Wavetek 3000 - M34 - Wide Oscillator Lock

THEORY OF OPERATION
3 .14 M33 - NARROWOSCILLATOR LOCK
TheM33 is part of a phase locked loop
for which the VCO is the "Narrow Oscil-
lator" in the M9W . The M33 includes a
phase
detector, mixer and
an electronic
"reference" switch . See Figure
3-19 .
3 .14 .1 PhaseDetector
Thiscircuit compares thereference fre-
quency to the variablefrequency which
represents the VCO
output .
IF
the
VCO
is
too high, for
example, the phasede-
tector puts out a more positivevoltage
which
is
filtered and
inverted
by
an
integrator and applied to the VCO (nar-
row oscillator) to lower the frequency
.
3 .14 .2 Mixer
The phasedetector can not
operate at
UHF frequencies so the VCO is mixed
with
1200 MHz CW . Thisprovides an offset
frequency
which is the variable input
to
the phasedetector .

The deviation
of this variable signal
from 2 MHz is
precisely the same as the deviation
of
the
VCO from 1198 MHz .
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W/DL
VCD
3-20
3 .14 .3 Reference Switch
3 .15 M34 - WIDEOSCILLATORLOCK
Figure 3-20 . M34 -
Wide Oscillator Lock
This circuit, controlled
by
the MODE
switch on the instrument front panel,
selects either the 2 MHz CW reference
for CW operation or the FM reference
(1
.5-2
.5 MHz) for FM operation of the
instrument .

The referenceswitch uses
a
hex
inverter to electronically route
the

reference
signals as well as to
guarantee that TTL levels will be
fed
to the phasedetector .
Thismoduleprovides the fine
tuning pro-
gramfor thewide oscillator in the
M9W .
Figure
3-20tis
the block diagram of the
M34 .
The letters A thru F relate the
signals
at the associated points in the
module to the graphs A thru F in Figures
3-18 and
3-19
.
TheM34 phase locks the
VCO to
1198 MHz plus the frequency indi-
cated on all six
front
panel
switches .
F . P .
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