Table of Contents
Advertisement
CHAPTER
3
P
R
IN
CJ
P_LJ_S_
OJL
_Q_P_E_R
AJJ O.N
BRIEF
DESCRIPTION
Introduction
3®1
Fig.
3.1
shows
a
simplified
block diagram
of the
signal
generator.
A
more
detailed block
diagram
is
Fig.
3. 2
at
the
end
of
the
chapter.
The
carrier
source
is
a
240MHz
to
540MHz
voltage controlled
oscillator
(VC01)
operating
in
a
phase-locked
loop
and
feeding a
bank
of
four
dividers
and one mixer
which
provide
the five
frequency
ranges.
The
dividers
operate continuously,
the
selected range being
chosen by
appropriate
logic signals
on
the inputs
to
the
succeeding combiner
stage.
From
the
combiner
stage the selected
signal
is
fed
through
an
AM
modulator
stage
and
level setting
attenuator
to
the
output
amplifier.
Carrier
Tuning
3 .2
Variable tuning of
the
r.f.
source
is
carried
out
by
an
optical type
shaft
encoder, manually
rotated
by
the
tuning (spin-wheel) control,
from
which
a
series
of
pulses
is
fed
into the
1
N
1
counter.
The
'N'
counter data
is
used
to
programme
a variable
divider
which
divides the
VCO
r.f.
signal
by
the
number
'N'
loaded
in
from
the
'N' counter.
This
divided
signal
is
then
compared,
in
a phase
sensitive
detector, with the selected
channel spacing reference frequency
(see
para
.3. 4).
Any
phase difference produces an
'error'
voltage
which
tunes the
VCO
to
bring the carrier
frequency
into
phase lock with
a multiple of the
channel
spacing frequency.
3.3
It is
essential
to
bear
in
mind
that
all
carrier
frequency changes
are incremental,
i
.e. the
VCO
frequency
is
moved
up
or
down
in
steps, the step size
being determined
by
the
channel spacing
selection
and
whether
FAST
or
SLOW
tune
mode
is
in
use.
In
FINE
TUNE
mode, however,
the
channel
spacing reference frequency
is
variable,
thus
providing continuous tuning over a
few
channels
(see
para. 3.
5).
Channel Spacing Reference
Frequencies
3-4
The channel
spacing reference frequencies
are derived from a
300kHz
reference,
which
is
itself
derived from
the
master
frequency
standard.
The
300kHz
is
divided
down
to
37.5kHz
which
is
then
fed
to
a
programmable
divider,
whose
division
ratios
are
t
10,-1
2
orTl5,
thus
giving reference frequencies of
3.75kHz, 3.125kHz and 2.5kHz
respectively.
These
basic
channel spacing reference
frequencies are used
as
references
in
the
phase
sensitive detector, as
described
in
para. 3
.2.
3.5
In
Fine
Tune
mode
a variable
300kHz
reference
is
used.
This
is
obtained
by mixing
a
5MHz
master reference with the
5.3MHz
from
a voltage controlled
crystal oscillator
(VCXO).
The
FINE
TUNE
control,
'pulls'
the
frequency
of
the
5.3MHz
VCXO,
thus
permitting a small variation
in
the
mixer
output frequency
and hence
in
the
channel spacing reference frequencies.
9081
&
9082
Vol.2
3-1
Advertisement
Table of Contents
