I alternate half cycles only of an alternating-current referlence voltage may comprisepa'half-yvave rectifier in series circu th the seursce o'f radiant gy r it may C0111 prise. a rgtarYjlight shutter disposed adjacentthe spark electrodes forming the analytical 'gap.

l foruse in spectrographic" analysis for the reason that-1m Further in accordance with thednverition the synichjronous, rectifier meansrnay be 'of the mechanical donble reed vibrator type, one reed being disposed respectively each ofithe outputcircuits' of the amplif ers and both of thierreeds simultaneously producing the switching operations at'a predetermined time. 1'

ment of the'invention; 7 a ,Fig. Zdi'agrarnmaticaHy illustrates'a modification ofthe sand 7 converted to d rect current for app i n r3110 s uring networks of the type shown in said Williams,']r.

PatentsNo. 2,522,976. 7

It was further pointed out insaid copending application Serial No. 157,541 that the use of alternating-current am plifiers of the type generally available are not suitable des rable i na s a e iatwd ced wl ieh'si na a td "to ratioditi erences betyveentheselected spectral lines or bands; Accordingly, in said 'copending'application' Serial No: 157,541 there vva's disclosed and vis included'here by reference a pulse amplifier system including a compensated a Pulse ampl fier having a feedbackcircuit including circuit elements havingimpedancecheracteristics which are ma terially different fo'f 'low *andjhigh frequency components of the appliedpulses for so modifying the amp'lifierscharacteristics as to produceagsubstantially uniformleifel of V invention applicable to the embodiment shoyvn finFigifl;

and

V Figs." 3-8 are graphs usefullin'explanationof the prob- "lems to be solved and the manner of their solution'by' the invention. r a V V V V :Referring to Fig.1 as exemplary of a spectrometer system utilizing the invention, light pulses fromthel a'na lytical gaporis our'ce 10 are admitted through entrance slit V :11 of's'pectrometer' 12 in'which a prism'pr'grating prosaid copending:application Serial Noi 157,541, the spec- 'trometer 'lzldirects' upon phototube's" 16 and 17 the set V lecte'ddines or hands of the spectrum; :Pho'totube'1j6 .maybejconsidered'as the'a'nalyz ing' phototube whereas phototube 17, inayibe' considered as the reference ,phototube." D i V 4 Eonpurposes" of clarity; insofar as possibl'ez like V parts inthe present application have been designated with; gthe's'ariiefiefer ence characters asth'e aforesaid copending applicationi'f Irithie spectr0meter'12 of the present applif cation, Fig. -1,fthere is interposed inzthe optical path adja:

'cer'i't' entrancesliti 11 a'cl ear polished qtiartz ligh t splitters V inithe" form" of aplate" 13"which transmits' radia nt energy to analyiin'g -phototube 16 and reflectsraclia'n tjenergyito reference phototube-li" The purposefiof the' li'ght splitter 'will 'be 'later'f'pointed out; 'As *disclosed in sa'id co- "pending applicatioh, the radiation-responsive devices and 17.,are' preferably photomultipliers and thes'e fhave V be'e'n:c onnectedin a ratiomeasuringfcircuitfor o ration 1 Q t 'r ?r r' 8 in e' rd exw th. he; relat v .:-values ofltheintensityfof the reference radiant energy d the radiant energy corresponding y/ith thes constituen f the: substance heing'analyzedf 'Aceordl gly, in"Fig; v #the ratio r'ecorder 18 is illustrated as havingfone input 7' circuit. 1 9 ;from phototube'fl andasecondfinpntcircuit w "2Qfrontphototube 16. 1

I111 the 'aforeinentiohed coperidingapplication-SerialNor 1 '157;5 41,"itj'was pointed o ut fcurrentg that is, ,thecurrentLfl Vvdurin he-abjsenceof p mr ti m l i q l s urce r ,t

a'ctri'sticsl Inthisi'cdnhtiori, I, V desirable'to have the "radiant energy "reaching the reference viith respect to the light actually; Vfpolished' qugrtg plate;13 "to. tHe analyzing 3211 16; of he spectrometeribecauise, the latter cell, is r viewing, only, a single line 'vvhichgcontains" littleradiant enei'g vghile i inelis v ei g 51 hav' ari extremely large i ratio, it,

p 1 tee-systems "utilizing multiple pulsesjfthat] is ip lsesprt'oducedf" V I The :mapner: ofjprodueing r'nultiple pulses is eiplained iiflinns Ba't'e'ntfNo 7 --source or analyzing gapi l fl 'is inc ided in ah gh-fre'quiicy j signal; betweeni'g'roupsof pulses in avoidance of spurious signals which would otherwise be introduced by change 'in V :phase operationof the rectifier in the regions nwhichsaid" substantiallyjunif'orm' signal level ihas"beenfestablishedr,

Similar amplifiers 37, 3 7ciand synchronous contact rectifiers '72, 72a illustrated inFig. 1 of said copending application Serial No: 157,541 have been illustrated schematicallysin Fig. lot this application'by blocks designated by a corresponding reference characters. The synchronous rectifiers72, 7221 may be of the polarized vibratorv type,

the construction .of'which disclosed'in coperiding appli cation Serial No. 725,465 filedlby Albert J. Williams, Jr issued October 14," 1'952 as 'U. '5; Patent No."2,6 14,l88

(and also disclosed in Belgian'Pa'tent No. 479,515)"; .The,

vibrators 7 2 '72rzmay'be either of the'normally closed-or the normally open" type, but preferably of the latter type. The reference'cell 17, While it may receive radiant'energy' from'onlya single line of the'spectrum, preferably andas' illustra ted in Fig. 1, receives radiant energy from a selectedband of the total'radiation of the 'eir'ci'ted specimenras more fullydis'closediin' cop'ending'application' sen-attire; 156,7 3, filed 'Agr tirg, "195 0, patent No. 2,734,418 issued February '14,f l956,;fo r fSp'ectrometerj w 7 ohri H.' Enn:sf The refreigfceband effectively 'viewedbyih V and ernbracesf the principal radiation fromdhecons'tituents; of the specimenyet i s limit ed' by the; response character-Y istic of the'receiver 'alone'ordn combination 'With an Qpti- 'cal'ffilter' 'to exclude-the" no -representative radiation due Q t components of the'ambient atmosphere,orduetospeci- 7 different excitation char-j efe'rencephotocell is undispersed' radiation ni n' cdmpenems of a preciab1y" t is tohe noted tliat it is channel oftheirjatio recorder 18 :considerably attenuated w re srsy 5' hus, a the" i s t reasonab e,

each half cycfle ofg' :1 refer we th es t e se a difficult to ad st he discharge circuit including a capacitor 21, a resistor 22 and inductor 23, and preferably a radio-frequency ammeter 24. The low-frequency charging circuit 25 includes a voltage step-up transformer 26 supplied from a suitable source of alternating-current supply 74. To adjust the charging voltage for the capacitor 21, an adjustable autotransformer 27 of the Variac type is utilized in conjunction with an adjustable series resistance 23.. The conventional Gil-cycle alternating-current source *of supply may be utilized, the sine waveform 76 being illustrated in Figs. 3-8. Variation of the voltage applied to the transformer 26 will predetermine, other thingsremainingconstant, the number of sparks which jump the gap 16 each half cycle or alternate half cycle of the low-frequency supply voltage. A control gap 29 is included in series with the analytical gap 10 to stabilize operation thereof, a stream of air being directed across'the control gap as g by means of a flow pipe 3110f insulating material.

7 As fully explained in said Ennspatent, when the voltage rises across the capacitor 21, it will attaina magnitude at which a spark will jump across the control gap 29, whereupon the energy stored in the capacitor 21 will be dissipated as an oscillatory high-frequency discharge in the" discharge circuitZi) including the analytical gap 10. The voltage across capacitor 21 will be quickly restored to zero. 7 However, the capacitor again starts recharging at a rate determined by the time constant of the charging network and therefore may again receive a charge adequate to produce a spark at the control gap 29 to initiate a further high-frequency discharge. The charging and discharging cycle repeats in each half wave of waveform 76 of the reference voltage 74 a number of times depending upon the setting of the control gap 29, the time constant of the charging circuit 25, and the available secondary voltage of the transformer 26. As described above, the spark breakdowns of the analytical gap 10 have ranged from 1 to 20 per half cycle of the reference voltage.

In Fig. 3 there has been illustrated the sine wave 76 of the reference voltage 74 of 60 cycles per second as well as three pulses 41, 42 and 43 of radiant energy directed to the spectrometer 12 as a result of three discharges across gap 10 during the positive half cycle of the reference voltage 74. During the negative half cycle of that volt age, there will again be three further discharges. As indicated by the broken line 120-cycle sine wave 45, it will be observed that the three pulses of radiant energy occur during the positivehalf cycles of a 120-cycle sine wave 45. Under such conditions, it will be desirable to have the synchronous rectifiers 72,7212, Fig. 1, '(disclose'djin detail in aforementioned'copending application Serial No. 157,541) operate at 120 cycles per second. With synchronous rectifiers 72, 72:: operating at 120 cycles per second, there will be obtained operation of the amplifier-rectifier combinations of Fig. 1 according to the waveform including the resulting voltage pulses 41a-43a illustrated in Fig. 4.

While the foregoing operation is satisfactory if only a single pulse or a small number of pulses are produced in each half cycle of the reference voltage 74 (illustrated as sinewave 76 in Figs. 3, 5 and 7) if numerous pulses,

in Fig. 1, the high-frequency dischargecircuit 20 may include a half wave rectifier 60 which servesto limit the discharge current through the gaps 10 and 29 only to the half wave corresponding with the polarity of connection of the rectifier 60 in the discharge circuit. Alternatively, the half-Wave operation may be accomplished by providing a rotary shutter (approximately 180 disc) 61 64 are mechanically coupled together for simultaneous operation to open and closed positions. Thus, when motor switch 64 is closed to energize motor 62 to provide half-Wave source operation by means of half-wave shutter 61, the switch 63 .also will be closed, thereby shorting out of the electrical circuitof gap lil the half-wave recti-' fier 60. Similarly, when switches 63 and 64 are in open position, half-wave rectifier 60 will be efiective'in the electrical circuit of gap 10 to produce half-wave source operation, and, accordingly, motor 62 will be deenergized thereby preventing rotation of the half-wave shutter 61. It is to be understood that suitable provision is to be made to insure that when switch 64 is in open position with motor 62 deenergized, the half-wave shutter 61 will be at rest in a position such that it will not block the pulses of radiant energy being directed from gap 10 to entrance slit 11. 1

By thus limiting the radiant energy pulses to either the 7 positive or to the negative half cycles of the reference for example as many as six pulses of radiant energy 5156 are produced in each half cycle of the reference voltage f 74 (illustrated as sine wave 76 in Figs. 3, 5 and 7),- it will be seen from Fig. 6 that two of the resulting voltage pulses 56a and 570 will be reversed by the synchronous rectifier 72, 72a and will subtract from the direct-current output signal of the amplifier-rectifiers. a

Operation according to the waveforms illustra't'ed'in Figs. 5 and 6 is undesirable and in accordance with the present invention there is provided means for applying to the input circuits of the amplifiers radiant energy pulses during alternate half cycles only of the alternating-current reference voltage 74. The alternate half-wave operation may be performed in various ways. For example, as shown voltage wave 76, the synchronous mechanical rectifiers 72, 72a of Fig. 1 may be operated at 60 cyclesgfor 'example, from the same source of alternating-current supply as supply 74 having sine waveform 76, Fig. 7, and the switching of the rectifiers may be phased for operation with respect to the reference voltage wave 76 to eliminate the possibility of radiant energy pulses which, inturn, may produce signal pulses at the amplifier. which, if prescut, would be reversed by the rectifiers 72, 72ain diminu tion of the direct-current output signals. As may be seen in Fig. 8, the switching of thesynchronous rectifiers takes place at the point of alternation of reference voltage wave 76. Thus, with operation according to the waveform illustrated in Fig. 8 there is avoided the possibility of inaccuracies due to change in the direct-current, output signal which would result in a change from the operation with some pulses, at times, subtracting from the direct-current output, and at other times due to mechanical rectifier phase shift, nctsubtracting therefrom. There is further avoided the need for a high degree of phase stability since by including the rectifier 60 in the discharge circuit, the synchronous rectifiers 72, 72a can shift in phase over a substantial period of time Without the possibility of the improper operation above discussed in'conne'ction with Figs. 5 and 6. There may then be as many radiant energy pulses 78 as desired during alternate halfcycles of voltage 74 represented by waveform 76, but none of the resulting V voltage pulses 79 willbe reversed by rectifier'sq72, 72a because their switching operations even with ,somephaseshift occur substantially during the inactive'half cycles.

- 'It is, of course, possible to have phase shift occur bee.v

tween the synchronous rectifier 72 contained in themeas:

' uring' channel and synchronous rectifier 72a contained in the reference channel which would lead'to inaccuracies in the system unless the amplifiers containedin the re f.

erence channel and in the measuring channel are compen sated, for example, in accordance. ,with :the compensation

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