M34 - Wide Oscillator Lock - Wavetek 3000 Instruction Manual

THEORY OF OPERATION
3 .14 M33 - NARROW OSCILLATOR LOCK
The M33 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 Phase Detector
This circuit compares the reference fre-
quency to the variable frequency which
represents the VCO output . IF the VCO
is too high, for example, the phase de-
tector puts out a more positive voltage
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 phase detector
can not operate at
UHF frequencies so the VCO is mixed with
1200 MHz CW. This provides an offset
frequency which is the variable input
to the phase detector . 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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3-20
3 .14 .3 Reference Switch
3 .15 M34 - WIDE OSCILLATOR LOCK
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 reference switch uses a
hex
inverter to electronically route
the reference
signals as well as to
guarantee that TTL levels will be
fed
to the phase detector .
This module provides the fine tuning pro-
gram for the wide 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 . The M34 phase locks the
VCO to 1198 MHz plus the frequency indi-
cated on all
six front panel switches .
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