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3-7
3.5.
3.5,1.
RF UNITS
1
and
2;
figs,
42
-
46
General
The
frequency
synthesis
is
a
combination
of
system elements
that
results
in
the generation of
many
frequencies
from one
or
few
reference
sources.
The
frequency accuracy and
stability
of the device
are
determined by
the accuracy
and
stability
of the
crystal
reference source
and
to
a
less
extend
the
cir*
cu
it.
The RF
oscillator
unit
is
the heart of
this
programmable frequency
generator.
The
unit
comprises
the
two
single
screened
RF
units
1
and
2
which
are
mounted
together
with connection board
RF
into
the
control
RF
box.
The
PM
5390
RF
synthesizer
is
based
on
the
principle
of
indirect synthesis;
fre-
quencies
are
generated
by
VCOs
(Voltage Controlled
Oscillators)
in
digital
Phase-Locked Loops
(PL
LI-
A
phase
locked loop
is
basically
an
electronic servo
loop consisting of
a
phase detector PD,
a low-pass
filter
and
a
VCO.
Its
controlled
oscillator
phase
makes
it
capable
of locking or synchronizing
with
an
Incoming
signal.
If
the
phase changes,
which
indicates
that the
incoming frequency
is
changing,
the
phase
detector
output
voltage increases or decreases
just
enough
to
keep
the
oscillator
frequency
the
same
as
the
incoming frequency,
preserving
the locked condition.
INPUT
SIGNAL
ViW
A>;
Hi
OUTPUT
SIGNAL
Fig.
2
Block diagram
of
Phase
Locked
Loop
(PLL)
Four
of
the
six
subranges
—340
to
510 MHz, 510
to
680
MHz,
680
to
850
MHz
and
850
to
1020
MHz^
are
generated
by
VCO
la
...
d.
To
get
always
the
same
dividing
range
N
in
loop
1
the
output
signal f1
of
VCOl
in
this digital
PLL
is
down
converted
by
mixer
1
to
50
MHz
...
220 MHz.
Mixer
oscillators
VC02
a
and
b generate
4
fixed frequencies for
mixing with
the
corresponding 4
fre-
quency
ranges of the
main
oscillator.
The
resolution of the
output frequency
is
1
kHz
(10
kHz
above
1000 MHz);
so
the
divider
N
must be
set
between 50 000 and 200 000
In
steps
of
1.
This
is
achieved
by
the
programmable
divider
1
(U2)
and
two
10/11:1
prescalers
(RF-U2),
controlled
by
port
3A,
unit
2.
The
frequency
ranges
from
0.1
to
170
MHz
and 170
to
340
MHz
are
generated
by
mixing
VCOIa
or
VCOIc
(assisted
by
VC02
just
mentioned) with
the
fixed
510
MHz
frequency
of
VC03.
For
this
the
output
signal
is
switched
to
mixer
2.
The
lowpass
filter
(RF-U1/U1
1/U12)
separates
unwanted mix^r
products.
All
3
oscillators
VCOl
,
VC02
and
VC03
are related to
the
5
MHz
X-tal
reference
oscillator.
Fig.
4 shows
the
different
oscillators
involved
into
the
frequency
generation
from
0.1
to
1020
MHz.
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