QEI 675 - Theory of Operation; General Assembly Descriptions

www.SteamPoweredRadio.Com
SECTION
4
THEORY
OF
OPERATION
4-1
GENERAL
Refer to block diagram
of
the
Exciter.
The
Exciter
circuitry
is
on
three
circuit
boards:
A2-FMO
and
Phase
Lock
Assembly;
A3-IPA
and
PA
Assembly;
and
A4-Power
Supply Regulator Assembly.
1.
A2
FMO
and
Phase
Lock
Assembly.
A packaged
on
carrier
direct
FM
oscillator
sub-assembly produces
the basic
signal.
This signal
is
amplified
by
the
FMO
buffer
and
control
amplifier
to a level
of
approximately
100
mw.
A sample
of
this
signal
is
fed to a high speed
divider
which
reduces the
fre-
quency
to
nominally 5
MHz
(fc/20).
This 5
MHz
signal
is
then pro-
cessed
by
the programmable
divider
whose
output
is
5
KHz
regardless
of
the assigned frequency.
The
output
of
an
8
MHz
crystal
controll-
ed
oscillator
is
digitally
divided to 5
KHz.
This signal
is
phase
compared
with the output
of
the
programmable
divider
and
the
re-
sulting
signal
is
filtered
and
used
to
control the
FM
oscillator
sub-assembly. Thus, the
programmable
divider
allows the use
of
the
same
high
stability
8
MHz
reference
crystal
regardless
of
channel
assignment.
It
should
be
noted
that
a phase locked loop required a phase
error
not a
frequency
error
to
generate
its
correction
signal.
Because
of
this-:-t"he
output of a phase locked
exciter
exhibits
long term phase coherence with
the reference
oscillator
without the frequency
drift
associated with
fre-
quency
locked loop
exciters.
·
Circuitry
is
included
that
senses lock
and
provides a signal
that
cuts
off
the control
amplifier
thereby
shutting
off
the
RF
output
until
lock
is
es-
tablished.
2.
A3-IPA
and
PA
Assembly.
This board contains a
two
stage
amplifier
which
raises
the
100
mw
output
of
the
A2
assembly
to
a level
of
20
watts~ -
The
transistors
are
of
the
emitter
ballasted
type
and
this
in conjunction with the
circuit
and
power
supply design allow operation
into
any
magnitude
or phase
of
VSWR
without
damage.
However,
good
operating
practice
dictates
that
this
VSWR
protection
not
be
abused.
Do
not knowingly
operate the Exciter without a proper
RF
load.
-11-