Wavetek 3000 - PLL #4 Operation

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-passamplifier .

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 thisloop operate the same
as those in PLL #1 .

In this case the
programmable counter is controlledby
the three "MHz" selector
switches and
the loop reference frequency is
1 MHz
.
PLL #4
The purpose of PLL #4 is to adjust the
WideOscillator in 1 kHz stepsfrom 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
byMixers #1 and #2 andcompared to the
reference (from
PLL #1)
by
the phase
detector .
A
difference in phase or fre-
quencycauses an error signal to tune
the Wide Oscillator until
both phase
detector inputs are identical
.
How
thisloop 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) phaselock-
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 frequencychanges
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 analogsignaldrives
the
WideOscillator 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
lockedoutput 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 referencefrequency
to mixer #1 were 1449 MHz, the
input
range to the phase detector would
look
the same except the entire rangewould
be
shifted 1 MHz to the right . Lock
points would thenbe 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