Research Department P,EPORT No. L.016 10th April, 1940

Serial No. 1940/5

'Work can-:ied out by -H.D. Ellis

Drawings Nos. L.016.1 to L.016.3

Summary:

General·

TOTAL HARiVrONIC ANALYSER

In Control Rooms and elsewhere where audio frequency equipment is used it is hig.b..ly desrrable to provide apparatus for checking the distortion characteristics from time to time Q For this purpose a han-nonic analyser which measures the total harmonic is adequate, but it rmtst be stable, simple and quick to operate, and, should require no extra equipment beyond a standard. tony source. The instruInent described in this report ope:mtes on the bal~mced bridge principle and satisfies the above requirements" besides being portable and mains operated. .

The design of apparatus for routine di.stortion measurements

provides a somewhat different problem from that of a wave analyser. In

the first place: it is necessary to indicate only the total swn of the • I

harmoniccomponpnts generated in an amplifier to deterraine whether or

not the apparatus has cieteriorated in service. Secondly, it is

essential that the apparatus slmll be simple in use, though not

necessarily in conception, so that it may be operated by relatively

unskilled engineers. Third j it must be capable of rapid manipulation

if it is to be of.any real value in maintenance work. Finally, it

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, shou~d not depend for its accuracy on a very .stable and distortion-free

tone source, or upon the use of clli~bersome filters. Briefly in the

wave analyser- the essentia1 features are accuracy rmcl individual

selection of harrnonic components, whereas in the routine dis

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it is particularly adaptable to the measurement of individual harmonic

components and does not lend itself to the measurement of the total sum.

Finally the balance mcthod was chosen. This method consists

in balancing out the fundamental tone and measuring what is left, and it

has several distinct ad.vrcntagcs. First, variations of frequency of the

tone source are quite unimportant since they affect both input and

output similarly cnd therefore do not disturb the h:Llcmce. Secondly

the test fr8quency may be chosen at will since there c:re no resonant or

filter circuits to restrict the frequencies which may be used. Thirdly,

distortion in the s'.~i.)ply tone only introduces a seconc, order error in

the result, as will be explained at a later stage.

The Balance Methoq,

The balance method consists in comparing the input to the

. apparatus under test with the output from it by means of a bridge

circuit. 'The difference between these two quantities is thEm a

measure of the dis.tortio:r: introduced by the apparatus. The test

circuit is. shown diagrarnma tic211yin Fig. 1. The signal levels at A

and B are arranged to be approximately equal for convenience; by means

of the attenuator shown, since this simplifies the design of the bridge

network.

The bridge netvlork itself must provide means for balanCing··

. out the fu..Yldamentc.l accurately, both inampli tude and also in phase, so

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that the output of fundamental at the detector point C may be reduced

virtually to zero. Tho output at C is fed via a calibratedattenuator

and an amplifier to the indicating meter, which must have a square law

character~istic if the true r.m. s. total of all the harmonic components

is to be recorded.

Suppose the source voltage at A is E and the voltage

appearing at B is E + E2 -+ E3 +....... representing thefundarnental together with the various hannonics generated. If the bridge is

properly balanced the output at C will be aE2 + aE3 -1- •••• , a being the

loss ill the bridge, and the calibr.q.ted attenuator may be adjusted until

a suitable indication is shovrn on the meter. This indication,

therefore, gives B.. measure of the r.m.s. voltage at C due to the

If the input to the

bridge from B is removed, the output at C will be aE and the

attenuator may be re-set to obtain the same indication on the meter as

before. The two settings of the. attenuator, therefore, give

iID~ediately a figure for the ratio Le. the E

percentage total distortion.

The Bridge Circuit

The balanCing network must provide both amplitude and phase

adjustment for balancing out. the fundamental. It is highly desirable

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that these two controls shall not be inter-dependent, Le. adjustment

of the c-w'1lplitude control shall not affect the phase s}lift Gnd vice-verso ..

, Unless this is achieved the process of balancing becomes laborious or

even exceedingly difficult, according to the degree of inter-dependence 0

By i::-ltroducing sufficient ettenuation the phase shift may be rendered

almost independent of amplitude cdjustmont. The BIllpli tude balance cells

for no comment save that it covers a mngs of 3 db. and there:fore links

up ,with the 2 db, steps assumed for the attenuator T in Fig. 1. It is

exceedingly difficult, however, to alter pho.se without altering amplit1J,de

and efter mnny circuits haQ beon investigated, given prolonged trial anci

finally rejected, the circuit about to be described W1C.S evolv89- which

satisfied all demands.

- Besidos the condition of non-interdependence of controls

mentioned above, it is essential that sufficient phase shift nhould be

available -to match that introduced by the amplifier or chain of

apparatus lmder test. At low frequency this may be considerable and in

certllin special Cllses may have any value between 00 and 360°, The

circuit finally adopted allows any value to be matched betwoen OOand

1800 ; ve,lues between 1800 and 3600 are ce.tered for merely by reverse.l

of one input lirw to the bridge.

The basic phe.se' shifting circuit used is illustrated in

Fig. 2. Referring to Fig. 2(e.), tone is applied e.t AB and C is the

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electrical centre point beb'men A and B. In Fig. 2(b), vrhich is a

vol tage vector diagrrun, the voltage across AB is flhovm by the vector

AB with C at the centre point •. Provided ha load is applied between C and

D the eurrent I which flows through RI also flows through X, so that

the voltage vectors representing the potential differences across

AD and DB will be at Tight angles ~1Sshovm. in addition their vector

SUlIl is equal to the vector AB. If the resistance RI is reduced the

relative magnituc1es of the two vectors AD and DB are varied accordingly,

while still maintaining their right-angled relationship as represented.

by ADI and DIB .. ,By simple geometry it follows that the locus of D is

a semicircle with centre at C so that the vector CD,which represents the

voltage betvleen C and D, remains constant in21nplitude but varies in

phase relative to AB. By varJing either RI or X, therefore, the

potential between C and D varies in phase relative to that across AB

over the whole range 0 0 to 1800 while maintc1'"ining co'nstant amplitude,

thereby satisfying all the demands for the circuit.

For use at audio frequency the capacity becomes too large to

be conveniently variable so that· the resistance is made the adjustable

arm. This can be reduced to zero but cannot be increased above a

fini te maximum, so that the full 180° cannot be cov~red on this one

control. A similar circuit is therefore included in the other input

line, whieh covers only a limited range and serves both to supplement

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the restricted :range and to provide a fine control. Since the amplitude

remains independent6f the impedances X 2"ld HI this circuit may be used

in the line;carrying the. hc-'lrmonics, but in the case of phase shifting

circuits where the amplitude is Et fUllction of the circuit impedances,

and thereforeoftbi'3 frequency of the h.D.nnonic, the~le circuits would

have to be confined to the pure tone line only.

,Although the output should not in theory be loaded at 8.11,

it is found. that if fLn impedance not lE-SS than lOB. (Fig. 2(0)) be

connected across CD, the behaviour of the circuit is not seriously

upset. . '1'he whole. ctr~ui t arrangement 'will be seen in the diagr&m Fig. 3·

To maintain a reasonable lmv of phase shift versUEl rotation oftho

variable resistances in the· tvv'O phase shifting circuits, the capacities

are varied in four steps selected by a ganged switch, each position

covering a frequency band. The appropriate position is then chosen

according to the test frequency selected. In order to maintain complete

separation, of, the uT():ne" 'and "Test" circuits (A fmd D, Fig.l) the two

circuits are, foqt9twP separate shieldedwindings of a transformer, a

third windingpickirtg off the difference potential and feeding it to.,

the detector circ:c!.i,to .

Calibrated AttenucJ.tor and Amplifier.

These tYro i toms call for no special comment 0 The attenuator

is, in fact, ,apotentiometer giving 2 dbo por stop oxcept near tho

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bottom, and is engraved direct in figures for percentage :b..armonic dovm

to 9.1%. The amplifier is str!l.ightforviaro. with two stages giving [-1

mHximwn gain of 66 db. from firot grid. to seconc~ anoc!.e. A gain control

is provided to standardise the·gain 2.ccoro.ingtb the level at which the

measurement is being taken.

Detector

In orde r that tJ:1e r.m. s. total harmonic shall be indicated

the eIetector must l)e truly square law. Theia - v characteristic of g

a normal valve gives a ver;/ close approximation to & square Lw! near

its bottoJ! bend, but it suffers from the disadvantage that the' maximum

signal which can be applied gives only a small rise of current compared

with the standing D.e. The latter can be backed off, but· all backing

off circuits are critical and apt to get ovt of adjustment~ and are

therefore to be avoided if possible. By using two valves with their .'

grids in push-pull andanod.es in' parallel, the oper&ting point may be

much closer to' the bottom bond and a true parabolic characteristic

obtained. The standing D.C. may thon:;fore be much reduced ana. the

, .

increase of current with applied signa,l bccoJT.l8s much "larger by comparison,

see Fig. 4 (a) and (b). Two AC/SP3valves 'are used for uniformity here,

as in the remainder of the apparatus'. lm adjustment is provided to . set

the standing D~C. in this stage to ensure'that the v&1VGS are operating

on the correct part of their cho.racteristics, b.nd· a pre-set bal~mcing

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control is also providedl:

The s~nsitivity,of;.- the detector is sl1ch that the standing

current (0.5rfLAj' is doubled when a signal level of --16 d1:). ,is appiied to

the primarybf the'inputtransformero

Operation

The sequence of ,events in the operation of the analyser is as

follows.

(1) Having pwitched on the SUP1)lies and allowed time to heat up

(say 1 to 2 minutes) set the calibrated attenuator to CD, tum the gain

control down to minimumJ and set the standing feed of the detector to

the liSet" mark by means of the zero controL

(2) Plug l1p tM.tone source, attenu[,tor, amplifier under test

and dummy, load and conne ct the t! Tone" ci rcui t ,of the, analyse I' ,to ,the

point A, Fig. 1 and the "Test" circuit to point B.

, (3) Set the, output level of the tone Source to the output : ': ,"., .

'level at which the amplifier under test is to be measured and set the 'o' ,"; I". ' ,

calibrated attenuatorto the stud marked IIFII • Break the IITesMline " '" - .

to the analyser by pressing the key marked IITest offlland bring:up the /' > • • • -.., ;

'gain till the meter needle reaches the '0' mark on the scale •. :Break

the "Tone" line and adjust T, Fig .1, until the meter reads as nearly

10' as pos8ible.

(4). Adjust the coarse phCCse shift knob when the meter should

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pass through a minimLm. If it does not, 'reverse one of the lines to

the analyse r. Continue to balance to a minimwnbysuccesslve

adjustments on the fine amplitude and phase controls~ increasing the

sensitivity as required by increasing the ce.librated nttenuator only.

(5) HaviY'.g obtained the best balance possible, retu.rn the

calibrated attenuD,tor to "FfI fild check the level of fundamental by

pressing the "Test off" key and adjust the gain control if necessary

to bring the needle to '0' . l1elease the,key and turn up the calibrated ~~.,,.,(:~,,!"Iit

potentiometer until the ',same reading is again obtained on the meter,

when the percentage distortion may be read off the scale. The meter is

calibrated in db. above and below the '0' mark, so tha.t if the correct

setting of the calibratedattenuatorlies between two studs the ,

percenta.ge distortion may be estimated with fair accuracy.

Accuracy

A jack isp:tovided so -thi3.t phones may be plugged to the output

of the detector amplifier and the fundamental balanced out by ear.

This facility is particularly-valuable vlhen distortion greater than 10% \

is being measured, when it will be found that the minimwnis difficult

to determine on the 'meter and maybe much more accurately determined

with the aid of phones.

If there is a negligible amount of phe.se shift in the apparatus

under test, d,istortion of the supply tone is unimportant since it is

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automc,tically balanced out? In P..ny c~se the error so introduced is .!,- . \ ..

only of the second or6er.. Some idea of the magnitude. of the error may

be seen from tlfefollOi~ingexample. If a perfectly linear ampl:i.fier is

under test which,)101uever, shifts the phas.e of the fundamentGl relative

to the secondh8.:rn1onic by 30°, Le. shifts the funclamental 50° and the

harmonic 20 0 , for example, then the hannonic indicated will be one half

that present irithe tone source.

The frequency characteristic of the detector amplifier is

sensibly fiat from 50 to 10.1000 cycJ,es/sec. and fcills off slowly above

this,. which ensures tho.t all harmonics of importance are recorded.

Application

.. The input' impedance to each side of the bridge network is

approximately 2.,000 or.ms up.der normal conditions " At the extreme limit

of the phase shift·coIltrol' the ,imped",,-nce of the tone branch may fgll to

about 750 obms but this will very rarely occur.

The apparatus will operate over e range of inputs from ..,.10 to

+30 db. whichirvill c:over most requirements in service and, as mentioned

earlier, Dny value of phase shift may be covered. Any test frequency ;,' ," ." .:--

maybe employedbetwe'en' 25 and 8,000 . cycles/sec. provided the ganged

frequency selEietionsYfi ten is set to the appropriatepcisition. There

'are four positionscQvering the bq.nds25. - 100, 100 - 450 J 450 - 2,000, ,

and 2,000-8,000 cyclos/sec.

1

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Each input line is completely separete from the other and is

approximately balanced about earth, so that the apparatus may boused

on balanced, unbalanced 'or ea:rthed lines at will.

Finally~ in cases where it is necessary to t&ke a meEcsurement ~ . . .. . , . : '

y,hen the pure output from the tone soure') is not av&ilab1e; the analyser

may still be used in conjunction with Cl. 10wp';:Lss filter. The necess[lr"J

pure tone is .obtained by tappi:'.1g off from the output of the apparatus

under test and passing it. through this filter. This method is

particularly valuable, for example, in order to meas~ire the distortion

pret1ent in recording systems, in fact, this analyse I' is the only

instrument on which any reliable figures 11flve been obtained. The

usual difficulty is the variation in frequenc:r of the fundamental tone,

but since this apparatus is unaffected by such viHiations of frequency

it is ideal for this purpose. ,

Existing Test Apparatus

Routine distortion tests are ~)t present tnade at transmitters

on the General Radio or Marconi-Ekco Noise and Distortion Measuring

apparatus. This apparatus uses the elimination method of remOving the .' .

fundamental and incorporates a high-pass filter. rts 'use is therefore

confined to tests at 400 cycles/sec. arid it requires El. very pure source

of tone. A further disadvantage is trl8.t the measuring circuit is not

square law, so that if two or more harmonics of approxin1atelysimi1ar

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amplitude are present the indication is not the true r.m.f'. sum.

This error is probably not. very serious.

As well as being suitable for all Control Room measurements)

the harmonic analyser described in this report J1'l[;,Y be used to replnce

the General Raclio Noise and .Dis-bortion Meter at tn,nsmi tters) for

measuring distortion at any audio frequency Rt will ccnd for mecsuring

hum and background noise. The indications give t~:'U3 r.m.s. velues J

E!.nd t~le operation is equally slmple. It vrould be usec~ in conjunction

with the Modu18tion~vloni to I' 0

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