CIRCUIT
DESCRIPTION
Page
63
Refer to the
Schematic Diagram
(fold-out from Page 67)
while you read this
"Circuit
Description."
The Dip
Meter is basically made up
of an oscillator and a detector
circuit. In the
injection mode, the oscillator
generates the signal which is injected
into the
circuit under test. Then the detector
circuit detects changes in the
impedance
reflected back
from the circuit
under
test
and displays these
changes on the
meter. In the
absorption mode, the
oscillator in the Dip Meter is used as a
Q-multiplier, but does
not oscillate. The
detector circuit detects the signal that
is
already in the
circuit under test.
OSCILLATOR
The Solid-State
Dip Meter
uses
a
balanced Colpitts
RF oscillator.
Transistor
Q11, a
plug-in coil, and
split-stator
variable
tuning
capacitor
C21
furnish the
phase
shift required
for oscillation.
Capacitors C11
and C12
are silver
mica
coupling capacitors
chosen for
their
inherent stability.
Resistor
R11
determines
the RF input
current
at
low
frequencies
and is bypassed by
capacitor
C1
3
at
high
frequencies. Transistor
Q1
1
is
a
uhf bipolar
transistor,
operating Class A.
Resistor R14
is
a
load resistor, while resistor R12
provides bias stability. Supply
decoupling and a good
RF ground is ensured
by
capacitor C14.
Variable resistor
R1 is a
variable supply control
for the oscillator and sets the
level of
regeneration.
DETECTOR
The
voltage
impressed across the
oscillator tank
(one of the coils L1
through L7,
and
C21)
is
amplified by a
broad-band
MOSFET
amplifier. In a
balanced
phase-splitter
configuration,
transistor 021
develops two
output voltages
across
resistors R24 and R25.
These
out-of-phase
signals are
rectified
by
series
hot-carrier
diodes D21
and
D22.
The
detected signals are
combined and
filtered
by
capacitors C25
and
C26
to
develop an
average positive-peak DC
voltage. This
DC
voltage is
indicated by
the Meter
which provides
mechanical filtering by
its
movement inertia.
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