Table Of Contentap 28 Ana Tegan gh Psey
‘A 200-Hz to 30-MHz Computer-Operated
Impedance / Admittance Bridge (COZY)
By LD, WHITE, RW. COONS. and RC. STRUM
(taste! veceved Septembor 12, 1980)
For the past fet: years the development of ferromagnetic compe-
nents, particularly far long haul transmission systems, has relied
‘heavily on large numbers of highly accurate impedance measure:
ments made on « compuler-ipurated inpedance/admittance bridge
(cozy) developed espectally for this work. cO2Y's accuracy and speed
table level of component development mat others posable. CO2F
ttutomatically measures complex impedance, temperature coefficients
of complex impedance, and disaccommadatian factors of feromag:
netie materials, providing aceuracies of 0.05 percent for inductance,
{L60 microradian for lass angle, and £20 parts per million for the
malt impedance changes associated with determinations of tomper-
ture coefficients and disaceommordation factor®.cOzY te easy (9 use
tand makes a mecsurement in 10 £0 20 seconds. Also, the calibration
of the bridge wits cqpacitonce and conductance standards can be
Checked automatically. Though developed primartly for ferromax
hee component wor, co2¥ 1s a eral purpose bridge, € measures
inductance, copacilance, resistance, and conductance over wide
impedance ranges at frequencieg between 200 Hz and 30 MIs, This
paper describes cozy's hardware, softcare, and performance.
A compater-opereted impedance admittance bridge (002¥) hasbeen
developed to have the following fees
‘© sate Frequency range— 20) Ilz to 0 Mfc in O.01-He steps
‘¢ milo impedance admilvenrerange-from a resaution of 0.1
ober for sell inpecances£0-& resolution of 0.001 piofarad
for mall admittancer
(¢igh acouracy-—high-@ unknowea can be measured to 0.05 pe.
ent for inductance/eapacitance and 80 mjeroradians for lacs
langle, Changes in impedance/admittance (with temperature,
time, shock,
rlion.
1 specifiabe signal level—vottage or current may he specified over
the nominal range of 0.05 ta vale for impedances lager thar
100 ohms snd 0.5 to 50 miliamperes for amallerimpeiances. The
achieved level is within £10 pero of che requested level acd is
‘eanured to £3 percent
‘relatively faat20 saconds pet measurement
‘easy 10 use yet fesible—the bridge has only a single pair of
binding posts to which the unlmown is connected and the user
ince Co specify only w test raquenry. Howover, tho uter can
-Apeciy signal level, fresaency nti, al various options for post
processing ofthe measurement results
the options for runs and postprocessing can be changed easily —
‘the software clearly separates the options from the basic mea-
‘surement proces.
‘© sutomatic aids for maintaining high accuracy—in particular, the
Calibration of the bridgo unit's standard can be automatically
shee
‘A mierocompater-controlled environmental chamber with an 1
sample capacity is appliqued to cozy. ‘The combined system provides
‘the folowing aditinal features
1 Highly aocurate aulomasie measurements of the changes of the
sample impedances/admittanoes ‘with environmental condi
‘ions—vemperatures way be specified to =0.1° Celsiue between
40 and #5" Celsius and rive humidities so 22.8 percent
Derwaen 20 and 95 percent, with a minimar dew point of 25°
(Celsius, Sooke times, signal levels, multiple frequencies, and en
ronmental runs can also be specified. Average time for a single
‘ensurement is 10 seconds
‘© Automatic meaurements of the disaocommadation factors (Che
crease in permesbiicy with time after demagnetization) of
{erramagnete materials peak demagnetization currents up to 2
amperes rth a IO-voke maximum, ean be specified,
coz was developed to provide the measurements required in fer.
romagnetic component development work. Large numbers of highly
accurate meanurements are required Lo evaluate meri, structures,
tal whole component aver thai oneratingenngen af frequency, signa
Trvel, and environmental condiena and to determine the effects of
‘ing, shock, nd vibration
"The moct crucial of the messurement requirements chat led to the
development of cory were: (L) «basis precision of sinificandy better
‘than ten para per milion co achieve the desired accuracies in measur
ing high values and emall changes in impedance, and (2) a measure
tment time rouch lees than a inate to provide the desired quantity of
vibration, en be menssred to £10 parts por
406 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1961
rmessurementa The measuring ayscems that come clozst to meeting
‘hese requirements are specially developed manual bridges and the
S0-Fi (0 250 MH computeroperaiad Usnemission meanaring
tem. The manual brides have sacsfactory precision butequite many
inlet and much rare and exprrtise for 2 measurement. The eam
pater operaled (rorsitsion ietoring sytem, on Tn athe ain, i
Aamply fast but haz a basie precision af approximately 100 parts per
millon.
‘nee the objectives for precision roquires bridge tochniquos; pure
tranamission messurements ary nol sutisialory. To achieve short
‘meapurement times requires aytomation, and because the amount and
tomplnity of bridge computations are large, the automation has co be
fdone with computer,
“The development of coz¥ rain new design futures and mek:
surement proceduses, co2's bridge difers markedly from msnual
bridges in vo ways. Fist, small impedances are measured with novel
bridge confiqurtions based on techniques previously used to calibrate
inductance salar Second, all siting of the bridge configu
tons and setting ofthe standards ie dane by’ new design of mercury.
‘wetted contact rea that requires very different design considerations
hac the wafer wiles used in manuel bridges,
‘con's measurement proces differs significant from mesrurement
proaeasea in manual bridges in three basic areas selecting the bridge
Configuration, balancing the bridgo, nnd obtaining the Inst 1¥ decades
ff due balance, To determine dhe bridge configuration Tor a measure-
‘ent, cozY ealculates an approximate value for she unknown trom
four teansmissior-ype measurements three of which use predeter=
amined settings of the bridge to provide a calibration of the system,
Halancing is done with an iterative process in which capacitance and
condiclance sandaris are changed, the Patio of the change in the
fcdilfance of thr olamdarda to the change in the ridge output ia
faleulated, and the ext change to be made in the standards iz
alcoated by multilving this ratio bythe last bridge output Th fi
Vis decades uf the balance are determined by mening the bridge
‘uipnt over a one-second period. I the final degree of balance hd
‘bor Timed by noise, this messurement provides inercused resolution
by nue averaging. On the other hand, ifthe degree of balance had
Tom ited by the Hnite sine ofthe smallest step ofthe standards,
Ue measurement provides interpolation hetween these steps.
"To provide information stable forse inthe meonmirement process,
‘the receiver must be phase sensitive and linear right down za
signa. This eccomplished by using heterodyne terhniques to prduce
‘Ben le sgnals whose amplitadee, including signa, represent orthogonal
romponents of the bride's eurpur signal‘ The goetraey ofthe repre
‘entation i one pereent
‘To achieve the required coutel of temperavare and humidity, the
covironmental chamber’ heaters, compressor, and humidifer water
trere pul under the control ofa icrocomputer using apelally devel.
ped firmware. A microcomputer rather than coz¥s computer was
sed tp enable coz%’s computer co be ire for general purpose mea
surements while the specified environmental conditions and soak Himes
ste being achieved.
‘This paper describes the hardware, software, and performance of
‘the computer-operated bridge and ofthe fclites added to the bridge
to provide temperature cosfficlent and disacoommedation factor mea
surements. Section II daseribea the bridge unit and other basic herd
‘ware. "The calibration ofthe bridge is coverod in Section Il. Section
IV describos the aoftvare, inciting the meamsrement process, inter
‘action with the user, and postprocesing of the measurement resula
Section V gives the measurement accuracy and discusses the soureas
of measurement unvertnnty, Seeion VI conver: the automatic wid
{or maintaining the eccuracy and hardware. The main features of the
hardseare and software for automatically measuring the disaecemme-
dation factors of ferromagnotic mutuals and the effects of wampere-
‘ure and humidity on impedance ure described in Section VIL Section
VIllis a summasy.
I BRIDGE UNIT AND OTHER BASIC HARDWARE
2.1. Gonerat
Figure 1 shows the basic hardware blocks for making impedance
rmessuremenc: a signal generator, bre unit, vollmetar connected to
the bridge unit, receiver, and alg to-gital converte, all controled
hhy a computer through an interface and test panel, igure 2 ie a
hotogeaph of cox when pot inte aervice, The bro and one-half bay
fabinet at che left contain from left to right Uhe signal generator
‘ceiver, and brige unit. Mounted op the horizontal tp surface of the
Thalbay are che bride's two binding poets to which the unknown is
connected. The sixbay cabinet on the rieht contains the computer
fand, st the far ond, the inverfues wal sh pone Inthe middle ia
tcleiypesriter In the backgroud al che end of the scbay eabinet is
"step-up" unit chat provides eamputer concollod admittance ballast
for automate calibrations ofthe bridge's admittance standards
2.2 erage unit
2.24 Over!
‘The bridge is unity ratio type with 100-ohm zeistoe forming the
‘ali arms, Four vonfigurations of the other te arms, accomplished
by automatic switching, arvusol w measure the ful-adraittance range.
408 THE BELL SYSTEM TECHINICAL JOURNAL, MARCH 1984
Lovmane J
“pO
=
oe
15.1 Mla eg of ampute operate impedence rd (nt)
igure 4 shows simplified schematics of these configurations. ‘The
capacitance siaslard, Cis in the A-D bridge arm for all configura-
tions the unknown, Ue, mney be in either the A-D or tho C-D arm
dnd thw conductance standard, Gi alway inthe arm adjacent to Ue
cozy: 200 He TO 90 MHz 408
‘a ‘or
0 ‘at
Pi 3 cnt
‘han 0.08 siemens, The
‘ig. db configuration iy weed for simdar-szed inductive unknowet The
“aval Inpedance” configurations ahown in Fig 2eand 2d are sed
for capacitive and inductive unimowns, respectively, heving suscopt-
ances typically larger then 0.02 siemens andor conductances larger
‘than 0.08 siemens. In these siall-impedance configurations measure-
‘ments are made with one of eleven calibrated capacitors, Cn series
‘with the unknown and a similarsized ballast enpacitor, Cra, in the
adjacent am.
‘As shown in Fig. 36, signal is applied tothe bridge by a transformer
4410 THE BELL SYSTEM TECHNICAL JOURNAL, MAFCH 1063
comecced between lw A and G comers the revever iz connected
‘etween the B and D comers; the volimlar ix vonnected scree the C
harm; and tha D vorner i rounded,
ach measurement rutin manipulating che cwpacitaace and
conutaclane standards to balance the bridge Iwien—an unknown,
Falanoe with che unkrnws rmected into a bridge aren anda reference
Tala with the unkaworn effectively out of he urm.‘The unknowns
simitience i computed from the lvitiance diference betwuun the
‘vo bales. Por the emall-advance configuration, shown in Fign
fa end 30, the unknown balance israade withthe sith in svi with
the urlinown closed and the swilah shunting the unknown upen (2
Shown. ‘The reforonce balanow is tude with the series switch upen
fd che shuntoy soit closed. For the smallimpedanes ennfigura
fiona, show in Fig, 22 wo sel, the ahunting switch is open for Che
funky halanee and closed for the veference balance
"The Iwilge’s basi blocks and evitchea aze shown in Pig. 4. Two
uanaformers aro required to cover the frajueney range. One 6 uscd
from 200 He to 101 Kz and he otber, rows IN! KE wo 90 Ml, Bork
fre double shielded and epecnly developed for bridge use The trans-
former inorsbield capsctanvey 0 pF and 15 pF. are large enough
requite thy complete daconncian ofthe unused transformer.
“Tho roto arm sesators, Ry ond Ry, are OOLpervvn! metal fn
resistor having very small parestic impedances so that thei resin
fncec wre fequeney indepen ell beyond the reouirementa ofthis
bridge The time enntant ofthe rasa wis ljasted toe within 10 px
ofr
The capacitance standard comin of eight decades covering 1.1 ¢
in QOL-pE ekopm The 1 0. and OMT per wep docudes arr wired
itn the C-D bridge arm, Sinco the capacitance standard is ieated as
hing in the AT) arm, thew diondes are operated in revere that is
‘heir “oer stings are thir swismom eapacicanee wtin
"The cwluctance suundurd covers 12000 pS in COL ys
consis of nine cones, Te frequency wl high Frequency versions of
the and 104 per slop decades ure neem to cover the feawency
range. "Tho 2000, an 100-5 and the lw equency 10- and Pp. per
[Rep decudie can be anieched ini either the A-D or C-7) rin The
‘omalniny conductanee decadks wn red into either the A-D or C-1
"The distribation of dasases mony te AD arm, the C-D arm, and
‘being itched ws based om considerations of simultaneously min
Imisng tho tolel admittance i the arma, ehe mumber of Tels al»
bridge crm, and the frequew'y sependencice ofthe decades
"The switched admittances ¥, through V> compensate for the
hanged in edmitames the ALD and CAI) arms that accompany
‘witching the conductance decodes from one ana to the other. Por
por. 200 He TO a MMe att
couple, ¥; compensates forthe esparitance changes associated with
‘oilching Ube 100 aed Tah WS devaes. When the decudes ae i che
‘AD arm, ¥; iin the C-D atm, and vice versa.
‘The ealbrated series capacitor, Cx, and the ballast capacitor, Cxa,
‘sed inthe emallsmpodance configurations, ay be set ta: 10, 30,100
‘00,1000, 3000 pF ued O11, 0.08 01,035 and Th patito,
x. is contsined within shield and the shield and capacitor can be
connected to the A, ©, or D comer. The ballast capacitors were
‘adjusted during prove-in au vba the bridge balances with low eettings
fon the capacitance and conductance sandards when the sec, SIP,
cross the binding posts i closed.
"The bridge was mechanically designed with the following objectives,
‘in mind: aa low as possible impedances in series with the various
‘ompenents the components in one 2m aielded from the components
{nthe other ams and the basic blocks in ech arm to have independent
lead vo the juncsion points with the adjacent arm and the aseociatd
generar o receiver connection, thus well defining each bridge comer.
‘The ratio resistors and the A,B,C, and D comer blocks are contained
fn central rigid structure. The eapacitance and conductance decades
dre connected to the A‘oF€ bloc and the D ck ih evil exes
All the clvuit componente mere selected for stability with cme,
Lemperature, humidity, and vibration In addition, the bridge temper”
ture is held constant to betcer than 20.05° Cla,
‘A critical creut component. developed specially for tis bridge, ia
the relay ted foe al switching. The relay containe a mercury witied
‘om itch, show i Fi, 6, with the leds wo Ube normally open
fontac made nf magbetie alloy and the Isa to the arataly led
‘eontacts made of plstinam With no power applied to the relay the
‘peo platinam leads are shorted by 1 har carried by the armature
‘When power is applied, this har moves to sor the two magnetic alloy
leads. The magnetic alloy leads provide part of the magnetic cizcuit
that permits the relay’ toe operated without estemal magnets, hich
‘would be too bulky. The platinum leads give a stable low-impedance
path for che critical crews: the alloy leads" resistance chenges 50
‘much with time aftr a relay isawicehed, dus to temperature changes
tstsed by lee! heating that thew levels ean be used onlin very high
iinpuance or enncrtial vince. A spetaly designed shiek and col
ssssuly elec vntatically ils the eapoule from it diving eal and
‘umounds the whole sseembly with an electromagnetic shield that
‘completes the magnetic eruit
"The switch ie very stable and reproduce. Menurements sith
rman brid showed tht the switches ese with variation of Ie
{han ten micro-ohis in series reise and one Chowsandth picofarad
in shont capacitance
ORY: 200 He 10 30 MH 413
However, the switch bat some dissdvantages, Tt must bw sed
‘upright ita elativey Inge with « horizontal center-to-cnver spacing
of Unre-unruens of an inl anal it hax fopP epee from the
Armature to the grounded abel ~
Figure 8 is a photograph looking down om the bride, So that the
Alas show, the cop civer a the eover of the shield surrounding the
ralia resistnrs Ive been removed The junction ofthe detector lead
‘with this shield is the B commer. The A and C corners are below the
high Uinding post, F, which is shoum."The D corner is below the A and
‘Creemers, Tae low binding onl, Z, is mounted on the top cover and
Jnana ts located shove the shild around the ratio resets
414 THE BELL SYSTEM TECHNICAL JOURNAL. MARCH 1081
