
5-11Principles and Technology of SQUIDs
edierence between the voltage developed across the unknown impedance Z and that developed
in th
e
se
condary
of a nu
lling
mu
tual
in
ductor
m is ap
plied
to th
e
in
put
of th
e
SQ
UID
ci
rcuit
. e pr
i-
mary
cu
rrent
in m is pr
oportional
to th
e
os
cillator
vo
ltage
an
d
de
ned
by th
e
se
tting
of th
e
ra
tio
tr
ans-
former(α)
. An ad
ditional
re
active
cu
rrent
is su
pplied
by a se
cond
ra
tio
tr
ansformer
(β) th
at
ca
uses
th
e
pr
imary
cu
rrent
to be pa
ssed
th
rough
a ca
pacitor
ra
ther
th
an
a re
sistor,
th
us
ge
nerating
a 90
°
ph
ase
sh
i
in th
e
vo
ltage
ap
plied
to m. e am
plied
o
-balance
sig
nal,
wh
ich
ap
pears
at th
e
ou
tput
of th
e
SQ
UI ...