Fig. 23
ohow~
a typical synthesizer
setup
using
three
OilICiIlators.
Two
of
the
oscillators produ<:e
the
on·
channel
frequency needed by
the
transmitter.
The
$4IIlC
bigh.
frequency oscillator used
with
a
dirrerent
low frequency oscillator produ<:es a frequency 455
KHz below
the
carrier
frequency
for
uie
as
the
receiver's local oscillak>r.
•
.15(4)
=
4·455
KHtl
."'"
•
~
Transmit
I
Tran
....
lt
T
.......
it
-,
I
,
"'.
+-
Mi~.r
,
Ooc:illetor
27.005
t---'"
,
132.1.5
,
,
, ,
Rectl
••
,
A_I
....
00'
I
C_
,
Receive
,
Ooc:illitor
MiKe'
,
I
,
26.1>496
f--"
,
132.7·6
,
(Qllonnel
,
,
I
,
"""
I
Otclilnor
I
5.695
0
5.715 0
1>.725
0
5.13f:i
0
6.1504 0
6.1704 0
6.1804 0
6.1904 0
32.700 0
32.760 0
32.110O
0
32.850 0
".960
Ot---'"
".960
O~
Fig.
23-o~iIlator
setup fo.
14
crystal syntllesizer.
PHASE .
LOCKED
lOOP
In operation, a
DC
voltage is applied
to
the
veo
which
causes
it
to operate
at
a frequency
dose
to
the
desired
output
frequency.
The
digital
countdown
cir-
cuitr. are set
$()
that
the
output
of
the
countdown
is
the
same
3lI
the
frequency
of
the
mllSt.er
oscillator
when
the
veo
'"
exactly
on
frequency.
output
r~quency
is controlled by
an
input
voltage.
In addition to the
two
oscillators,
the
PPLL consists
of
$lOvernl
digital countdown stages, and a phase com·
parator.
The
amount
of
countdown
is controlled by
external programming lines connected
to
th<J
channel
selector switch.
The
phase comparator.generates a
corre<:tion voltage
oUlput
if
the
phase
of
the
two
input signals is
not
the same.
•
~-
.~.
,--
~
Coo,........
Otci"'",
eo.
.."'....
lI.
v
....
1lI
Ret
......
~.
I-f
,_.
00<;11"",
,
~~
OJ,ide,
Some
23
channel transceivers and many
40
channel
transceivers use Programmable Phase.Locked Loop.
(PPLL) circuits to produce the necessary internal fre.
quencies.
This
system usually requires only one
or
two crystals for transmit
and
receive frequencies
Fig.
24-Programmable
Pha$l!"wched
Loop
block
dwgram.
A typical PPLL is shown in
Fig.24
.
It
consists
of
a
crystal controlled master oscillator and a voltage con·
trolled oscillator
(VeO).
The
veo
is designed to
operate
at
the
desi~
output
frequeo<:y. Its
elCact
It
the
two
inputs
to
the
phase
comparator
are
out
of
phase, an error
in
the
veo
frequency is indicated.
This
phase
error
results
in
II
correction voltage
output
from
the
phase
comparator
which is applied to
the
Yeo.
This correction voltage causes
the
veo
to
shift
frequencica until
veo
is operoting
at
the
desired
fft!-
quency.
26
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