Heathkit HD-1250 - Injection Mode Theory

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.