Ramsey Electronics COM 3 User Manual

prescaler goes back into the synthesizer chip,
where itis compared to the 500 KHz reference.
5.3 5-7MHz Fine Loop PLL
The fine loop runs from 5 - 7 MHz in 500
Hz steps, it consist of frquency synthesizer IC U6
which is a MC5155. Similiar in operation to the
MC5156P1 that is found in the coarse loop, it
receives its input data through the same three
lines; clock , data, and enable. The clock and data
lines are ignored except when the enable input is
high. The data line tells the programmable dividers
within the MC51555 what to divide by. The data
input, clock, and enable come from the
microprocessor on the front panel board.
IC U23 which is a divide by two, takes the 16 KHz
reference which is available within the Com 3,
divides it by two to produce a 8 KHz reference
signal to U6. internal to U6 is a divide by 16
counter, which divides the 8 KHz signal to provide
the 500 Hz reference which is used inside the
MC5155. The output of the MC5155 is available
on pin 6, which is a three-state output. Being a
three-state output it can be high, low, or open
circuit, that output is smoothed out to a constant
DC voltage by U5 and its associated circuitry.
U5 also performs the function of a level shifter, in
that the VCO requires more than 0-5 volts to tune
it,
so
the
level
is
shifted
to
the
5
to
7
volts
required
by the VCO.
The VCO consists of transistor Q3, coil
L12
and
varactor
diodes
D19,
D20.
You'll
note
that
all
the
components
within
the
fine
loop
run
off
their
own
5
volt
regulator,
the
reason
for
that
is
to
keep
the
various
reference
frequencies
used
within
the
Com
3
which
may
be
present
on
the
power
supply
bus
from
modulating
the
oscillator.
The
VCO
oscillator
which
is
used
within
the
fine
loop
has
to
be
very
clean,
since
that
is
where
the
FM
modulation
takes
place.
The
loop
filter
used
in
the
fine
loop
is
designed
to
be
fairly
slow,
so
that
.
modulation
can
be
simply
added
to
the
control
voltage
which
goes
to
the
varactor
diodes.
Since:
the
loop
is
so
slow,
any
modulation
appearing
on
the
VCO's
output
frequency
is
not
taken
out
by
the
loop
.
If
the
loop
was
faster
it
would
in
effect,
vary
the
control
voltage
to.
take
the
modulation
back
out
of
the
loop.
However,
this
is
not
the
case.
You'll note that the FM modulation is applied
through the resistor R69, and modulation
sensitivity adjust control R70.
5.4 1024 MHz Offset PLL
The 1024 MHz oscillator consists of
transistor Q8 and associated components which
form a VCO that runs ata frequency of 1024 MHz '
with an output level of approximately 0 dBm. The
output of the oscillator drives mixer X4, through a
pad, and is also coupled to U10, which is a 1.25
GHz divide by 256 ECL prescaler. Since it is ECL,
the output level is only 600 mv p-p. Transistor Q10
is used to raise the 600 mv p-p to a TTL level of
approximately 0-4 vpp. The amplified output then
goes into one stage of ህ11, which is a
buffer/amplifier,
and
then
fed
into
the
phase
detector portion of U11.
ሀ11 is an exclusive OR circuit , this will
provide
a
high
output
whenever
the
two
inputs
are
- different from. each other. When the-two - inputs
are out of phase, the output goes high forcing the
oscillator
to
move
in
frequency.
The
other
input,
the phase comparator portion of U11, comes from
another
buffer
/amplifier
stage
of
U11
which
is
driven
by
the
4
MHz
crystal
standard.
The
output
of
U11
will
be
a
series
of
pulses
when
the
loop
is
in
lock.
These
pulses
are
filtered,
then
sent
on
to
the VCO as a dc voltage representing the error
voltage
needed
to
maintain
a
locked
loop
condition.
In
a
locked
loop
condition
the
average
VCO
output
would
be
1024
MHz.
Therefore
U-
11's
output
is
run
through
ឧ
lowpass
filter,
consisting
of
components
C98,
C99,
R157,
and
R158.
These
components
form
a
two
pole
filter
which
filters
the
pulses
coming
from
U11
into
a
constant
DC
voltage
to
control
a
varactor
diode,
CR28.
Since
the
1024
MHz
oscillator
uses
a
reference
of
4
MHz,
its
output
spectrum
is
very
clean.
A
clean
spectrum
is
absolutely
required
when
mixing
down
to
another
whole
band
of
frequencies. ; |
Notice that a separate five volt supply is
used to run all the components in the 1024 MHz
offset
loop.
This
is
done
for
two
reasons;
one
is
to
provide
a
clean
five
volt
signal
to
the
VCO
and
phase
detector
so
that
they
are
not
modulated
by
any
of
the
other
frequencies
found
on
the
power
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5-3

Ramsey Electronics COM 3 Specifications

General

Brand	Ramsey Electronics
Model	COM 3
Category	Monitor
Language	English