AN ELECTRON SWITCH. by C. DORSMAN and S. L. de BRUIN.

Transcription

1 SEPTEMBER AN ELECTRON SWITCH y C. DORSMAN nd S. L. de BRUIN An pprtus is descried with which the time function of two different mgnitudes cn e oserved simultneouslyon the fluorescent screen of cthode-ry oscillogrph. A numer of prcticl exmples re discussed where such investigtion is of vlue, inter li, the simultneous registrtion of virtions t different points of mechnicl system or of the current nd voltge conditions in trnsformer. With the id of cthode-ry oscillogrph, such s the GM 3152 recently descried in this Review 1) the imge of n electricl tension cn e represented on the fluorescent screen of cthode-ry tue in reltion to second voltge. If liner time se is used, the curve of the vrition of the voltge with time cn then e projected on the screen. In ddition, third vrile is ville, viz., the intensity of the cthode em which cn e modulted in reltion to third mgnitude. In this wy two mgnitudes cn e simultneously represented s function of the time, displcements in phse in the mgnitudes under mesurement eing reveled in the projected imges. But this method does not revel ny smll vritions in the third mgnitude, so tht it ecomes essentil to devise n pprtus for registering two curves simultneously showing the vrition with time of two different voltges. Apprtus designed for this purpose hve often een descried III the literture, nd their underlying principle hs lso een discussed in this Review 2). The present rticle gives description of n pprtus, the soclied electron switch GM 4196, which hs een devised for this purpose, together with detils of certin of its pplictions. A view of the mesuring outfit, consisting of the electron switch nd its ssocited cthode-ry oscillogrph GM 3152, is shown in fig. 1. Method of opertien of the electron switch A circuit is shown in fig. 2 with which the time functions of two electricl tensions VA nd VB cn e represented simultneously. These two voltges re impressed on the control grids of the two pentodcs LA nd LB, nd n lternting voltge of R o SAJZi---+- VA 0-1~ Fig. 2. The two voltges VA nd VB to he oserved simultneously y the cthode-ry oscillogrph 0 re pplied to the control grids of the pentodes LA nd LB. The switching voltges SA nd SB vrying rectngulrly with time re pplied to the screen grids. The nodes of LA nd L B re connected through common node resistnce R to constnt positive voltge. Fig. 1. Mesuring rrngement comprising n electron switch, Type GM 4196, nd the ssocited cthode-ry oscillogrph, Type GM cycles frequency vrying rectngulrly with time is pplied etween the two screen grids. This voltge is of such mgnitude tht the two vlves LA nd LB lterntely either function in the norml wy or re in such stte tht no node current psses through the vlve. In consequence of this, current psses through the common node resistnce R which flows lterntively through LA nd LB nd is hence controlled in turn y the tensions VA nd VB t the control grids of LA nd LB. The voltge through R thus fluctutes with fre- 1) Philips techno Rev. 4,210, ) Cf lso Philips. techno Rev., 3, 154, 1938.

2 . 268 PIIILIPS TECHNICAL REVIEW Vol. 4, No. 9 quency of els etween two vlues which re determined y VA nd VB respectively.lf, now, the node voltge is pplied to the first pir of deflecting pltes of the cthode-ry oscillogrph 0, while liner time se is pplied to the other pir of pltes, n imge of the type shown in fig. 3 will e projected on the screen. Fig. 3. Oscillogrph imge otined when two oscillogrms re, registered simultneously with the id of the electron switch. Actully the numer of impulses is, of course, much greter thn sketched here. To ensure tht the light spot trvels sufficiently swiftly etween the projected curves (fig. 3) of VA nd VB, the mplifier must e suitle for mplifying rectngulr oscilltion of cycles per sec. It must, therefore, e le to mplify not only the fundmentl wve of cycles ut lso'hve roughly the sme gin for lrge numer of hr~onics of this fundmentl wve. In the cthode oscillogrph GM 3152 the gin remins, I ' prcticlly constnt up to 10 6 cycles, so tht the hundredth hrmonic is ~otyet severely ttenuted. of',3"23"s:!r Fig'. 4. Circuit for generting the switching voltges SA nd Sn vrying rectngulrly with time, t the nodes of the pentodes Le nd LD, whose control nd screen grids re epcitively coupled with ech other. Fig.4 shows the circuit used for otining the lternting voltge vrying' rectngulrly with time nd which is required for the screen grids of LA nd LB. In principle this circuit represents multivirtor s designed.y Arhm nd Blo c h. Between the "two correspouding pentodes Le nd LD two condensers re connected which link the control grid' of ech of these vlves with the screen grid of the other vlve of the pir. The stte of equilirium, which would e otined if the voltges of the control grids for the two vlves were equl, nd likewise those of th~ screen grids for the two vlves, is found not to e stle: If, for instnce, t' certin moment the' voltge t the control grid of Le is too high, the current through Le will increse, so, tht greter mount of current will hve to e dissipted y the screen grid resistnce of Le, which will reduce the screen grid voltge, of Le nd owing to the cpcity coupling lso cut down the control grid voltge of LD. A lower current will then flow through LD nd the screen grid current of LD will fll, nd hence the voltge t the screen grid of LD will rise. The presence of the. cpcity coupling will then lso rise the voltge t the screen grid of Le. The condition of. the circuit hence will e lile, since we strted with the ssumption tht the control grid voltge t Le ws lredy too high. The increse in the voltge t th~ screen grid of LD is limited y the node voltge to which the screen grid is connected over resistnce. If the screen grid voltge of LD cnnot increse further, the control grid of Le will ' dischrge itself through its resistnce lek, with the result tht the whole process will e repeted in the opposite direction. The relxtion time of this oscilltion is hence determined y the cpcity nd the lek resistnce of the control grid.. If now the node voltges of Le nd LD, which vry roughly rectngulrly with the time, re pplied to the screen grids of LA nd LB, these vlves will e working or idle in turn, so tht the mplified voltge VA' nd VB re' pplied lterntely to th~ deflecting pltes of the cthode-ry tue. Since the oscilltor circuit lies etween the screen grid~ ~i of Le nd LD, while the reversing voltges re tken from the n 0 des of these vlves, the oscilltor is prcticlly unffected y the voltges which my e pplied to the vlves LA nd LB nd which re to e registered y the oscillogrph. Assume tht the voltges VA nd VB to e registered hve constnt vlues nd re equl to one nother nd tht chnging-over tkes plce instntneously y mens of pure rectngulr node voltges of Le nd L D,' the, totl node voltge of LA+L8 tr-~--~+---~~--r- -t 3"23"28 Fig. 5. Anode currents 'of the pentodes LA nd Ln if they re incresed nd decresed linerly with time.

3 SEPTEMBER 1939 AN ELECTRON SWITCH 269 LA nd LB will then lso e constnt nd there will e no indiction on the screen of the chnging- t 3'23'2f! Fig. 6. Actul shpe of the time functions of the node currents of LA mi LB nd of LA + LB. over opertion. This would still e the cse if the node currents of LA nd L B during chnging-over vried linerly with the time s shown in fig. 5. smooth curves ecome visile on the screen. To otin this result it is not enough to mke the switching signls purely rectngulr, so s to otin swift chnge-over from one imge to the other, ut, in ddition, the frequency of reversl per second must e mde so gret tht the lrge numer of smll dshes pper to the eye to merge into smooth nd continuous curve. If switching frequency of cycles per sec. is employed, the oscillogrm of function with periodicity of 50 will show prcticlly no reks, s my e seen from fig. 8. Furthermore, the tenth hrmonic of 50 cycles, whose frequency is 1/20 of the switching frequency, will lso still hve stisfctory outline. If switching frequency lower thn cycles were used, the sudivision of the oscilltion of 500 cycles would ecome too corse, while if the periodicity were mde very much greter thn cis difficulties would rise in otining switching signls with sufficiently rectngulr S2S'6 c Fig. 7. The shpe of the switching voltges SA nd SB is shown in fig. ). The shpe of the sorillogrm for VA = Ve is shown in fig. ), nd for VA ~ Vn in fig. c). Actully, however, the flnks of the node current impulses of LA nd LB re lwys curved (fig. 6), so tht the totl node current which psses through R, nd hence the tension cross R, will show smll troughs. The voltge etween node nd erth, which is pssed to the oscillogrph, will therefore revel smll peks t the moments of chnging over from one of the voltges to e registered to the other, oth when the two voltges re equl nd when they differ (fig: 7). These unvoidle irregulrities cn, however, e minimised y mking the current impulses s steep s possile, when the peks re rely visile, s shown in fig. 8. Shpe of projected curves The curves registered simultneously y the oscillogrph when using the electron switch re mde up of short dshes which must follow ech other III such close succession tht firly shrp nd time function, since the cpcity of the currentcrrying prts gives too low n impednce for these high frequencies. Fig. 8. With the electron switch two curves re otined which to the eye pper to hve continuous outline. Registrtion of mechnicl oscilltions To register mechnicl oscilltions with cthodery oscillogrph, these must first e converted into electricl voltges, for which e.g. n electrodynmic system cn e employed: n electricl coil moves

4 270 PHILIPS TECHNICAL REVIEW Vol. 4, No. 9 in the field of permnent mgnet (fig. 9), the coil eing ttched to the oject whose virtions re under investigtion. Fig. 9. A coil oscilltes in the field of sttionry mgnet with north-pole N nd south-pole S. In the moving coil electricl tensions E re induced which re proportionl to the velocity of deflection v. In the moving coil n electricl voltge is induced which is proportionl to the velocity provided the deflections sustined re not too gret. If the oscilltions t two different points of mechnicl system re imprted to two different coils, oth oscilltions cn e registered simultneously y cthode-ry oscillogrph with the id of the electron switch, thus permitting direct visul comprison with respect to mplitude nd phse. Phse displcements etween currents nd voltges It is frequently importnt in the exmintion of electricl plnt to determine not only the time functions of vrious currents nd voltges ut lso to know the phse reltions of these mgnitudes. With the id of the electron switch, oth current nd voltge my e projected simultneouslyon to the screen of n oscillogrph, so tht the phse reltions re directly visile. In n issue of this Review which ppered lst yer, the comined oscillogrms of currents nd voltges of gs dischrge lmp were reproduced nd discussed. Another exemple my e mentioned here, viz. the current nd voltge conditions in circuit contining trnsformer with n iron core so highly sturted tht the secondry voltge remins prcticlly constnt when lrge vritions occur in the primry voltge. The principle of opertion of stiliser of this type (fig. 11) hs lredy een,,2$27 Fig. 11. Circuit of trnsformer with highly-sturted iron core, s result of which the secondry voltge remins prcticlly constnt, lthough the primry voltge my fluctute widely. VI nd V 2 re the primry nd secondry voltges respectively, nd i l nd iz the primry nd secondry currents. Z2 is the lod, which is principlly cpcittive (n pprtus for node-volts). S2S19 Fig. 10. The lrge mplitude represents the mechnicl virtion of n electric motor, while the smll mplitudes reproduce n lternting voltge with stndrd frequency of 500 cycles per sec. descried in this Review 4). If pure sinusoidl voltge is pplied to the primry side, voltge which is hevily flttened (fig. l2), i.e. with considerle mount of third hrmonic, is otined on the secondry side owing to the high sturtion of the iron. The curve of the primry current lso indictes pronounced third hrmonic, which, The voltge induced y mechnicl virtion cn lso e registered in conjunction with nother mgnitude on the screen of the cthode-ry tue; thus in fig. 10 the virtions of n electric motor (with lrge mplitude) re compred with n lternting voltge of 500 cycles s norml frequency (smll mplitude). This comprison shows tht the frequency of the fundmentl wve of the virtion is 53 cycles, so tht the motor is running t speed of out 3200 r.p.m. The fundmentl wve of the oscilltion lso contins numer of notches; this oscilltion of much higher frequency hs een produced y one of the moving prts ruing t some point or other. The frequency of this disturnce is 530 cycles. Fig. 12. The sinusoidl curve represents the primry voltge impressed on te trnsformer; the flttened curve in ) is the secondry voltge, nd the peked curve in ) the primry current, which oth hve pronounced third hrmonic. 3) Philips techno Rev. 3, 156, ) Philips techno Rev., 2, 279, 1937.

5 SEPTEMBER 1939 AN ELECTRON SWITCH 271 however, ugments the mxim, s my e seen clerly in fig. 12. Furthermore the figure revels tht oth primry current nd secondry voltge re somewht delyed with respect to the primry voltge. Registrtion of pulsting direct voltges As ordinry electricl mplifiers only mplify lternting voltges, n oscillogrph will usully revel only the A.C. component of pulsting direct voltge. One of the min dvntges of the electron switch is tht it enles us to study lso the directvoltge component with the oscillogrph, since it converts the direct voltge into n lternting voltge of the switching frequency used, nd for which the mplifier of the oscilltor hppens to e prticulrly suitle. In the oscillogrms in fig. 13 the two curves show the zero line of the voltge nd the pulsting direct voltge of source of node voltge. With low lod of 100 millimps the ripple of this direct voltge is oviously only smll (fig. l3), while pronounced pulstion is shown with hevy lod of 500 millimps (fig. 13). Fig. 13. The oscillogrm of pulsting direct voltge s well s the zero line cn e registered with the electron switch. ) The voltge of n node-volts supply revels smll ripple t lod of 100 millimps. ) At lod of 500 millimps the curve shown in ) cquires hevy pulstion. AN ELECTRICAL MEGAPHONE y J. de BOER A portle electricl voice mplifier, the "Portphone" Type No. 2831, hs een devised to otin greter rnge of the humn voice thn when speking normlly without directive ids or when using n ordinry megphone. This pprtus increses the intensity of the sound energy 30 to 100 times the gin relised with n ordinry megphone. Hence, the rnge of the voice is 5 to 10 times lrger with the "Portphone" thn with n ordinry megphone. In ordinry speech the rnge of the humn voice is not very gret, nd vrious mens hve therefore een devised for incresing the rnge to which the voice will crry. The oldest of these ids is the speking tue or megphone which ws invented out the middle of the seventeenth century y the Germn divine K i r C her nd the Englishmn Morlnd. Its ction consists in imprting directivity to the sound wves so tht they re concentrted into nrrow em with smll solid ngle. Recent developments in mplifying technology hve, however, rised the question whether etter results in the trnsmission of the humn voice over gret distnces could not e relised with the id of simple electricl mplifier. While retining the concentrting effect of the megphone horn, n pprtus of this type would moreover provide source of sound which is more powerful thn the humn voice. In constructing such simple VOIce mplifier, the first requirement is to rrive t mximum convenience in use. The loudspeker, horn, nd microphone should form compct unit which cn e conveniently crried in the hnd, while the electricl mplifier my e ccommodted in seprte cse. The "Portphone", Type No. 2831, which hs een designed on these generl lines, is shown in fig. 1; the cron microphone is mounted in the loudspeker which lso crries horn nd hndle with switch. The flt cse contining the electricl mplifier is crried y strp; its totl weight is 6.8 kg. nd in ddition to the mplifier lso holds 2-volt ccumultor to furnish the filment current nd to feed the microphone, s weu s dry tteries for the grid nd node voltges up to 150 volts. The electricl gin is pproximtely 40 deciels. The cousticl gin is somewht lower owing to losses in microphone nd loudspeker (cf. t the end of