Injection Mode Theory - Heathkit HD-1250 Manual

Page
44
3.
Refer
to
Figure 2-2
and
position the end
of the meter
coil close
to,
and
directly
in line with, the
coil or inductance
to be
tested.
4.
Slowly
turn the tuning
dial
from one extreme toward
the other. At
the point of resonance, the
meter
indication should drop
abruptly,
and almost immediately
rise again to the nonresonant meter
level.
5.
Carefully turn the tuning
dial back
and
forth across the point
of
meter dip
until
you
locate the lowest point of indication on
the
meter
scale.
Slowly move
the dip
meter
coil
from
the
coil
under
test
to
reduce the
coupling, and
re-dip the meter.
6.
Read the tuning
dial
scale
on
the color that corresponds with the
coil
you
have chosen, and
note the frequency
in megahertz.
Injection
Mode
Theory
Refer to Figure 2-3
as
you
read the following information.
"In the
injection or oscillator
mode
of operation,
the Dip Meter
operates
as
an
absorption
trap.
As
shown
in
Figure
2-3, test circuit L
t
-
C
t
is
not energized and
has a resonant
frequency
f
0
.
When
you
tune the Dip Meter
to that frequency, mutual
coupling
between
L,
and
the pickup
coil
causes
a
high
circulating current to
develop
in
the
test circuit. Simultaneously,
a
high impedance
is
reflected into
the
oscillator
circuit
and
degenerates or "dips" the
level
of
oscillation. This
change
in level
is
detected and
then indicated on
the instrument meter. The
resonant
frequency
of
the
test circuit
is
then read
directly
from
the
calibrated
dial.