.a . .,

RESEARCH DEPARTMENT

U.H.F. HIGH ADMITTA.."1CEBRIDGE. TYPE BG/IO.

Some Notes on Manufacturing Difficulties.

Report No. F.018 Serial No. 1947/1

• Work oarriedout by:

D. Maarice R.H.Minns

Introdact1on'

RIPORl' NO. F.Ola Septem~e~, 1947 •

. SERIAL NO. ·1947/1

UoH.Fo.· High Admittance Bridge, . Type BG/lO

Some notes on Manutaoturing Ditticulttes.

1· / Attaohments: ! .

RDjG/10l and Figs - 1 "and 2. "~

. Thi:s repo.rt tollows Research ,Report F.016.and deals with .events':tollowing . the decision, 'to construct four Type BG/10' U.H.F •. R1gh Admittance .;Brldges in the· Workshops .01' Research De:partment.The d1ffioultieetexperienceQ. j:;:n the '. . manufacture' 01' ·these ·bridges are described and thei-easons.forsome 01'. tbe rather considerable ,precautions are given.

1'ortomano e

, ' . Two models 01' this bridge were originally designed and made iIJ. the Rcfsearch DepartmeIi. t. They covered the frequency range 150 .kC/8 to 60 Mol s and were , capable ot measuringcapac1ties.tromO to 250 ~ and those values ot induct8IJ.ce which would resonate with any value in the same capacitance range at the frequency

,of meas~reme~t, with an accuracy 01' not worse· than 5fo. Tb,esebri~es were also _ 'capable ·ot measuring conductances. b$tween.l and 100 millimhos with the same .

... degree· of accuracy. Lower conductances may be measured with the same aocuracy but the frequency range 1s somewhat restr1oted,the minim~,conductance which the

_ bridge will mea.sure is 0.1 millimhos. Slightihy, greater erl:'OrE3than those just • quoted may result it the bridge.is used to . measure the maxiiiJ.Wnsusc~ptanoe and

oonductance ot which it ls capable. ,Tais is.· due to the vary l:lea:vy loading on the bridge. Tlae· bridge may be' used with the tront ternu,nel connected. to. oase tor the measurement ofunbalancedlmpedences;and the accuracyiSl;lti:J.l within 5% whether these impedances are trulyunbelanced or floating •. 'The accuraCY,1s diminished to lo%1t ,measurement 1s made ot impedanoes· having.·their electrical centre . connected to caSe. This deterioration in accuracy occurs,: however, only at' the higher trequencies.

, At detailed description ot the. oircuit and construction ,of t.he bridge is given·in Research Report F.015.

.. 2 ..

, Historical

The first attempt at thf? iIl,ari\lfact~re o.f 'Qne of these bridges by an outside organisation was made by Messrs. Scientific A.coustics in 1943 and under M.A.P. Oontract No.iCT/1067QLO~.6a; . a. S1J.IllInsry of . the~l.'work ,is given in Research Report N.· F.016. At the end of that report it was shown that 1norder,to be suocessful the SCient1t1c:Acousticis bridgerequ1~ed some modifications. These were, subsequently made b.u1;,tP.~ bridge was still neither meohanioallYl" .. • sound nor stab.le and it was not possible for Research Department to gi-ve it"'.. ,>

, ·type .apprOval. It. meeting was held between representatives of the BBO, Scientific Acoustics end the Ministry of Supply. The decision of a subsequent meeting of the I.R.M.C. was to oancel the oontract on this firm. .; .. : ...... :. e

During the time when Scientit'i.Q.'Ac()ust1cs ware' working onth.1r madel of the bridge,' seve~ ·o'rganisationsbeoenie interested en'd considered the. ' .. ' . ~ossibili ty ot making these bridges for their own laboratories' use.- ~ese

: interested organisations included ·the N~P .. L. Radio Division,. G.E.C. j'., , .. '

P.ye Radio Ltd. and British Insulated and Callenders Cables Ltd. The first three ot the above conoerns have made considerable progress; models have been seen by Research Department and advioe and lielp has been given. ~n their manutacture. In return. Researoh Department has gained some exper1enceot those troubles likely to arise1n'themanufaoture of-these bri'd'gesby -" outside organisations,·' Oallenders Cab~es',on the other hand, deci(led not to oon~truct a bridge themselves but to' sub-'oontraotthe work to . a suitable .firm •

. " . . ~.' . At about this '.' time, Messrs. Wayne-Kerr Laboratories' Ltd. Bubmitted·:.a:·· .' ,

medium-:wave multi-ratio bridge, BBC· type BG/12, produo.ed from a Researc~ -.; '. .... Department design and wHh some assistanoe 'from Researoh Department' staft.,: ... ,> • Th-is model produoed slleha good impression in Researoh Department . that· :!_ , Callenders Cables were recommended to 'approachWayne-Xerr Ltd. with a :v1ew,.,· .. : ... to their attempting the manufacture of' a' U.H.F. high admi ttanoe bridge;' .,

·type BG/lO.No· drawings-were given 'toWayne-Kerr at this stage as they felt ' 'that they couid'design a bridge on·the basis of the informatiori,available."· ·Meanwhile, it was 'alsodec1dedthat .before .any manufaoturer ·received thej' '.

'. cancelled M.A.)? oontract·for. the fitty U.H.F.· high admittanoe bridges.BG/10,: tour of ·these should'bemadein Researoh Department workshops and'thetourth''''.;

. model; oomplete ·with a set ot detailed soaleddrawings and test specifioation, should be lent to the oontractor.·,· :...

These four bridgashave now :been made' in Researoh' Department ·.workshops. There were two main reasons for this course ot action. lI'irst~y ,:four bridges were needed to meet BBC requirements and, seoondly, their oonstruction, assembly and test would reveal those difficulties likely to' be enooun':t;ered by outside manufacturers. This proved to be th6 case and the diffioulties

- 3 ..

encountered are detai~ed in the rest of this report.

Manufacturing Notes.

,It will be assumed in what follows that the reader has acquired, detailed familiarity with the bridges in question.

1. Terminal blocks. To keep the capacity between these blocks as constant as possible, it 'is necessary that their inner surfaces be as flat as it is practicable to make them, assuming the employment of very skilled labour and a thoroughly well­equipped instrument workshop. Before electro-plating they should be flattened by being faced on a surface ,plate covered with fine abrasive paper. After electro­plating they should again be inspected for hollows and general surface irregularity 'and, if necessary, the surfaces in question should again be flattened!,

The only' reI\laining source of capacity variation, between these .terminal blocks is the mica spacer.' Thepermi ttivity of this may well, vary between the limits 4 and 7. It has been found by experience that the average permittivity necessitates a thiokness of approximately 0.003" but in view of,' the wide variations in, , permit~ivity it would not be good,commerical practice to specify tolerances on this thickness, therefore, the mica spacermustbe sele.ctedbY trial anli error by testing the bridge electrically, and replacing the' mica until: the capacity dials' enable a null output to be obtai~ed when they are in the' required angular position.

Some small adjustment of the null balance position is possible by bending the busbars which connect VC.l with VC.2 (Figure 2",RD/GI01). If this adjustment, ,does not enable a sufficient incremal t of cspaci ty on the "standards" side of the

" bridge ,then a 2;..8 IoijlF air trimmer ,condenser may be connected in parallel with It VC. 2. , No greater value of cepaci ty should be added because of the additional load thrown upon the leakage inductances inevitably present in the bridge.

2.' Main Busbars. These are' designed to connect the standard condensers and resistors to the transformer unit with a minimum of series uncontrolled inductanQe

, and, also a, minimum of shunt c,?-paci tanoe. This, means that the busbars must be close together b,u t not so close as to, cause c,rippling shunt capacitance. T.he actual busbar assemblY inciues~ion has been designed as a compromise between too much series inductance and too much shunt capacitance. The spacing between the busbars is maintained by two mica wash~rs 0.005ff thick. The same :,requirements of oonstancy of capacity between the busbars applies as in the case of the terminal,bloc1;:s but as the terminal block capacitance is, adjusted to balanoe the busbar capacitance (in the impedance ratio 1 : G_ of course), it, will bo the busbar capacitance which determines the'loading on the transformer leakage inductanoes. This capaci tanoeis adequa,tely specified by the dimensiop.s of the mica spaoing washers. It is, perhaps, not et first obvious that the diameter of

· if

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the mioa spaoing washer is qu1te as 1~portant as its thiokness. Finally, the oapaoitanoe of the "standerdsff side ot the bridge is affeoted by ,the oapaoitanoe between the two oopper foil leads whioh connect the transformer assembly to ,the main busbars, AS the relative positions of these two copper foil leads have' to be set atE;!O Mcls it will be neoessary to re-adjust the busbars oonnecting Ve.l with VO~2at 150 kcls in order to maintain a null output on no load with the oapacitance dials at the required angular position.

3. "Variable condensers. The conneotions between VC.l and the main 'bushars havee been very carefully designed and upon their accuracy rests the success err the' bridge. Very careful manufacture is here required pa~tlcularly as nochebk of dimensions can be made once the bridge is assembled. VC.2 has a higher impe'danoe e ,and so the leakage inductance of the busbers connecting it to the maiIibusbars' is not important. The reason for the distrene spaoer between the subsidiary bus­bars connecting'Ve.2 andVe.l is to prevent miorophony" 'The remaining variable oondenser, VC.3, ls ueed to set zero and its conneotions are not so important.'

4. Variable resistors., 1fhen the first of the four bridges was' received for test' the circuit of ,the resistance measuring network was as shown in Figure 1, in which it will be observed that VR.land VR.2 are oonnected across the 'whole bridge winding of the input transformer. It was found that when making measurements of low oonductanceat 60 Mc/s, conSiderable errors ,were experienoed. These errors were reduced to less than 5% for conductances equal to or greater than 1 milliI!lb.o, by moving the bridge. circuit to that Shown 1n F1g.2 in which it will'be Observed that VR.l and VR. 2 derive their E.M.F. from the same portion of the bridge winding of the input transformer as does the unknown impedanca. This circuit modification, ,also neoes~itated t~e reduction of' the 5000 ohmresbtors connecting the sliders,~, of the varlable reslstorsto the bridge winding of the output transformer to ,., 1500 ohms and 1000 ohms. The somewhat lower value of ,R. 24 as comp,aredwi th R.25· enables a greater range of variation to be obtained on the zero set conductanoe knob. A further improvement was made by encl .• sing the E.M.li'. supply lead to' It VR.l and VR.2 in a copper tube. this tUBe b@ingoonneoted to thebridganeutral wire. It was found that the shape and phYSical'position of'th1s tuO& had soma effeot on the acouracy when measuring lowconductanoes. No satisfactory . explanation has been found for this phenomenon. This ohange of circuit ~ffected a considerable improvement but :ineasurement errors may still exceed 5%. if ' ,. conductances very muoh smaller than 1 :mil11mho are measured.

5~ Drives.; The drives to all the continuously Variable elements of the' bridge are single ratio ball type epioycl1cdrives. These require oareful mounting and spindle' alignInen ts must be done with thatpreoision of manufacture whioh would':be required if there were no flexible couplings employed at aU.The COIllI!l.On' impression that flexible couplings may be used to "take up'" spindle ',mis-align;' menta has been found to be quite erroneous for this type of work.

, ... 5 ..

6.00ntiu.ctanca drums. There are two drum.s of re si stors providing the O""J.OO millimho and 0 .. 10 niill1mho ranges. 'An attempt was made. in the first instance to arrange that the drums could be mQved axially along their shaft,s so that the drum spoon contacts should pe easily aUgnedwith the busbar contacts. This, however, led to two difficulties. ,Firstly, the adjustable mounting was not sufficiently rigid and, secondly, the drums had to be very carefully, re-adjusted each time they were ,removed for inspection or to gain access to concealed parts of the bridge. The final arrangement consists of demountable drums held to a fixed flange on the shaft by three screws; no adjustment is provided~

The body of the drums was originally made of loaded synthetic resin (R.M.70), this rather low grade material being used on the grounds 1{hat if its high frequenoy resistance was low it was, nevertheless, constant and could easily be neutraJ.ised by the zero set conductance potentiometer. Experience has shown, however, that the spoon contacts were not always rivetted to the drum mater.ial with the same pressure, and so the shunt loss resistance was not constant for each conductance drum angular position. In these circumstances it was decided to use a material having a negligible high frequency loss conductance, and distrene was the obvious choic'3. This material requires somewhat more careful workmanship as regardsrivetting and soldering to tags mounted on it - particularly the latter, as its melting temperature is rather low.

7. General Assembly. When the first of the four models was submitted by the workshop to the Measurements Laboratory for testing it was found that the bridge chassis was distorted when it was screwed into its screening case. This distortion ,was sufficient to upset the 'bridge both electrically and mechanically and was overcome by having end-plates, one on each end of the bridge. These plates are large, -thick and rigid.,

S. :Oalibration. Detailed instructions for the calibration of these bridges are given in Research Department Test Specification RD/G 101.

The capacitance dials are calibrated with the zero at one end of the scale but the no load null position is arranged to occur at approximately mid scale. For calibration purposes, therefore, this null position must be moved to one end

, of the scale either by increasing the capacitance on the ffstandards" side by about 30 ppF or decreasing the Capacitance on the unknown side by about 250 ~F. Both methods have been used though the letter has the advantage that it can be carried out without removing the bridge from its case, by connecting across the unknown terminals that inductance which will resonate with 250 ~ at the frequency of calibration.

... G ...

Theoonductance dial D.5 (Figure 1, RD/G 101) is calibrated at 150 kc/s wlththe aid of a resistance box and in certain cases may present trouble. This'takes the form of an electrical baoklash within the potentiometer and may be due to movement of the carbon on the carbon track'as the wiper ,passes over it{',The net result is that two points on the dial can be obtained ',,' corresponafilg with one conductance value accordfDg to the direction of :,"",' approach.; The distance between these pOints varies wi thindividual ~tentio­meters but in certain cases may be so great as towarrantthelr rejection.

DM/JEH ,CH.L. Kirke)

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\;-.

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I I·

5 ...

6 •. Conductance drums. There are two drums of .. resistors providing the 0--100 millimho and 0-10 millimho ron.ges.· ; An·attempt was made in the first instance to arrange that t1:e drums could be moved axially along their shafts so that the drunlspoon contacts should be easily aligned with the busbar-'contacts.ThiS, however, led to two difficultIes. Firstly, the adjustable mounting was not sufficiently rigid and, . secondly, the drums had to. be· very-carefully. re-adjusted· each time they were removed for- inspection or to gain access to conoealed parts . of the bridge.· ~he final arrangement consists of demountable drums held to a. fixed flange on the shaft by three screws; . no adjustment is provided •

. The body of the .drums was originally made of loaded synthetic resin (R.M. 70), this rather low grade material being used on the grounds that if its high frequenoy resistance was low it was, nevertheless, constant and could easily be neutralised by the zero set conductance potentiometer. Experience has shown, however, that the spoon contacts were not always rivetted to the drum material with the>same pressure, and so the shunt loss resistance was not constant for each conductance drum angular position. In these circumstances it was decided to use a material having a negligible high frequency loss conductance, and distrene was the obvious choice.. This material requires somewhat more careful workmanship as regardsrivetting and soldering to tags mOunted on it - particularly the latter, as its melting temperature is rather low. .

7 • General Assembly. i'Jhen the first of the four models. was submitted by the workshop to the Measurements Laboratory for testing it was found. that the bridge chassis was distorted when it was screwed into its screening case. This distortion was sufficient to upset the bridge both electrically and mechanically and was overcome by having end-plates, one on each end of the bridge. These plates- are large, thick and rigid.

8. Calibration. Detailed instructions for the calibration of these bridges are given in ~esearchDepartment Test Specification RD/G 101.

The capacitance dials are calibrated with the zero atone end of the scale but the no load null position is arranged to occur at approximately mid scale. FOr calibration purposes, therefore, this null position.must be mo~ed to one end of the scale either by· increasing the capacitance on the "standards" side \ by about 30 ~]' or decreasing the capacitance on the unknown ·side by about 250 \4lF. Both methods have been used though the latter has the advantage that it can be carried out· without removing the bridge from its case, by connecting across the unknown terminals that inductance which will resonate with 250 \4lll' at the trequency of calibration.

,­,

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Whe conductance- dial D.5 (Figure 1, RD/G 101) ':ls calibrated ·at .150 kc/s' with the aid.pt' a resi stance box and in oertain cases may present. t.roubl~~ . This takes t~efor;m of an electrical backlash with1n th~ potent1ometer ~nd may pe due to movement of the carbon on the carbon track as thew1pe;r passes

·1 . I:, .... . .

over ~,titf: [fle net result is that two :points on the dialcanbe obtained .. ' j. ,,' corresponding with one conductance value according to the. direction of approacl+. The distance .between thesepo1nts varies with individual potentio- . meters but in certain cases may be so great as to warrant their rejeotion. e

(H.L. Kirke)

. ''; '~" :

ISSUE

22-9-+1.

IN

STANDARDS

UNKNOWN

FIG. I.

STANOAROS

10001\. I ST. TURN

UTRAL

UNKNOWN

FIG. 2.

I

OUT

-t +-. OUT

.... ... _" .... ARCH BBC U.H .F. HIGH ADMITTANCE B~loc.E. TYPE SCr/tO:==:'=: REPORT

1-----1 SOME NOTES ON MANUFAC.TURING OIFFICULTIE5_ F.018

2,3p.1946

230 100 1946 17. ~47

.. U.H.P' HIGH ADNTTANCi: BRIDGE B(;/lO TRIMLiING ALI~TS,

.~

l'ART I.

OALIBRATION AND OHEOK u:.ASUREliENTS PROCRWRE.

SWl1Il8ry.

Part I. Pert 11.

Pert Ill. Part IV. Part V.

Transformer Tests. Mechanifal inspection, A.F. Trimming, ~ calibration and earth symmetry. U.H.F. Triulling Complete A.F. Check ~easurements. U.H.F. Oheck !.eesur.ements.

- - - - - - - - 000 - - - - - - - - -

Before a transformer unit is 6ssen~led in a bridge it must be submitted to the ¥easurement Kogineer who will test it to the test. specified in Re.15. These te~ts shall be conducted on the transformer unit without either .the bUfi-bars or the unknown terminal blocks. A subsequent test may be made tor the convenience of the workshop in which the transformer unit, cOKplete with bus-bars end unknown temlnal ,blocks, is again subrr.1tted to the Mea.surement xngineer who will check that there are no shorted turns or short circuits between the unknown terminal blocks or between the bus-bars by obtaining bridge balance py connecting across the bus-bers such capBcity as is ~ecessary _to 'balance the capacity between the unknown terminal blocks. These check measurements shall be done at 150 kC/s. ·No accuracy shall be speci:t1 d.

PART II.

1. ~eohanlcal inspection. When complete the bridge shall be sub~itted t o the 'easurement will ii at of all submit it to a minute and detailed mechanical The bridge shall conform 10 aLl details to the drawings. Such following shall receive extre-Bttention:-

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RillI'! r who ins !'lction. tems as ~he

Slow motion drives shall present no otservable back-lesn (this means th~t in Bpi te of the presence, in SOlf. cases, ot fleXible cou.plings, spindle a11~~t8 ~~st be of the ve~y ' first olass workrr,anship). rhe ~u stion o~ . lrtecoanical back-lesh shall b examined with the brid~e both in and out its. case end this will, therefore, necessitate electrical checks of the back- ash. No ®'$.q ~ ~ ~·,dfi~~i~.~ part of the bridge chassis shall occur as it i8 being screwed into its case. The w~y in which the condu~tance drum contacts locate with t he bus-bars as the drums are rotated shall be obs ~ed with great care. These contacts must rrake good contact but shall not be so bent tha~ they do not ride up on the bus-b~r contacts. Variable condensers shall be checked for shorting vanes. .'

The spacing between the busbars shall e checked to drawing with the aid of

2:3.9.46 2:3.10.46 17.4.47

"T

~.

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2 -, ..

teeler gauges. Th1e spaoing ahall be oheoked all along that portion ot the bus.bars which leave a .004" gap between them. A .004" teeler gauge shall be used as a "go" gauge and 8 .005" feeler gauge shaU be used as a "no go" sauge. There ahall be no visible spaoing between the two oopper foil etrips which oonnect the unknown terminal blooks to the beke1ite bridge piece joining the transtor.mers other than that oaused by the mioa spaoing piece between them. The anti-miorophony Distrene spaoer between the bus-bars oonneoting VC.l and VO.2 (r1g.2) shall be oheoked to be tlrmlJ in position halt-way between the two var1able oondensers and held in position by adequate tension between the two bus-bars.

2. Lel. Tr1mm1ns. 'l'his shell be done at 150 kc/.; The br-idge ,snust be SO odjusted that with the oapaoity dials D1 and D2 at mid-soale (Fig.1) and oondenser VO.3 at slightly more than halt tull oapacitance. a reactance zero balance ls obtained (at 60 Mc/s this zero balanoe will shift and in order to keep it within the range of the condenser VC.3, it ls necessory slightly to ottset for the L.r. trlmming);thls zero balance to be obtained by adjustment ot the trimming condenser TO.4. It this ls not possible it w111 be neoessary to alter lhe thickness ot the mioa spac.r betwe n the two unknown terminal blooks. 'l'l1e nominal thickness of th.e mica 8p8per ' .0 OOS~ _u", ~t _, b. '­Jaeae .. 8ary to reduce it; to some value between O.OO~"i and 0.003". DuriAg the .­course ot this trimming prooedure U w1ll, 01' course, benaoesaa.ry to obte in • conduotanoe- balance on the bridge, otherwise tine a<lj.q.stment at oape.e~tano8 ~r1aUon w111 not be apparent. . -

<,

I. ' 1.. ... Oalibre.tion 61. aliall be done at 150 ko/s.

30 1 Oon~otenoe Drums. 18ch oonduotance position in eaoh drum shell be checked by oonnectiag across the uninoml ~er,m1na18 ot the brl~ a su1t ble reslstan9. box capable ot glTi.ng an aecur8cy.:.p.t three sign1fioant tigures tor each drum position. Hormal resistano, bdxea suitable tor this purpose aay present conslderable Inductance on the low res~staQC •• alues and 1 t .may be neoessary to oompensate tor thls by connect1ng an external condenser in parallel w1 tb the resi stance box aoro8"'s tlle unknown terminal

" The reading obtained roJ' each conductance drum positIon shall DOt depart I' trom the nominal value by more than rf/:,. During this oonduoMnoe drum oheck

the tront terminal block ot the bridge shall be oonnedted to the earth te1'll1nal •

. 3.2 Coq.t1nuoual, YBriable oonduotanoe oalibration. AS betore the tront .lID1nal. 01' the bridge shall be oonneoted to the earth terminal. Oontinuously variable oonduotance dial D5 ('ig.l) to be rotated fully olockwi.e. A penall mark to be marked on this dial opposite the poin' where tbe oentre line 01' the ourser appears. This pencil mark to be marked "O~. Obtain a null output by trimming dial D7, rlg.1 and with dials D1 and D2 at about mid-seal. trim dial DO. Oonneot a suitable re6ist8~O. box aoross the unknown terminals. Calibrate oontinuously variable conduotanoe dial 1n steps Ot 0.05 mill1mhos. '!'his shall be don. by making two separate oalibrations, one obtained by always moving the dial in ' an aJ1tl-olockwise direct10n and the other by always moving a cloolarise direotion. This method ot calibration wiU 1nd1 ete a oertain un~v01dable error tor each penoil marked

23.9.1946 230 10.1946 ~7 •• 7

(1 sheets) - 3 - T&sT S~C. RD/G ),01/3

oSltion on to- a1 1. Trr~ dial shall be scrutinised and that position iVi ng t.e greete er ~ h~ t 1s marks widest apart, shall be

tran fer ... d to t ' Cll - er, ~b 'JS inscrllr1ng the curser with three lines, two error i Ir. i ti .... 1 n~ nd one ,ntl"al line. These error 11 i t1ng lines .,,111 1n': .. ~st the r..3 '(ill'.wr. arror obte1n!:tble y an unskilled op rato. If th~ dista. e etv h two error 11 1 ting line. on the ~ r ~r exceeds 3/l6~, th poten lonete r shell be rejected.

3.3 ~e11brst1on of 0 - se pF aial ~l, 11 • 1. The front terrr,in ' of th !'idge sh II be connected to the earth tel 1n31, rotat111 dial Dl "ully

• • I

anti-clockwi se. ~eke e en~1l rr£rk on it op~osite th6 curser centr~ line. Rot te dial 1 in a 10ckw1se direction tor 5/16" olon its periphery, mIke e pencil 11ne underneath the curser line. ~3rk his pencil line "0". Connect a standard varietl condenser !3cross the unknown terminals of the bridge. It ia convenient to adjust this ~ondenser to such a value that it is in the linear range of its

. ~ capacitance variation, say 100 ~l. Connect in parall 1 with this conuenser en inductance of the order or 3 mil11henries. Obtain a null OU~Fut fro~ the bridge ty rotat1n the ~onductance zero adjustor D7 and, 1f n~cessary, ell of the cO:ld'..l~tanee dials D3. 1>4, D5 end the capacitance dial 2 and the capacitance zero adjust~r ~6. Calibrate diel Dl every 10 pr moving it in a cloc~Rise d1rection until it meets ~ the s top. ~t every lCO p? pos1t1 n return the standard veriable condenser to its initial velue and the bridge dial Dl al50 to zero end check that null output is obtai d. This calibration shall be done with an accuracy of not wors than~.

3.4 Calibration of 0 - 70, pF dial D2, Fig.l. This shall be done in a s1~lar rr.anner to the ca ibratlon of d1 1 Dl. This ca11bration shall be one et every 5 p's and the acour~cy she not be worse than ~:

1 , .4. 36rth Sj"U£letrt • .... unit consisting of 8 100 OM resistor in :a:rellel itb. 8 500 pr cape! 'to shall be made up ana this nit s.1e1 b c nnect d in series with tin 1 le

f unit. Thes two units she 1 not differ ro one another in any respect y .f, rr.ore than 11·. ith thl::l unknown terl!lina of th bridge float1n and 'ith t e

conductance dials D3 ana and D5 t Z 0 and the capaciten e dials Dl and ~2 at 250 p~ d 35 pr r~sp ~tively, a nu utput shal be obtained from the brl dge by en a d ustrlent of diRl D6 an 7. The tree ends of the sy etrical ' net~ork described above shall be CODD ted ~cros the unknown terminals of the bridge and e null output shall be obtained by adjusting dials Dl, ~ and D5.

·These dial reedings shall be noted. The centre point ot t is symmetrical network shall then be conneCted to the 'earth terminal and a null output from the bridg shall be obtained by adjust cnt ot dials D4 and D~ and the tri~ing condenser TC.l, F1g.2. This means th8~ the lack ot reactance symmetry in th bridge is compensated tor by the trimmer condenser TC.l. The dials D4 and D~ should not require much re-adjustment because the resistive symmetry ot the bridge with respect to earth should be quite good. The ohange ot conductance read1ng 1n dials D4 and D5 wh1ch may t place whe ,the centre point of th symmetrioal network 1s connected to the earth terminal shall not exceed l~.

/

23.9.46 ::'~.lO.46

17 •• 'I

(7 sheets) TEST SPIC. BD/a lOl/4

The d1 ls Dl, D2 and 1)5 may now be rellOved and engraved. AtJ soon as the d1als are returned troll the engrevers, tl t to the bridge and prooe.d. .to Part Ill. . . PART III

1. Ad ustment o~ loo Tt.l The tollowing trllft11i1ng shall be done at 60 MO/IS w1 tb both unkl101lD. terminals floating. Ob'ail). null output trom the brl6ge w1 th dial Dl at the 250 p" position. Measure the U.R.I. capaoity standerd than measure a 30 ohm Welwyn resistor. NOte the peroentage errors ot measurement ot "these two quant1t1es and adju.t tha trimm1ng loop T~l, P18.2 until they are both equal and es small as poss1ble. It has been found by expertenoe that these two cond1tions 81 818 occur at the same time. or course, each time th, tr1mming loop TL.l~ alt red a null output from the brl~e et be obta1ned with no load oonnected across the unknown term1nals. n this lr1mm1ng procedure baB been compl ted; with d1als DG. D4 and D5 at 0 m1111mhos, d1al Dl at 2~0 pr and dial D2 at 35 pr, the adjustment ot dials D6 and f1'l must be oapable ot g1ving null output.

2. Obta1n1 null out ut on no load with the front ~own e pr1dge connected to tbe earth terminal.

It may be found that this is ho' possible, but can be made so y oonnecting a series c1rcuit B.2S and 0.1 11g.2, trom earth to the same busbar as conneots w1 th the trlmm1Dg loop TL.l. The apprOXimate values ot R. 26 an4 e.l are respeotively 0000 oluDa ud 20 ~, but it may 'be naoeaeary U'1 other values. It should be noted that the 'bridge will normally be d~11v.red , t the .easuramD~ Ingineer with ~.26 and ~.l already fitted to · it.

3. Ad ustment of TC.5 i th unlmown terminals floating obtain nUll output from the bridga in th ppro,r1ate manner. Oonneot a looOohm elwyn resistor aorOS8 the unknown

terminals; set dial D.5 to 1 m11l1mho. Trim TO.5 tor null output wbile adjust'1ng D.6. Remove the 1000 ohm resistor trom the \ll1kDOwn t rminals and ra-adjust the zero trimmers D.S and D.7 for nu~l output. Re-connect the 10000ha resistor acros9 the unknown terminals and , that dial a5 correo'll da , 1 milllmho. If it does not,re-adjust'TO.5, ~-obt8in null output and at.

I

• on uotanoe L.r. Cbeck Measurements. ith a suitable resistanoe box aheck the oalibration. ot 05 in 0.1 m1ll1 step~

.. it any of the readings have a greater ern>r than that indioated by the l1m1tlq soribe marks on the ouraer, this error shall be l"8corded. '!'he record shall ­indicate the amount by whioh this error exoeeds the 11.1t5 shown by the scribe marks. No suob r~orded error ahall exc. d ~

2. ca oitanoe L.F. Oheck Measurements. he oalibration ot dial D shall be checked at every 50 p'I.' i:Yery' errol" whioh

exoeeds ~ shall be reoorded. No suoh recorded error shall exoeed 5~ Dial D2 sball ,be checked in exaotly a1m1lar nn r to d1al Dl 'but the oheok pOints ah U be, at every 10 pr. Any srror which axoeed~ 1!fc shall be reoo rded end no reoorded error shall be grea er than 5'1..

The U.H •• standard conden er shall b. m' asured at

23. 9~ 1946.' 23.100 1946. 17. 1 .... ' 7

,e I

sheets) - 5- TEST SPlitC. IW/G 10,..,5

60 o/s and the results recorded. The eTl"Or shell not be worse than A 30 o~ elwyu resistor ah 11 be measured et 60 k_/s and the result shall be reoo~ded. 1'h error shall :not be greater than

2a; rtg S11!1llu,try oheok at 60 cls. _ it shall be made up by the parellel oombination of. a 100 ohm resistor and

50 pP oapacitor. This combination ehall be connected in series with an identical one whioh shall differ from it by not more than 2%. This series parallel oombinat1on shall be meaeured at 60 Mc/a with its ce~trepo1nt

.floa 1ng and the re di~8 shall be noted. When the oentrepo1nt 1s connee ed t ~he earth tenniool ot the bl'1dge 1 t will probably be necessary to re-adjust the d1als D2, D4 and D5, the d1~erenoe in readings obtained when the centre point' 1s earthed shall be not greater than l~. '

!!!!I iach .~ridge shall be issued with a U.ll.F. capacity standard, a 30 tl and a 1000 A lelwyn res1stor and a sheet record1ng the oheck measurem&n; errors as set out 1n iarts IV and V. ' .

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23 . 9 46.

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£~~-::c o~ ~ 5~ t'~ i ~ ~ 8~:O'i e. ~ ~... 0 0.0 0 EO 4)(J'i'-" -SogS-::

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FIG. t

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\1) U ... c:: ;;; I ~~:-:-:--=:-:-::--:-::~:-=-----::-::-=--_=-=-:-r::-=--.--_ __ -L7-=5H"-!.!:E=-=E~TS~ <"0 -;;; r-: .~ (; ~ t '- U.H.F." HIGH-ADMITTANCE BRIDGE BG to B.B.C. RESEARCH DEPT.

;. cc g a. TRIMMING AL'GNMENTS,CAlIBRATION DRN.J>1 CH . "OJG.IOI/6 - & CHECK MEASUREMENTS PROCEDURE. TRD·Sl APP.R "

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23 . 9 . 4G. 17~ 4- ,47.,

~ TC!5.

FIG. 2.

UH.F. HIGHADMn:TANC£ B~IDGE BG/fO B.~ . C. RESEARCH

TRIMMING ALIGNMENT5t.CALIBRATION I DRN.~~ CHo- RD G fOt/7 C.HECK MEAc.:,U~EME:N 5 PROCEDURE. T~D. j . APP. •