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Potomac Instruments FIM-21 - Page 18

Potomac Instruments FIM-21
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SECTION
3.
THEORY
OF
OPERATION
3,1
General
The
theory
of
operation
of
Field
Strength
Meters
FIM-
21
and FIM-41
is
explained
in
this
section.
Included
in
this
section
is
a
general
descrip-
tion
which
is
referenced
to
the
associated
block
diagram.
3.2
Block Diagram
Description
Refer
to
Figure
3-1,
Field
Strength
Me
ter
FIM-21/41,
Functional
Block Diagram.
The
FJN
can
be
considered
as
being
made up
of
seven
major
sections:
(1)
A
loop
antenna
(2)
A
very
stable
AM
superheterodyne
receiver
(3) A 20 db
per
step
RF/IF
attenuator
within
the
receiver
(4)
A
calibration
oscillator
(5)
A
gain
comparison (nulJ.)
circuit
(6) A
panel
meter
(7) A
regulated
power
supply
A
shielded
loop
antenna
is
used
because
of
its
directiv.i..ty
char
-
acteristic
and
its
freedom from hand
capacity
effects.
Signals
picked
up
by
it
are
fed
into
the
receiver,
and
the
carrier
to
be
measured
is
selected
by
tuning
the
receiver,
using
headphones,
or
the
panel
speaker,
and
the
panel
m
eter
as
tuning
indicators.
Both
the
audio
and
the
DC
to
operate
the
meter
are
obtained
from a
diode
rectifier
at
the
output
of
the
last
IF
stage.
T
he
RF
attenuator
is
set
to
the
range
which
prov.ides
a
reading
between
the
upper
and
lower
limits
of
the
meter,
and
the
instrument
is
rotated
for
maximum
pickup
in
the
loop.
The
six
steps
in
the
attenuate~,
plus
the
10
to
1
range
of
the
meter
,
mak
e
it
possible
for
the
instrument
to
measure
signals
over
a
range
of
10
microvolts
to
10
vo
l
ts.
With
the
loop
-
receiver-meter
section
of
the
field
meter
just
described,
it
is
possible
to
make
comparative
or
relative
field
measure-
men
ts
over
a wide
range,
and
the
shape
of
the
radiated
pattern
can
be
de-
termined
with
accuracy.
However,
the
FCC
requires
the
broadcaster
to
make
absolute
m
easurements
of
the
signal
intensity
at
any
point,
in
volts
per
me
ter,
and
for
this
purpose
the
calibration
oscillator
is
includedo
3-1

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