GB Patent 556855 - Synchronisation of alternating current sources particularly applicable to common wavelength broadcasting systems

Description

PATENT Sft CIFICATION

Application Date: March 31, 1942 No 4307/42.

Complete Specification Left: March 2, 1943.

Complete Specification Accepted: Oct 25, 1943.

556,855 PROVISIONAL SPECIFICATION

Synchronisation of Alternating Current Sources particularly applicable to Common Wavelength Broadcasting Systems I, R I Av N Iox CALVERT, a British Subject, of 34, Streatlham Close, Leighan L Court Road, London, S W 16, do hereby declare the nature of this invention to be as follows:The present invention relates to the synviiehronisation of alternating current sources Although of wide application, it is of particular use where it is required to maintain the frequency of a slave oscillation generator in step with a master oscilla tion which has been trans-, ritted from a distant point Where the aecillations to be synchronised are of, hiib ifrequency, as is the ease in common wav-elengthl broadcasting where the oscillations are of carrier frequency, it -is customlary to transmit from a master station to a slave station over a line a reference tone which has a frequency which is a known sulb-multiple of the master radio frequency Although the reference tone received at a distant slave station will remain constant in its mean frequency and phase relatively to the master station, there will often be randomn changes, of phase al-out the mean value owing to interference on the transmission line These changes are particularly bad when, as often happens, the reference tone is transmitted along a speech channel and sharp band pass and band rejection filters are used.

At the slave station, in order that a comparison mayv be made between the master reference tone and the slave oscillation, it is, necessary either to multiply the reference tone or divide the slave oscillation W Vhere the disturbances referred to are considerable, multipljication of the reference tone is unsatistactorv because the fluctuations would be multip)lied as, well as, the reference tone.

It is therefore necessary to divide the slave oscillation The present invention is particularly concerned with cases where this is done so that a comparison of the oscillations is made at a relatively low frequency for example about 1500 cycles per second.

The present invention utilises a method of maintaining a slave source of electric lPrice 1/-l oscillations in synchronism with a master oscillation, which consists in combining the two oscillations in two modulators, 55 the phase relation of the two oscillations being different in the two, modulators, and utilising the outputs of the modulators to control in an appropriate sense the frequency or the phase or both the frequency; O and the phase of the oscillations generated by or derived from the source.

In one method according to this invention, two relays are employed and these are so, arranged that the order in which 65 they are actuated is dependent upon whether the slave frequency is greater or less than the master frequency and the order of actuation determines the sense in which a frequency adjusting device, con 70 trolling the frequency of the slave source, is moved.

In another method according to the invention, two pairs of modulators are used, and the method consists in combin 75 ing the master and slave oscillations in these two pairs of modulators, the phase relation of the two oscillations being different in the two modulators of each pair, and utilising the outputs of the 80 modulators to control in an apropriate sense the frequency or phase or both the frequency and the phase of the oscillations generated by or derived from the source 85 The invention also provides apparatus for use in carrying out the methods set forth.

The invention will be described by way of example with reference to the acccm 90 panying drawings in which: / Fig 1 is a diagram illustratinig' a s.imple form of frequency control, / Fig 2 is a, diagram illustrating/a preferred arrangement according to 'the in 95 vention for giving frequency control, Fig 3 shows a circuit for hpplying oscillations to the circuit of Fib 2.

Fig 4 is a diagram to illstrat e the manner in which the circuit 'of Fig; 3 100 operates, Fig 5 is a vector diagram for explain ing a, preferred arrangement aceording to-.

the invention, described with reference to-5.56,855 Fi-s 5 to 10, wherein plinse variatiou is made use ol to control the frequency, and which includes, means whereby the phase of oscillations derived from a slave oscillator is controlled, in this case iiieideiitally to the operation of controlling the frequency.

Fi- 6 shows, the eircuit of a comparator in which the phases of the oscillations are compared.

Fig 7 is a diagram to assist in explanation of Fig 6, Fig 8 shows a preferred device for lgiving phase shift, Fig 9 shows how the device of Fig 8 can be applied to the Present invention, and Fi- 10 shows a circuit,-, conillinin the circuits of Figs 6 and 9.

Referrin to, Fig 1, the master frequency is, fed directly from teiiiiii-i-,tl 1, into, two modulators MODl and MOD 2; thet slave frequency is fed front terminal 2 directly into 310 D 1 9nd through a 90 ' phase shifter 3 into MOD 2.

Let the voltage of the master frequency be assumed to be A sin mt and tlizit of the slave frequency B siii pt Then the inp tit to MODl is A sin (d + B sill pt, anti into MOD 2 is A sin ot + B sin (pt + OT A sin wt + B cos pt, sill ce sill ( O + 2 cos, 0) The, lawer sideband at the output of MODI is 0, cos (u)-p)t and at the output of 310 D 2 is C, sill (")-p)t, where C, and C are proportional to the product A B and to the canstants of iniodulation.

These outputs have the same frequency but they are in quadrature aud colisequently they can be used to drive a twoljlla-se motor M Since cos O cos, ( 0), whereas, sin O = sin ( 0) tb ere will 'be a reversal of the direction of the motor field as (t,)-p) passes from positive to negative Thus the motor 31 will rotate in one direction for (,)>p and ill the opposite direction for (,)<p, and it can therefore be used to drive a frec I Lien(,v control on the slave source The rotor will turn with an angular velocitv of (W p) radiansilseconel, so that tkeslave frequency, will approach the, master frequency -,ym-ptotically.

This system apparently offers a simple so'llitiou to, the problem of aufomatic enntrol of freciueney but in practice has several disadvantages, mostly of In i-nfrument f ype.

As, the motor requires a cert-iiii anioulit of power to drive it, the lliodulators mart be followed ty ampliflers These m-iit be of the D C Type as the diference frequellc is sin low, nd tll,,ef(ii e the c-oreices the i oii;i),, field take the lorni (it varying D C z(s opposed to true A.C Am,ijilialziii(e of the tnel lu cur 65 rents will produce fixed poles, whiell will cause the rotor to tall ill jerks lf the unbalalice, is sufficiently rent the rotol.

will tend to slick at a 1)01 e aild 111,1 referee to move at all The iiiii)lititi's inust 70 lw balanced not on Iv statical Iv bill dynaIllically Tlji, is diffivult Is it i not Possible to _get 11, plire Sil)ll 'sol( 1,11 -,iveforni froni1 lie Z 11swh l low frequency 75 The phase on the Elle iolw appeal, Ull(lil)lilli'lje(l At the output of the modulators, tol if all uscillatioll A sin is inodiflated witly tanother 13 sill the lowel Sidelland ii, C co, 80 P is illipo -sililetoremove the disturl): -, 11 (e,; by Lisill filters.

beea-nse if a filtel is beill drivell ill the steady state altd tfle drivill Jolted"(, i, suddenly charitiesl ill Idiase, there is at 85 the outwit o f the filler N cliall-e ill phase -Syllose 1 lia-nitilde is (letel-iiiiii Ii llot Oilly l)v the iii,-t,,iiitj-ide of tli(, _fillase change of the driving voltage hut ty the, ( liararteristies of the filter, Ill(, filfer also 90 cause,, a chan-e ill Filters, therefore, do not iiilj)io-je inafters wol can actually thein worse He'lee else, line tone, are -,Diversto the rotatin fleld, causing (-oi i esj)(iiidillo95 variations ill its velocity The rotor can be Greeted froin followitig in s.vililiitlivonlyby of dainpillg.

There is, however, a proctical Iiinitation to the aniotint of (laiiii)iiil, that can fle 100 iniposed set fl Y the j)t-)ver available to drive the niotoi, and ill pra Mice when the (Ii zktiirl),iiiee; zire severe flie rotor camint.

be prevented froin r,,iuriiiil unevenly.

Fiii,,?,llv, there is a ( 1;fficiiltv, 105 that is the -Iowiiesz of oi)ei,,-ttioii Thi; is best 1)a exaniple thils O lisle 1,11)e l-C(lllellcvof 1000 p: S,cull lliat it is (lurched to 'keel-x the slave to,x-ltliiii 1 i)ai t in 108 110 (if the master frequency Whell the, slave is on the Yei,(, of its folei iiiee tb f.

fliffereuee, fieqtieliev is 1 cycle ill 101 seconds, or 1 evelo iii 2 lfouled Tii Ill e sv,feiii iiif defensible(l the nintni 1111151 llot 115 do more tlian 1 revultilion fir 2 S 1)oitis if flie e enliffitions fo 'he fistful(,(l for if t did the slave fie(iuifvi(,v W 0111 ( 1 lAte ontside tolerance To practice for c reasons Oven Allege, til, illotol 1111 ( 1 1) ot 12 ( be allowed to Court (-)l The 11 v niore tli, in -,il-oif 3 t-)ai f in 108 per revoliltion so that the s Inve inust have :1 fi-eclilelic-, of nt l'LlQt 1 part ill 101 -Cor a Period of 9 Ironies 125 A vsteiii to This invention, 6, 835 which overcome, most of the practical (if the inotor system ( 1 Fig.

A, is lllit, ti-itiefl iii Fi- 2 Althatigh it is le,; seiiiti- e than the, arra Buement of Fig 1 iiii, is usually llot, a elioii,: disc becatise, where (Treater accuracy 1, tn of control is required a,vtelii responsive to phase V'11 1 ation or controlling the, frc( 1-tiet)cvwill be -used as will he described later.

to Fi- 2, two anode relay, A, A are comlected as shown to two Clector relay ' S, -bird S',2 alid 5 vith two.

illotol 1-el;iv, M, alld M -are coilres I)eetivel,to Ilie W, W, o T a illoi Gi M stlell tbat il the sviii(iiii W, is, the iiintor 111 Ofie directioll, W Ililt if the will(l illn," W, i the motor turns in the x)ljl)osite dilection The motor is drivers l)v ciiireiit fed at 4 and can 'be zi"iaiiloe(l to a(l just, the i iiiiiilip: element, of Ait, slave sotirce The 1)11 rl)o,e of the collpectiolls Alown;, +O cable tile, angle of the, rotation of the motor II to depend -upon Ill,' order ill, which relay, and A 2 are 1 ( ttlzltefl 1,rtli always However, being required to lie before t 116 Illotor m-ill rull.

Thus first fliat the order of actuation i A, A 2; when A is actuated, a V;l-c-nit i, made frora earth through a lj,,ttteii,,, B, through the right hand contziet,; of relay A, through the winding, of the relay S, and the left hand confacts, of the relay 52 (which are closed tiivtil 52 is actuated) -ick to, earth The relay S, -;- therefore,,chatted The relay A-,, is sect actuated,,air(l tltlioii,-, li the battery B, is then collected through the right liand contacts of relay A 2 to the relay 52, lAid, latter is not actuated hecatise the left fland of the relav 'S',, are, thell (pell However a circuit is closed from 4.5 through a battery B,, the left hand colltacts of relays A, and A 2, the right haild cuntacts, of relay S,, the will(lin Z-I of the lliotor relay M and thence flack to earth The W'ludin W, is therefore enerpised Similarly if the seq-ttence of zletuation of the aucide relay % is A, A,, the W,, will te, enirgised.

For the amde relay, A, A% there array be used the circuit shown ill Fig 3 ' Ma 4 el oscillations are applied through terminals MI to a tr,,xiifoliyier T.

Illcl lave ios(,illtitioiis, are applied (at.

leriiii Dalg SI to it trin,foiiiiei T, B, means of a eondenser C and a resistatlee R, the master freq-tiency voltage at Iii is made o lead that at a, whilst by me,-iiiq (if a condenser 02 and R the master fieq-tieiin voltage at ( 1, is made to lop 1)v an equal amomit on the voltage, al, a 'The voltages at b and ( 7 are N 30 ( 1 11lated, with the slave voltli( e with the aid X re-; 1)e(,iively sill of rectifiers -X, and ' d the rectifier o,-iitpiit, appear across collItensers C, and, C, iii the grid circuit, of valves V, and V,, the circle circuits, oi 10 syllich colitain the alicide A, Cantl A, re.spectively of Fig 2.

Let the voltage at tfle e(,oii(l, irv of Ti h(, represented -1) A sin vit airsl the voltage at the secondar-v of T be repi esented 11-Y 75 13 Sill pt The it if tle phase lead iiiii o'duced fiv C, l R, is O' n-nd the l,a,), introduce It, l C, isC, the across Ill( reetiflor cirenit will 'he A sill ( t + 13,it) Ikti and the, volt,i 9 e across X-f-', will be:

lA, sin ((,)t0) B -dii ptl In order to silliplify the e-X; 11 ilizilioll we shall tlint A'=B flint the rectifiereircuils at peak volt S,5 Illeters, o tliat tliere will lie a D C.

G, equal to the i)(?iilc voltage at 7) and a D C voltae C, (,quit to the Peak voltagge at ( 1.

Now considers the state of Sybell 90 the slave frequellcy i, equal to, tile 111-154 elfreq Uency and wl;(-i e flie input voltage from thislave (i e file tlin secondary of T 2) laid any arbitrary plinse angle (p Vector addifion the 95 voltal-e b as: & = B, 2 l 1 + cos (q, 0) alld the penk voltioe at ( 1 11, = B,/2 r 1 _ cos ( 6 + o) L The oiiclle, i tliocle vultal-P of the V, 100 is therefore:

c,, =_ 13,/2 l 1 +uo -E and of V 2:

c = B /2 l 1 + cos ( + 0) E where E is trio voltage oll' the (;,rid bias 110,5 battery E collision to both Y Ave',' The flias volt,t- e E is imide -reatly to exceed the cut-off voltage the Furtive V, will then take ano(le lrev ever bo t 01 110 5111 all rall-e, ot Vallies of ( 1,1 1,17 the valve 2 will take mode 1 1 ill over a,iiiall raise of (i- il)otil Illen the cirenit em be to give the conditions shown ill Fig: 4, which show, the anode current, 111 112 f(l the 1 15 twol valves plcitted o Tilt clotted line 1,, i e preoe 7 if the mimmum curreni required to airline flie Airline relavs and the dotted line I, the etil"ent at whir'l, these lelav, will bold ( 11 120 otice, they have, been made.

5565855 is nearly correct, j, J, chatiging very If it ha, not clianged enou h to break one of the anode relays lay the j:liiie the inotor has completed its run the 65 Selecior reizi- m-111 still ice made, and the inot or will i;ii)l,-e a secoild rein in the salne direction In la-t, the motor Nvould conlimie runnin mitil el had shifted far to Oreak one of lhe, I iiode relays, 70 liy which time the slave freqiieiiey would ij all prollability have 1,(-eii ovei,-,i(Iiii ted.

The anode i i-itiys are easily broken when 11 W 11101 ol to rtill hy providing oil Ale Illotor call)-Switell a secolld contact 75 which applies all additional Ilizzis voltage to bias 'lie valves to beyond cutoff the cirenit, must te so li(f iii-i IC(I 111 a; there, i Q; j 10 all-le O (," at both anode relay, ( all make simul 80 1 Z 1 fl eollsl v Ii llii, is not done the valves Alin lie s-s N-itu Jie(l Nick into circ-nit, after i,, rim of the motor, -,it such an angle and the relay which had the children operation illotor.

The motor 31 can be of a simplc and (-Aiding tylie, the relays may lie of staodard type ill(l require no adjuslinent 1 lod the (,ice,,) current of ' the anode relays A, 90 strict A (i e the difference between the required to Diake the relay and that required to break it when litde) is lstreet o that the adjustment to flee input circuit is not critical 95 Where -31 eater accurate of control of frequene is required than can be obobtained by flie ai Tzingelileji'k' jail descrited, control ol frequency in response to variation in phase can Tie provided ill 100 a maimer which will be Described b Y way of examplewith reference to Fi s -)'to 10.

The principles iiii(lel-lyill", tfle method of jilt,tqc to l)c,ire indicated ill Fi- here the vector OA 1 05 i.eiji eseiii the slave volttt,,e acid the vector OB 1 he master voltal-e These volta-es are seen to be in quadrattire The resultant of voctors OA and OB is shown af O C If now th pli,"e of file slave voltage chan-es 110 thro 11,Al angle O to -,ill Inerease ill slave frequency, the vector OA moves anticlockwise to OX 1 and she restiltarit be(.oiiie, O C 1 -",llicit i, -realer than O C.

Similarly, if the fieqiiei)er decreases 115 1 v file samp amomil, the Yeefor illove-; to 0 A'I and the iesiiltiit O C 11 will be sect) to be less than OC.

The increase ill(l derrease in the vector sum prod-nced by the of the 120 vector OA to O A 1 Acrid O All iel)ectivelv are, each used to, actuate a relay Each relay when actuatel serves to, do two firstly to tlecrea,e or increase the slave frequency b y a predetermined 125 amount, and Reconcile to introduce a pbase Ew ot cyclist s-ij 1 lieconStirti Dll j G= 2 Ien Z,11 ZUE'le 1 1 reached, the anode A evillmake.

AS-11 ell all allof i- 1-elclleil, the l-alter A, bill lbrace Both i-elly, -W ill llold oll ulltil tlie i, 5 V 13 en A Il 1)reilk At z i i v a 1 1 O' c -L x 1 1 A, will 1 k If the aii,,-le Loth relavs will remain firoken imitil ili 2.- -(; 5: is i-eii I)Pcl wben the 11-I-ll'lflfl lie startfli P llairlh 1) alld Il ZltlLeell 2 Idle cen rt, vei sed It i ilo clear arts,, 114) lil ItlT Mli d the startillo llably1 c fl, c y oil (i, increase, or It mav that this is 11 ( 31 lle(-ezszlr;lv the cas c with stir aij,, le o-O Drift illit 11 owevel the oill " eff(c- is that Yell,YA, maize set:ill ol m at fail amde before the relay A llas 111 Lide.

'I'lle Illotor -,visit 3,ot lie offected lilifil the Z 111,0-11 e lws increased -iifii( ieiitly to imake first A, and tli-ii A,.

If the slave frequency does not equal Die sinister, fre(menv l Able is SAID, 'reatel:

flizin it the el Vert i; tlie;acne o all-e and file A 2 oil, A 2 wid A, on, A, on If the slave frequency i, less Aloft the Ireq-vency file effect else ame as 35:isles zealot Divers the,ecji-ience, A, oil, A, 111 ( 1 A nil, A nil.

-witch a few m-actical epic, vteiii can e "ilv lip madi, to o ive " 'reC 111 Plic V -11 centres The Attire (Contract, (if the rjetifiel eirellif' Ear),l tbe finie of trio relpvs iii-tif lie le,;, tban il),, citric taken hv oiie c-ele of the difference frequency Thi:, i Qz not , difflimilt Colldi1; 011 to fulfil a; the V,ml 4,5 be made apl)roxiiii,,ifelcontrol ic Asserted nil.

are l)aside(l fnix;viteliiiig, the, motors -M (Fi 2 2) ffi:after it lia, Trill for a 1)ieclefi Eiiii,"tied time, stroll 1 zaloft+ -'o,-yl)iiij)l, thi; trip clip device, for example 2 VZ',11able condenser, for -idiiisliijn the freC 111 PDCOf fir(' Ca P l)C 1 -ed to lia -c eb lreel the freqtieijcl,v t suit,,iblp,iii( 3 tilil adult the vid (Y f' Pie (if the Junior mav Ye effecied hy circuit of Vaill 1 ,kvifeli drivers Ly 11) mo for Tbore imif also lie stein, -,, provided for liie)1,iii1 he anode relay-, as soon as flie motor acrid; to rtin when tlip grave frequency 556,8 bo lag or lead of C , or slightly more, in the slave irequency circuit.

fig U is a diagram of a circuit whereby the anges shown in Fig 5 can be used o -actuatute relays A 1 and Ag.

Ma.ter oscillations are fed to terminals M 11 of a transformer T having four \econdary winlings e,, e 2, e, and e 4 The slave oscillations are fed at SI to a transformer T= having three secondary windings fl l and /f The whole arrangement of transformer T 2 is shown within the dotted rectangle 5 but only the secondary windings are shown conneeted with the modulator circuits of windings c,, e, etc The winding /2 is, common to the modulator circuits of windings e 2 and c, Each of the four modulator circ'nits e 1, lA, c, f 2,, etc contains a rectiflier x', r etc and a condenser C,, C 2 etc.

The modulators are coupled as shown to 1 wo valves V, and V which have anode relays& A, and A 2 in their anode circuits and are normally biased beyond cut-off by a bias voltage applied at 6 The modulators control the grid voltage of the valves V 1 and V 2 in a manner which is hest understood from the diagram of l'ig, vhere the transformer windings and their associated rectifier circuits are hown against the corresponding vector d(liagramis Thus, windings e, and fl, together with the rectifier X 1 and condenser C 1, make the point P positive with respect to L; windings (e and /2 with X, and (C, make the point L negative with respect to G Then, if the voltage across C( is equal but opposite to the voltage across ', there, will be no potential differen'c bletween P ali t G This condition is obtained when the input voltages are iii quadratitre If the slave input nsow starts to lead in phase, as indicated by an anllti-lockwise rotation of the vectors f, f and,f, in each of the vector diagrains, the sum vector of cl /f will increase and that of c /2 will decrease This means that the potential difference across C, increases vlhile the potential difference ',loss C decreases, and so, P is made positive with respect to G Similarly it an be shown that Q is made negative with respect to G by the same amount If the slave input lags instead of leading in phase the effect is exactly the opposite; i.e P is made negative and Q positive.

Thlis, starting irom the quadrature posii Oni if the valves atre biased to cut-off it is clear that the ariode current of V, will rise as the plhase of the slave is advanced.

Meanwhile the current of V will remain zero, whereas if the phase is restarted the current of V -will rise and the current }f V remain zero.

The ac-tivation of lthe vtalve V, and hence of the relay Al is arranged, in a manner to be described, to introduce inll the circuit of the slave oscillations a predetermined phase lag of O, and also to decrease the frequency of the slave oscil 70 lator by a predetermined amount Similarly the activatfion of the valve V introduces a phase lead of 6 ' and increases the frequency of the slave source by a, predetermined amount The circuit of Fig 6 75 is adjusted so that the appropriate anode relay is closed when the relative phase of the inputs is changed by substantially the same angle O It is not essential that the correction applied should be exactly 80 equal to the error which causes the anode relay to close so long as the correction applied is, great enough to release the anode relay.

Owing to the fact that when the input 85 frequencies are nearly equal, as they usually are in practice, it takes a considera Hle time for their relative phases to change by say O = 5 to 10 , it is permissible to insert smoothing circuits with 90 long time constants between the modulators and the valves V and V The resistances R and condensers C of Fig 6 constitutes these smoothing means Since the potentials across the condensers C 95 cannot change quickly, the fluctuations of the output voltages of the rectifiesl circuits, due to rapid phase disturbances on the reference tone, cannot appear at the grids of the valves and the operation 100 of the system is therefore entirely unaffected by them.

There is a second need for the time delay circuits R, C, which is to enable the motor, which effects frequency adjust 105 ment as will be described later, t o run long enough to close the hold on circuits of the motor relays after an operation of the phase restoration circuit.

The phase restoration circuit by which 1 i O the phase corrections are applied has to blie of special desigm, for it must introduce either a phase lag or a phase lead after each operation of an anode relay Now it is probable that over a long period of 115 tihme the mean frequency of the slave oscillator will drift in one direction and so the phase restoring means must be capable of inserting phase correction continuously in either direction; that is to 120 say, the phase shifters must have no l.imit to their travel 'There are two ways in which this can be done, either there may b)e one phase shifter which when turne(l clockwise, say, alwavs introduces 125 phase lag, and when turned anti-clockwise introduces phase lead; or two, phase shifters one of which introduces phase lead nd the other phase lag It has been found simpler for mechanical reasons to 130 D adopt the second alternative, and suitalle circuits have been developed using standard Post Office rotary line switches for the purpose.

The paeferred form oi phase shifting network is that illustrated in Fig 8 The phase shift produced between the input terminals 7 and the output terminals 8 of Fig 8 (a) is O ' tan-e C= 152 provided that the reactance X, of the condenser C, is high compared with the input impedance and provided that the output impedance ishigh compared with Xc The circuit hathe advantage that the output voltage is independent of O The way in whiech all unlimited change of phase can be produced with the network of Fig 8 (a) will be understood from the following:

When R 1 = 0, the output terminal j is connected to the point,q and if X, is, highl compared with r there will be a negligible phase shift through the transformer and the output voltage will have the same phase as the input voltage As Rs is increased the output voltage will lag, the input voltage until R=o' when if will lag by 180 , provided X, is small compared with the terminating impedance.

This circuit thus gives a phase shift of from 0 to 180 In order to continue the phase shift from 180 to) 360 , the connections to the condenser G and resistance R are reversed as shown in Fig 8 (b).

The circuit of this figure gives a phase 36 shift of 180 when R-1 = 0, for then il is connected to h' As, Rs 1 is increased the phase of the output again lugos until it reaches 360 when Rl=o Thiis colnpletes the cycle and by switchien back to the circuit of Fig 8 (a) the range from 360 to 340 can be covered and so on In order to get a continuous phase lead the procedure described is reversed alnd the circuit of Fig 8 (b) is used first with 4 f i l _x R 1 is then reduced to zero and the circuit is switched over to that of Fig 8 (a).

The way in which the circuit of Fig' S can be controlled by means of standard Post Office rotary line switlches is showm in Fig 9 The whole phase restoration unit shown between input terminals 9 and output terminals 10 is arranged in the circuit of the slave oscillations between the slave source and the phase comparator of Fig 6 The oscillations first pass through a device 11 for introducing phase lag' then through a stage of amplification 12 then through a device 13 for introducing phase lead, and finally through a second stage of amplification 14.

The devices 1 1 and 13 are line switches each having foullr banks of contacts 15, 16, 1,5 and 161 The, contacts of the lanks 15 and 151 are strapped together and tlhe contacts of the banks 16 and 161 serve to provide the variable resistances 1 t and 1.1 of Fig 8 (a) and 8 (b respectively.

The condensers C' and C'); also correspond to the conlensers of Figs 8 (a) and 8 (b) 70 respectively 'The terminals 17 are connected to a source of H T voltage the negative t ermiinal of wvhich is earthed.

Each actuationl of the relav A, of Fig.

6 is arranged, with the aid of a suitable 75 ratchet nlechanisnl, to move the fiour contact arms of the device Il forward one step corresponding to tle spacin g betwveen two adjacent contacts Similarlv eaich actuation of the relay A, of Fig 6 is 80 arraloe(l to move forwardd one step) the arms of the device 13 The device 1 1 introduces at each step a)hase lag of O whilst the device 13 illtroduces at, each step a phase lead of With t'ventiy 85 five contacts in each balnkl O will equal 7.2 The ratchet nmechanismls are stuch that whichever of the relays A, aod A, is actuatteld muft l)e made and broken before the contact arms are moved forward one 90 step.

The way in which the circuit of Fig 6 is comnbined with that of Fig 9 and also serves to introduce a fre(quency adjustment is shown in Fii 10 The terminals MI 95 are fed with the master tonle and the terminals SI are connected to the terminals 10 of Fig' 9 and thence to the ,lave source The phase comparator PC is that shown in Fig 6 with relays A, and 100 A These two relays actuate selector relays S, and S respecti-ely w-hich in turn aetuate motor relays M 1 alnd IM, respectively Actuation of the relay M, energises the lower winding of the mo)tor M 105 ani(l actuation of the relay M, energises the tupper windin The motor M serves to drive a tuning element, such as a variable condenser, of the slave source and may lbe of simple type;uieh as is used in 110wireless receivers for p)ush butt on tuning.

An alternlating eurllent supply is conneeted at terminals 4 W Vhen the motor is actuated by the lower winding a decrease in frequency is prlodueed and tvice 115 er,s' The inofor i a M that in Fig 2, ha.s automatic mneas (suech as a cam switch) for switching it off a predetermilled time after it has b 1 een started.

The relays A, and A, thro h the 120 selector relays S, and S, actiate line sw;tches 18 and 19 r-spectively which drive the eontact arms of the devices 11 and 13 respectively of Fiqg 9 Closure of thle relay A, (say) does not immledliately 125 cause a phase shift to le introduced I ecause, as already stated, the relayv must ble both nmade a;ld broken lefole this occurs However, when the line switch is made it opens a contact which is in series 130 l with one of the inputs to the phase coiniarat or PC' Now, with omly one input to the phase comparator tile irput circuit is balanced and there is no output fromi thle rectifiers so that the anode cirrent of V\ falls to zero, breaking A 1 The rate ait v-hi, h the anode current falls is determined a' the time constanlt of the smoothing Circuiit It/C (see Fig 6).

Tlis timne constant should bie long enough for the anode relay to stay onl for alnout two seconds (say), because when the motor M starts to turn it takes one or two seconds for its cam-operated switch to close and until it is closed the hold-on circuits to the motor relays are not completed.

It is useful when inaliing adjustmlents to the circuit to have a manual control over the slave frequency In Fig 10 this is provided for by a switch 20 which gives req-uency control and a switch 21 which gives phase control Raising the levers of these switches produces a decrease ill requency or a lag in phase and lowerin, themn produces the opposite effect As sliownl it is desirable to include inl the energising circuit of each mnotor relay M, and M the back conticts o' the other, so that both these relays cannot be mnade simultaneously The terminals B, are connected to one terminal of a D C.

source, the other terminal of which is earthed.

l The secondary which calln be achieved by the last-descrited system of control is limited because the slave trequency lmust necessarily be adjusted in steps, and betwveen steps the slave oscillator runs free.

More accurate results may ble obt tained by usiing a vernier control which controls the slave frequency betwveen the steps of the motor control Thus, supposing tlhe motor changes the frequency in steps of one part in 10 g, the vernier has a maxiimum control of one part in 10 S and operates continluousl v through the angle 7 O .

Thus, starting from the balanced condition with the vernier at zero the operation is as fellows Suppose the slave frequency is high, the phase of the slave input to the phase comparator advances wxith respect toi the phase of the reference tone, the anode current of V, rises and thle vernier starts, to decrease the frequency in 55 proportion to the rise in anode current.

When the phase of the input has changedl by 7 2 '0 tile vernier has reduced the slave frequency by one part in 1 Hs At thi, poin t the mnotor operates and intr oduces a 00 step of one part in 10 I, and( both the vernier and the phase -ire reset to zero.

The process is then repeated.

The vernier control may be a, small variable condenser operated by a moving 65 coil movement having a differential winding with one halt in the anode circuit ot the valve V 1 and the other in the anotlde circuit of the valve V,, so, that tlhle capacity would be increased ly a rie in 70 current of V, and decreased lby a rise in current in V The control obtlined(l in this vay is suche that, starting at any arbitr N time, the frequency control is proporrio'ial to the 75 angle tuin ed through b 1 y the vector representing the slav-e input fo thme phla e c-oiparator with respect to that of the reference tone At fir t sight this looks like a systemn of straight p)iase ' control 8) and it might be suspected of liabilitv to hunting; but in point of ifact it differs from the straight" phase" e control in two very important ways First, the control increases contilluouslhy withl increasing 85 angle, whereas in the straight " phase conltrol system the control decreases againl if the angle exceeds 90 U-which is the cause of the instability Second, there is no limit to the miagniitude of the control, 90 since the mnotor can be made to do as man:y steps in eithler direction as is neee-sary; whereas in straight " phase " conltrol the control cannot exceed the value obtained at 900 95 i Dated this 31st (a-v of Marchl, 1942.

IRIEDDIE & G 1 ROOSE, Agents for the Applicant, 6, Bream's Buildings, Londonl, E C 4.

COMPLETE SPECIFICATION

Synchronisation of Alternating Current Sources particularly applicable to Common Wavelength Broadcasting Systems I It AY Ml OND CALVERT, a B 3-itislh Subject.

of 34, Streathain Close, Leigham Court Itoa(l, Lonidon, S AV 16, (do hereby declare the nature of thlis invention and in what manner the saile is, to lbe performed, to be particularly described and ascertained inl afnd by the followin O sta'emelnt:The present in Venti't 1 relats to the synchronization of altelrnatilgn current sources Although of wide al fipicatiom 105 it is of particular use wherie it is, required to maintain the freq(lueicv of a slave oscillation generator in step with a master oscillalion which his bleen t I-aismilted froin it distant point Where the oscillation to 13 c synchronise(l are of frequency, its is the case in common wavelen-th brmideastin,, m-here the oscillations ale of Carrier 'ieclueijey, it is to ii-,tilsiiiit froill it 111 a Stel:

tzitioil to a slave station Over it line it reference tone xs-jjieli Iiii; a f l equene which i , it stil)-j)j-,iliiljleof the master radio frequency Altbough the reference tone i-e-ci-e('t ikt it distikat slave ,1 itti(J 11 will renjaill colizt-I-Iii t in its ilicall fi-e(liieiicv and pliase relittively to the master tation-, there Baill oficii be randoni, chan-es of phase sir ont the jocan value to interference on the i)ii s-loji fine These are par-')Airl s-lieti, as often lialipens, Ific eferelice lone i transmitted zilon i speech cliannel Acrid sharp;Iit(l filter's are tisetl.

At the slave station, in order thitt it c(jilili Lirisoii may 1)e made liet-vecit the mabter referelice, tone and the Live oscillation it is licee-;sal Yeither to intiltiply the je'ereiic(- tone ov (livide 1 he slave Where the (listtirliiiice-, referred to are plicatioll (i the reference tone is f,I( t(l v tile lliteltiatil I 11,, wolild lie iiiilItil)llcl 's well as 1 he lone.

It is therefore uecessai v to divide llic Aitye o The Iii e -;uiit invention is wil b.

where this is;)lie so lli;tt a ( ojiii)i Ii-ioji of the o,,5 clllatioiiz is made at it relatively lo,\fl'P(IUCDC 11-, for C-ikill 1)1 e,tlotit 1 500 ( Y( le per secolifl.

Tbe presew ii)-jeiitiiiii a, mulliod (if it 'Llye of (sematioll ill it initsicr miliell consi A, ill ( oj))l)illiji" the two ill two lilt)( 1111 i Ii(Il.

the relatioll ol the IX 5-o (ixf ill-lltioiisAlerting(lifferentin arrest l-o am 1 ille oi 11)111 S of' tile -1 1 1 _ P 'T 1 It I)I Bail it fl lr cirr 1 Tdt S 1 xi ( ( i 1:, 1 rf Aill i lit z 1 111 oe 1 ISC the (f Ille latiolls f"etlejiite(l l)v oi delived froll) Ille -(Ballets.

Accordin to bills hivention two pall,' 1), lil(cliilit(lrs site us A,;bill the 111 e 111 A ill tileIllit Ielalldsl;lv(, llation, it) these lwo paiis, of modu71 ( l) o %-zlv that Ille Sells(, of llie Ail ',-c, f),Ililli"tioll is 111 c tw O Modulators fif each l),Airs, antl the C)1111)11 t-, of 111 e pilirs of in iti) or botli tile -1111 ( 1 flit, of till, os(, illz-tfioll T froll) the Tlip wilwil, 1,1 Wii:Ili S (if G 5 -,11:1 k lic llcd 1 m 111 to conll-ol ill;ill zipplolipiitte scuse the (f the slave 1);eijlitioll Lv said bource and Asu lo into the oscillation derived from said sonree itiol ztll)lle(i to sitid of pbase correctimis of 70 the Aspirin order is clip pliase -ili iatioii that illitl-,le(t i;il operation, the fretillulif ol the sieve o,cillitll()11.

to 1-11 c 'iivelltioll A 1 Z 11 otit iltis niciliod include,,, two 75 pairs of iii(i(iiillttoi:s, mcans for fee,11110 c both of said lo each of said in such it maimer that the Of 1 Le slave stroll is reversed as the Modulators (if each pair, 80 I k-fi relays adapter to 'iie actitated respec1 i \-ch l) Ale outputs of sai(l two pairs of 11111 it motor fol adjusting lh,, freclitency of said source, whell act-itted, servill to 85 Sz)l(i Motto to rm, il) opposite Senses The mliv also iiitltid(, -Veritable comlections said sourre and salil pairs of iioilulitoi, opt-rable l) v the 90 outpitts (if said two jmir, of moditlators lo pply -tions ill prelletermined pliase suil,es to flip slilve oscillitfiolls li Illit'fl to sitid of illo(fil Izitol,b, respecFhese iind offier fealm-es of' Air(, ilivelll:( i, will bc describe(l, m-a\ of -lill lo all ill o', ille ' Ille Provisional Specification, in 100

1 V 11 i ( 11: F 1 p 5 is it Vectol (Iiit,,-lztiii for explaillthis Fi,- G shom-S flic f-retill ( 11, a comthe Hillsides of flic Oscil 105 ,I re to El f,( ill e-xl)lzllli(if Fig 6,' li(lxs-s Z) prelef-red (levice fol givill phast, shift, 1 1 110 F i 19 shows lio- all's device of 8 call 1 c inclined to Ille invell tioll collihilling the 11 (,71;t, ol, FigsG 115 Tbi, principle, the inherits(l of pliase c(ij)ti ol 1,o l)c descrilied iiilicite";ill, here tile vector O A 11 te slave illid the OB tile it,stel 120 T Jje ( vollages arc seen 10 bc ill The of vector, OA 013 is l O C If ' noss Ale phase o' fie Aave 11)1,oll(, 11 all fli sill iii( i ezie lit slave fre 125 0 A iiiiti( 1 o(,k14) 0 A 1 wol Ific become W w Iliell zi, illim 0 C if 11 W l Itbe hallic z Iiii(;Liiiili(, to ()lliki)(lIfic130 556,855 resultant O Cn will be seen to be less than OC.

The increase and decrease in the vector sunm produed by the swinging of the vector OA to OA 1 and O All respectively are each used to actuate a relay, Each relay wlien actuated serves to do two things: firstly to decrease or increase the slave frequency by a predetermined amount, and secondly to introduce a phase lag or lead of O , or slightly more, in the slave frequency circuit.

Fig 6 is a diagram of a circuit whereby the changes shown in F'ig 5 can be used to actuate'relays A, and Ao.

The master oscillation is fed to terminals MI of a transformer T, having four secondary windings e, e, ee and e 4 The slave oscillation is fed at SI to a transformer T having three secondary windings / 12 and /f The whole arrangement of transformer T, is shown within the dotted rectangle 5 but only the secondary windings are shown connected with the modulatior circuits of windings el, e, etc The winding f, is common to the modulator circuits of windings eo and e, Each of the four modulator circuits e,, f, e 2, f 2 etc contains a rectifier -rj, a etc and a condenser C, C etc The modulators are coupled as shown to two valves: V, and V 2 which have relays A, and A 2 in their anode circuits and are normally biased beyond eut-off by a bias voltage applied at 6 The modulators control the grid voltage of the valves V, and Vo in a manner which is best understood from the diagram of Fig.

7, where the transformer windings and the'r associated rectifier circuits are sheswn against the corresponding vector diagrams Thus, windings el and /l, together with the rectifier X,, and condenser Cl, make the point P positive with 4,5 respect to L; windings eo and /2 with X 2 and C 2, make the point L negative with respect to G Then, if the voltge across Cl i, equal but opposite to the voltage across C( 2, there will be no potential difference between P and G This condition is obtained when the input voltages are in quadrature.

If the slave input now starts to lead in phase, as indicated by an anti-clocklwise rotation of the vectors 1, / and f in each of the vector diagrams, the sum vector of c, f 1 will increase and that of e Lf will decrease This means that the potential difference across C, increases while the potential difference across C decreases, g O and 'so P is made positive with respect to (G Similarly it can he shown that Q is made negative with respect to G by the same amount If the slave input lags instead of leading in phase the effect is {', exactly the opposite; i e P is made negative and Q positive Thus, starting from the quadrature position, if the valves are biased to cut-off, it is clear that the anode current of V, will rise as the phase of the slave is advanced Meanwhile the current 70 of V will remain zero, whereas if the phase is retarded the current of V 2 will rise and the current of V, remain zero.

The activation of the valve V, and hence of the relay A, is arranged, in a 75 manner to, be described, to introduce in the circuit of the slave oscillations a predetermined phase lag of 9 , and alsso to decrease-the frequency of the slave oscillator byv a predetermined amount Simi 80 larly the act ivation of the valve V introduces a phase lead of 00 and increases the frequency of the slave source by a predetermined amount The circuit of Fig 6 is adjusted so that the appropriate anode 85 relay is closed when the relative phase of the inputs is changed by substantially the same angle O It is not essential that the phase correction applied should be exactly equal to the error which causes the anode 90 relay to close so long as it is a substantial part, thereof.

Owing to, the fact that when the input frequencies are nearly equal, as they usually are in practice, it takes a con 95 siderable time for their relative phases to change by say O = 5 to 10, it is permissible to insert smoothing circuits with long time constants between the modulators and the valves V, and V 2 100 The resistances li and condensers C of Fig 6 eonstitute these smoothing means Since the potentials across the condensers C cannot change quickly, the fluetuations of the output voltages of the rectifier cir 105 cuits, due to rapid phase dist urebances on the reference tone, cannot appear at the grids of the valves and the operation of the system is therefore entirely unaffected by them 110 There is a second need for the time delay circuits R, C, which is to enable the motor, which effects frequency adjustment as will e described later, to run long enough close the hold-on circuits of the 115 motor relays after an operation of the phase-restoration circuit.

The phase-restoration circuit by which the phase corrections are applied has, to be of special design, for it must introduce 120 either a, phase lag or a phase lead after each operation of an anode relay Now it is probable that over a long period of time the mean frequency of the slave oscillator will drift in one direction and so the 125 phause-restoring means must be capable of inserting phase correction continuously in either direction; that is to say, the phase shifters miust have no limit to, their travel There are two ways, in which this 130 1 556,855 can be d(lone: either there may be one phase shifter which when turned clockwise, say, always int roduces phase lag, and when turned anti-clockwise introduces phase lead; or two phase shifters, one of which introduces plhase lead and the other phase lag It has been found simpler for melchanical reasons to adopt the second alternative, and suita ble circuits have been developed using standard Post Office rotary line switches for the purpose.

The preferre(d form of phase-shifting network is that illustra ted in Fig 8 The phase shift prodtuced between the input terminals 7 and the output terminals S of Fig 8 (a) i O = 9 tai-so CR f= provided that the reactance X of the condenser C, is high compared with the input impedance and provided that the output impedance is high compared with X_ The circuit has, the advantage that the output voltage is independent of O The way in which an unlimited change of phase ean be produced with the network of Fig.

8 (a) will hle understood frolm the followilg':

A When R(= O (, the output terminal J is connected to the point gq and if X, is higohl compared with r there will be a negligile phase shift through the transformer and the output voltage will have the same plhase as the input voltage As R is increased the output voltage will lag the input voltage until 1 R= when it wvill lag by 180 , provided X, is small comDared with the terminating impedance.

This circuit thus gives a phase shift; of from O to 180 ' In order to continue the phase shift fronm 180 to 360 , the positions of the condenser C(, and resistance R, in the network are interchanged, as shown in Fig 8 (b), or another network incorporating such an interchlange is substituted The circuit of this figure gives a lhase shift of 180 ' when R l= 0, for then j 1 is eonnecte(dl to hl As I 11, is increased the phase of the ouput agaiin lags until it reaches 360 when 1 = = Tlis coampletes the cycle and by switchlling hackl to the circuit of Fig 8 (a) the range froml 360 to; 540 ' can be covered and so on In order to get a continuous phase lead the procedure described is reversed and the circuit of Fig' 8 (b) is used first with Rl Rl is then reduced to zero and the circuit is switlched over to that of Fig- 8 (a).

The way in which the circuit of Fig S can be controlled hv means of standard Post Office rotary line switches is shown in Fig 9 The whole phase-restorationii unit shown between inpult terminals 9 and output terminals 10 is arranged in the P,: circuit of the slave osceillations between the slave source and the phase comparator of Fig 6 The oscillations first pass ilrougll a device 11 for introducing phase lag, then throughl a stage of amplification 12, llhen through a device 13 for intro 70 ducing phase lead, and finally throughl a.

second(l stage of amplification 14.

The devices 11 and 1 3 are modified telephol(le type rotary line switches each ha yinlg four b-anks of contaetts 15, 1-6, 15 and 76 1 l J The armns ca'r Ving' the mnoving contacts are caplable of continuous rotation over a compl)lete circle, whilst the fixed contacts in each case occupy only a semicircle as is usual for S viledles of this type 8 ( O The four rotating arms are melhanically -coupled in such a way that those of banks and 16 connect with the fixed contacts at the same time as those of 151 andl 16 are disconnected, ant(l-rie rer sa All fixed 8 E (ontacts of bank 15 as vell as those of bank 151 are strapped together and the contacts of the banks 116 and 161 serve to provide the variable resistances R, and I 51 of Fig 8 (a) and 8 (b) respectively 90 The condensers C and C,'J also correspond to thle condensers of Fios 8 (a) and 8 (b) resl)ectivelv The terminals 17 are connecte(d to a source of H T voltage the negative terminal of -which is earthed 95 Each actuation of the relay A, of Fiog.

6 is arranged, with the aid of a suitable ratchet nmeclhanism, to move the four contact arms of the device 11 forwvard one step corresponiding to the spacing between twxo 100 adjacent contacets Similarly each actuation of the relay A of Fig 6 is arranged to move forward one step the arms of the device 138 The devicee 11 introduces at each step a phase lag of ' vwhilst the 105 device 13 introduces at each step a, plhase lead of 6 ' With twsenty-fi-ve contacts in each bank O wvill equal 2 ' The ratchet mechanisms are such that i-hichever of the relays A, and A, is acetuated must be 110 made an(l broken before the contact arms are moved forward one step.

The wa y in which the circuit of Fig 6 is combined xwith that of Fig 9 and also serves to introduce a frequency adjust 115 ment is shown in Fig 10 The terminals MI are fed withl tell master tolne and the ternfinals SI are connected to the terminals 10 of Fig 9 and thence to the slave source The phas, compllarator 1 C is 120 that shown in Fig 6 -ith relays A 1 and( A_ These two relays;actuate selector relays S, and(l S respectively which in turnl actuate motor relays M 1 and M respectively Actuation of the relay M, ener 125 gises the lower winding of the motor M and actuation of the relay M, energises the upper winding Tile nmotoir M serves to drive 'a tuning elemuent such as a variable con(denser, of the slave source 130 556,855 and may be of simple type such as is used inl wireless receivers for push-button tuning An alternating current supply is connected at terminals 4 When the motor i, actuated by the lower winding a deerease in frequency is produced and vice circta The motor M has automatic means (.ulch as a cam switch and hold-on contacts onl the relays Ml 1 and M) for maintaiming its operation after the release of the relays A and S and for switching it off a predetermined time after it has been started.

The relays A, and A 2, through the 16 selector relays S, and So, actuate line switches 18 and 19 respectively which drive the contact arms of the-devices 11 and 13 respectively of Fig 9 Closure of the relay A, tsa y) does not immediately cause a phase shift to be introduced because, as already stated, the relay must bie both made and broken before this occurs Howvever,, when the line switch is made it opens a contact which is in series 26 with one of the inputs to the phase comparator PC Now, with only one input to the phase comparator the input circuit is balanced and there is no output from the rectifiers, so that the anode current of V, falls to zero, breaking A, The rate at which the anode current falls is determined byv the time constant of the smoothin g circuit RI/C (see Fig 6).

This time constant should be, long enoug'h for the anode relay to stay on for about two seconds (say), because when the motor M starts to turn it takes one or two seconds for its camin-operated switch to close and until it is closed the hold-on circuits to the molotor relays are not coinpleted.

It is useful when making adjustments to tle circuit to have a manual control over the slave frequency In F'ig 10 this is provided for by a switch 20 which gives frequency control and a switch 21 which gives phase control Rlaising the levers of these switches produces a decrease in frequency or a lag in phase and lowvering them produces the opposite effect As slhown it is, desirakle to include in the energising circuit of each motor relay M, and M, the hack contacts of the other, so that both these relays cannot be made simultaneously The termninals Bl are connected to one terminal of a D C.

source, the other terminal of which is earthed.

The accuracy which can be achieved by the last-described system of control is limited because the slave frequency must necessarily be adjusted in steps,, and between steps the slave oscillator runs free.

More accurate results may be obtained by using a vernier control wlhich eontrols, the slave frequency between the steps of the mnotor control Thus, suppo sing the motor changes the frequency in steps of one part ill 10, the vernier has a maximum control of one part in 101 and operates con 70 tinuously through the angle 7 20 Thus, starting froim the balanced condition with the vernier at zero the operation is as follows Suppose the slave frequency is high, the phase oof the slave input to the 75 phase comparator advances with respect to the phase of the reference tone, the anode current of V 1 rises and the vernier starts to ' decrease the frequency in proportion to the rise in anode current When the 8 () phase, of the input has changed by 7 2 the vernier has reduced the slave frequency by one part in l O s At this point the motor operates and introduces; a, step of one part in 10 s, and both the vernier 85 and the phase are reset to zero The process is then repeated.

The vernier control may be a small variable condenser operated by a moving coil movement having a differential wind 90 ing with one half in the anode circuit of the valve V 1 and the other in the anode circuit of the valve V, so that the capacity would be increased by a rise in current of V 1 and decreased by a rise in 95 current in V.

The control obtained in this way is such thai;, starting at any arbitrary time, the frequency control is proportional to the angle turned through by the vector repre 100 senting the slave input to the phase comparator with respect to that of the'reference tonlle At first sight this looks like a system of straight "phase " control, and it might be suspected of liability to 105 hunting; but in point of fact it differs from the straight " phase" control in two, very important ways First, the control increases continuously with increasing angle, whereas in the straight 110 "phase " control system the control decreases again if the angle exceeds 90 which is the cause of the instability.

Second, there is no limit to the magnitude of the control, since the motor can be 115 made to do as many steps in either direction as is necessary; whereas in straight phase " control the control cannot exceed the value obtained at 90 .

Having now particularly described and 120 ascertained the nature of my said invention and in what manner the same is to