39-31Magnetic Field Measurement
In the servoed type shown in Figure 39.25a, the magnetic eld being measured and the RF eld are
co
axial
. e fr
equency
of th
e
RF os
cillator
is mo
dulated
wi
th
a x
ed
lo
w-frequency
os
cillator
. i
s
ca
uses
th
e
RF to sw
eep
th
rough
th
e
La
rmor
fr
equency
. If th
e
sw
eeped
RF os
cillator
is no
t
ce
ntered
ab
out
th
e
La
rmor
fr
equency,
th
e
ph
otocell
ou
tput
sig
nal
wi
ll
co
ntain
a fu
ndamental
co
mponent
of th
e
RF
mo
dulation
fr
equency
. e ph
ase
of th
e
sig
nal
re
lative
to th
e
mo
dulator
os
cillator
de
termines
wh
ether
th
e
ce
ntral
RF is ab
ove
or be
low
th
e
La
rmor
fr
equency
. e ph
otocell
ou
tput
is ph
ase
de
tected
to pr
o-
duce
an er
ror
vo
ltage
th
at
is us
ed
to dr
ive
th
e
RF tow
ard
th
e
La
rmor
fr
equency
. e RF ca
n
be me
asured
to determine the