Wavetek 3000 Instruction Manual page 25

To provide phase locking in 1 kHz steps,
the PLL #4 phase detector's reference
from PLL #1 is adjustable in 1 kHz steps
(10 .000 to 9 .001 MHz) . This causes the
Wide Oscillator frequency to change in
1 kHz steps in order to keep the loop
locked .
PLL #3
The purpose of PLL #3 is to stabilize the
Narrow Oscillator at a frequency of 1198
MHz .
Figure 3-6 showsa simplified block dia-
gram of PLL #3 . This loop operates in
the same manner as PLL #1 and PLL #2
except that it does not require the use
of a
programmable counter . The 1198
MHz output from the Narrow Oscillator
is combined in a mixer with a 1200 MHz
crystal controlled signal . This pro-
duces a 2 MHz difference signal . This
signal is fed to a phase detector where
it is
compared to a 2 MHz crystal ref-
erence . Any difference in the input sig-
nals will produce an
error voltage
which is applied to the Narrow Oscilla-
tor (VCO) to correct the frequency error .
To provide
FM
modulation, the
2 MHz
crystal reference applied to the phase-
detector in PLL #3 is replaced with the
output of a VCO as shown in Figure 3-7 .
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Figure 3-7 . FM Circuit
CRYSTAL REFERENCE
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THFORY OF OPFRATION
With a O volt (0 deviation) input to the
VCO of Figure 3-7, the output is 2 MHz,
therefore the operation of the generator
is unchanged except the frequency accu-
racy is reduced because the reference is
not now crystal controlled .
A modula-
ting signal of +5 V
to -5 V will cause
the VCO frequency to shift 0 .5 MHz above
and below the 2 MHz signal . Since the
instrument's RF output signal is phase
locked to this reference the output of
the generator will be FM modulated with
a maximum deviation of .5 MHz .
All
the reference frequencies for the
phase-locked
loops are derived from a
single 40 MHz crystal source by means of
appropriate multiplication or division .
Figure 3-6 . PLL #3
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