Wavetek 3000 Instruction Manual page 23

Figure 3-3 shows a simplified block dia-
gram of PLL #2 . PLL #2 operates in
the same manner as PLL #1 with one ex
ception . The circuit includes a mixer
and band-pass amplifier .
The purpose of
this
additional circuit
is to offset
the 1448 to 1487 MHz output from the
VCO to 8 to 47 MHz . This offset is
necessary in order to make the frequency
compatible with the programmable counter
and phase detector circuits . The other
circuits in this loop operate the same
as those in PLL #1 . In this case the
programmable counter is controlled by
the three "MHz"
selector switches and
the loop reference
frequency is 1 MHz .
PLL #4
The purpose of PLL #4 is to adjust the
Wide Oscillator in 1 kHz steps from 1198
MHz to 1718 MHz as the front panel fre-
quency selector
is adjusted from 0
to
520 .000 .
The Wide Oscillator frequency is offset
by Mixers #1 and #2 and compared to the
reference
(from PLL #1) by the phase
detector .
A difference in phase or fre-
quency causes an error signal to tune
the Wide Oscillator
until both phase
detector inputs are
identical .
How
this loop locks on a particular fre-
quency can best be explained in three
steps :
1) phase locking at 40 MHz in-
tervals across the band, 2) phase lock-
ing at I MHz intervals, 3) phase locking
at
1 kHz intervals . Figure 3-4 is a
simplified
block diagram of PLL #4 .
To understand locking at 40 MHz inter-
vals, assume temporarily that the ref-
erence frequencies
from PLL #1 and PLL
#2 are fixed
(10 MHz and 1448 MHz re-
spectively) .
Figure 3-5 shows the fre-
quencies
throughout the loop for
this
discussion . This step of the PLL #4
explanation can be described more clearly
by considering the entire Wide Oscillator
range rather than discussing single fre
quencies .
The Wide Oscillator covers
the range
of 1198 to 1718 MHz as the
Output frequency
changes from 0 to 520
MHz .
(Figure 3-5, lines A and C .)
THEORY OF OPERATION
When the Wide Oscillator range is heter-
odyned in
Mixer #1 with 1448 MHz the
difference frequency which is produced
ranges from 250 to 0 to 270 MHz . (Fig
ure 3-5,
line E .) This signal is
then mixed with a 40 MHz comb (all har-
monics of 40 MHz) in Mixer #2 . (Figure
3-5, line F .)
Taking the difference be-
tween line E and F yields the repetitive
frequency
range from 0 to 20 to 0 MHz
as
shown in line G . This signal is
fed to the phase detector .
The reference to the phase detector is
10 MHz but the loop will not lock on
every 10 MHz output of Mixer #2 shown
on line G . Only the 10 MHz signals to
the immediate right of the 20 MHz signals
on
the graph are the proper phase to
produce lock . Therefore at every
40 MHz interval of the output frequency
an input to the phase detector
would
allow the loop to lock .
Section 3 .2 .1
explains that an analog signal drives
the Wide Oscillator to within three MHz
of
the proper frequency . Therefore,
although
there are 14 possible lock
points on line G, the only one selected
will correspond to the analog-tuned
frequency of the Wide Oscillator . The
unit as
described so far is capable of
phase
locked output at 0, 40, 80 . . .
520 MHz . The following is an explana-
tion of locking at 1 MHz intervals .
To allow
phase locking at 1 MHz inter-
vals, the reference frequency
to Mixer
#1 is made
adjustable in 1 MHz
steps
over a 40 MHz range
(1448-1487 MHz) .
If, for example, this reference frequency
to mixer #1 were 1449 MHz,
the input
range to the
phase detector would look
the same except the entire range would
be shifted 1 MHz to the right . Lock
points would then be possible at output
frequencies of 1, 41, 81 MHz, etc .
Being able to
change this reference in
1 MHz
steps allows phase locking from
0 to 520 MHz in 1 MHz steps .
3-5