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RESEARCE DE? A1?!ME~.

Notes on the Development of a Speci­flcationfor Very Wide Band Radio FreCl~ency Transformer Core,~'

Report No. F .017 Serial No. '1946/3

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Research Department

WOr'kcarried out by'~, D.~urice R,Il,:'Minns

General

REPORT NO. F. 01 'L Aprii, 1946 ,,_t-.

Serial No. 1946/3

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'. AddendUm M.1063,' M, J,Q~;3 :1." M:,LI.39

/ The design ,by Research Department of very widobanq radio 'f'r~q1lency t'railsforlUers ms expanded considerably duriTIg the past few,', ,yori.~fl. It, has Iiow become ,possible and des it'ap 10 to place the ~e~hnical ll:i;'~coduro on ~l :HI'lll ongplOeriIif? bas~s. In the noarflltm::e ,~t ' i.$, .,h(n~Gdto publISh Cl report doal1ng Wl th all tho necossa:ry deta+.J,-s of,

,,<ie~ignsd that this wOrk may be undertaken by a widorgroup of engineers J., :than~is possible at present. The major fioldwhich' still remail1s >in:

: thq '·roalms of' research and devolopment i.8 the. application oft}l~ 'principlcs'oflowpower' transformQl' design to' that .. of high ,PQlmr' t~nns.formors.:; . . ' "

{to, .

Coros: of

Ono 'or' -Gho first subjects to bo doalt .with was the question of suitable iron corO's~ " For tJ. long timo rhomo,taJ" a product at the Telegraph',Construction and Maint,enanc'G . .company • was used. It was thon foundtha.t mumotal, manufactilredby the, same f+r.m, was more suitable. As the limits of performance were gradually tightened,; it was 9bsoriod that the mumotal toroids" possef,lsod' cmmc:toristics of which the variability was too grod 'for stable dosigns. It was clear th8.t the nk,1,nufactur'ingpr,ocossos wore not subject· to sufficient control. An effort \'ms.mt~doto' 'obtain' from the manufacturors a copy of their Olfin internal oloc-liricalspecification. Thisfailod, and it ~s bolievod that apart from mcchdnical dimon?ions ,no such ,spocification oxisted. It thus devolved upon Research Dopartment to draft such aspeci:fication. This S?oc'ification, which is 'thosubject lilutter of this report, has had tV-TO groat adv8..t.'1tages, and 'ono tempQrnry disadvantage,. The formoraro tha~. the .. samo performance can bo expected from. any ,number of transformers

. constructed to a Hesoarch. Dopartmol1t specification. calling forcbros to \ the buyingS1?OCifi'Oc'l.tibnM'~1063,' 9Jlda:lSo thatvtehave novtboen ablo

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to f'ind alternative suppliers,' namely Standard Telephones and Cables Limited. The latter is that both firms are having difficulty in meeting the specification, with the consequence that the present supply' of cores is small. The difficulty,that the makers are having, in ' meeting the specification is surprising, in that it calls for cores wi th a performance only about half as good as truat of murneta,1 f;lamp+.o.s: of s~mei"'s.:Lx Years ago, and is less exacting than that of two s~mpji 'S'\l,bmfH9'~;:by; S.T~ and C~, It is noteworthy that we have \ had. t'()~l~

,T.lh and-M., suitable measurement apparatus for them to set Up controls of their processes.

,Specification - Theory; \

The 61ect:r'ical specification imagines the construction of a • one ohm to one' ohm transformer with an assumed total loss of 8 db. at the low frequency end of its pass band. This 8 db. loss is obviously made up of a 6 db. loss incurred when connecting tho two ono ohm terminating . resistors in parallel and 0. further 2 db. insert. ion loss due to the shunt resistance and reactance of the transformer windings and core. It has been found by experience that a 2 db." insertion loss is not excessive for radio frequency transformers. The losses in the Windings of radio frequenCy transformers are practical1y always nogligiblo. except occasionally at the high frequency endS of the pass bands, Bb, that this 2 db. loss may be considered to 'be due to the core material only. As the transformer'will have negligible 'leakage inductance at the low" frequency ~imit, the equivalent circuit diagram of this one ohm to onc ohm transformer is effoctivelyas shown in Fig, 1. , . .

FIG. ~.

--....,~·t-~·-----·--·

"

o .. 8 db~

,---' - ~. €I ·2 =- --e 2 .. 5- 5

'where a is t,he shunt resistance andX' the shuntreac,tance, due to .the core. From Fig. 1 we ha,ve ..

- 3·,,-·

a

1 f a - ..... ~.~ 2' ·.1· ..... : ...

5 --~--------

ii ···l

a

fl + a

I'-_·· .. _a ...... , · ••••• 00 •• " ••••• · ••

1 .L,' 2a

t

.... : ..

whero a is the c 0l\{9lax impedance 0::' Rand jX,' in parallel, i.o.,

jRx a -

R .J. jX

Ptltting 2 into 1 a:nd inverting

li --,4" '

.L 2 f;'t~' *)2 .,

Solving 3 for R

...... tJ ............. ~' ............ q •••• ~ •

.",. \

(1)2 X ... •••••·•••••• •••••••• 00 •••

1.

2.

3.

• I

(4 ~f5---;--g(;~} . ') .. ',' .. ~ ....... '. ~ ...... ... '. 4. where R

Q '-X

From physical considerations wo know tbn. t

so wo must take only, the . .J. sign in front. of the, radical. modulus of the "core iiiJ.peq.ance ll a is. Now the

I~l Substituting 4 into 5

2

. . . - ',' ' . • • • • .. .. • .. " ...... 0 ............................ ..

)' ,.' '···.·· •.••• ':.00 •••

5 ..

6.

,

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"

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This gives the modulus of that open circuit impedance required to produce an 8 db. total loss for any value 'Of Q. at a given frequency when the transformer is connected botw0ena one ohm source and a one ohm sink as in Fie. 1.

" '( "Eq,untion 6 is reproduced graphically on sketch M.l139 (whibh 1.s ~ncluded in 'the specification. In the intorests of, c1nrity 10.1 is ,ealle~ I z I in the specification). , ' , ' ,,'

It is tho custom, in testing transformer coros, to measure the R and X, of the coro under investigation by' winding any suitable number of turn$ onto it; from this ~s obto.ined ~ QndZ. The ratio of the former to the lo.ttor gives the Q. ~rom knowledg~ of the Q tho appropriate value of fal (= Iz I) is found from M.l139. The actual e impedance (modulus) of the coro under test is \

1·" 1 Z '=' 2 "~-----

:;~2 1 t-"

X2 .'- ............................... 7.

l?

where n, is' the number of turns. As Z is the actual impedance per turn squared, the number of turnS nocessaryto use on n ono ohm to ono,ohm tr:nnSformer in 01'0.01' to obtain a 2 db. insertion loss is

, IM "r =j Z

••.•••••••• ~ ............ O ••• ~ •• Q ••••••••• 8 •

'The be,tter the core material, the should be ns small as possible. discussion i~ shown on M.I063~1.

loss turns should be required, thus I'

The prnctical application of the above'

Of course, in theory, this test Sllouldbemndo at whatever frequoncy is fixed as the lower passband limit of every transformer it is proposed ' to design. In practice, so many tra.n.sformeps start atfroquencies around the lower limit Of the broadoastbands that thethroe froquencio,s 50, 106 and 150, kc/.$ :wore ,chosen. It is pertinent to ask why three test f~equencios are c~osen. The answor is that many coro materials vary so widely ~nd have 'such variably shaped curv~s of. r against· froq~ency tbIlt it issomotilnos only possiblo to fix on one particular c:ore material by kn()wing~t least three values ot I' for three different frequencies.· , ",

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I The actual test values for r, shown on M.1063.l, were chosen after very extensive measurements of various core.materials and shapes. They are a compromise between what was required in order to satisfy many past transformer specifications and what could reasonably be expected from future core ma1;erials. It is hoped to be able to improve on. these figures when the manufacturers have better control over their metallurgical processes.

The mechanical specification is shown in sketch M.1063, which is simple and straightforward.

The electrical specification, based upon the foregoing elementary network theory, is set forth in a form suitable for manufacturers on sketch M.I063.1.

DM/VRM -I

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(H.L. Kirke)

[Will the reader please alter any transformer specifications in his possession prefixed "RE" to rea.d "Gore: to M.1063" when the core originally called for was ~umetal R282l:J

r~r-______ -r ______ ~ ________________________________________________ --, l

6 -10 -44 ISSUE 2-IG-4-4-5 iSSUE 3 27. 4.4'3

fo----

fo----

.J; ~ '250" + ·015 ~

--000

1. TAPE THICKNESS TO BE NOT MORE THAN -00211+ -0005"

2. CORE TO BE SU1TABLY DOPED DURING WINDING

RLD.203 SCALE:- TWICE FULL srZE

SPECIFICATION FOR HIGH PERMEABILITY , B.B.C. RESEARCH DEPT.

SPIRAL WOUND MAGNETIC TAPE DRN.W.Y.A. M.1063 HIGH fR.EGUENCY TRANSFORMER.CORE APPD . }.~.

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Iasue 1 11-4-46

la I 21.'04 •

f---

Cl} '-:o c: or. - Q)

-- ()-ot;: (!) ;:

C I/i ~ 0 - <U e ~ E "0 '5 r:: "- 002

\- 0 + e.. :0

c. ~ 0 ) c.

~ :l E: \-r,) ... c

( 0 ~u

ELECTRICAL SPEOIFIOATION roR CORES 1'0 M.l06S

1. A oonvenient number, ]1, ot turns ot 24 Se".g. enamelled and single silk substitute wire to be wound onto the toroid to be tested.

·2. The resistance and reaotance, series or shunt, Re and %a, or Rp and Xp to be measured to an acouracy ot not 'WOrse than "5% at 5OKc/s., 100 KC/s., and 150 V'. - .

s. Oaloulate the quantities Q and Z as shewn below.

4.

6 •

8.

Q :: ~ Z :: ~ Ja;.. 2

or Q: 1m. %p

From c:1raw1ng LllS9 determine the three values ot IzI tor the tbNe stated trequencies by means of the three values ot Q oaloula.ted 1n paragraph 3.

For each ot the three stated frequenoies determine the ratio

Thil ra.tio to be les8 than or equal to the values given in the table belcm.

r =JI:I Frequenoy in 1.0/ ••

1.-7 50

1.67 100

1.6 150

An agreed pero_tage ot oores from eaoh oonaigmD8Dt to be tested. It any rejects are found, in this sample a .fUrther sample to be taken and so on lDltil a. sample i found in which there are no rejects. ,

B.B.C RESEARCH DEPT. SPECIFICATION FOR HIGH PERMEABILITY SPIRAL

omm MAGNETIC TAPE HIGH FREQUENCY TRANS­FORUER CORE.

t--~l-l_. ---4 M.1063.1 AWD·

DAT A. S H F.CT N o . 5 tOGARITHMIC)( M ' M tJ C=VC::-L-=I!j,---------~--~-~--o----------------~R='-. -.-.'~r Huu~t:. AhllILL.lH'I ow,

This specific fon is the pronertv ~he BnlTISH Br. DCASTI G C2R~()R~~I'J,

and may not be reproduced or d "~IOSE'rj 10 a third p3rty In any form wftl10tJI tile written perm!sslon of' I""')rpomtlOn.

r. I.

0 0

I 0 t

r y _': rIp I J . I

-, t

o 0 0 .-, 0 0

" 0

/0 cv

I ~ IYJI139 :>

0 0 (j 0 0 0