Ramsey Electronics COM 3 - Page 27

5.19
DISPLAY
DRIVERS
The
display
drivers
are
40
pin
devices
(MM5451)
that
take
a
35
bit
serial
word
from
the
microprocessor
and
from
this
produce
a
constant
current
output.
The
microprocessor
loads
each
one
of
these
serially
and
at
the
end
of
the
36
th
data
pulse,
the
display
drivers
store
the
data
and
output
it
in
a
parallel
form
to
control
the
various
LEDs
of
the
display.
5.20
AUDIO
FREQUENCY
COUNTER
The
audio
frequency
counter
function
is
performed
within
the
microprocessor.
It
measures:
the
period
of
the
audio
signal,
then
averages
it
over
4
count
cycles
and
displays
the
result.
ህ
7
(acmos
quad
nand
gate)
suitably
amplifies
the
signal
and
low
pass
filters
it,
then
applies
it
to
U13
(a
dual
D
flip
flop),
which
divides
the
signal
by
two
to
provide
a
clean
waveform,
insensitive
to
edge
jitter.
This
Square
wave
then
goes
to
the
gating
circuitry
of
U
5,
then
to
the
microprocessor
through
pin
37.
The
microprocessor
internally
counts
the
signal
and
then
displays
it.
5.21
RF
FREQUENCY
COUNTER
The
RF
frequency
counter
also
draws
upon
the
.frequency
counting
capabilities
of
the
microprocessor,
but
has
two
external
prescalers.
The
first
prescaler
U
6
is
a
1
GHz
divide
by
256
(SP4550)
unit
.
After
passing
through
this
prescaler
the
signal
is
amplified
by
01,
applied
to
gate
U5,
and
then
continues
to
the
second
prescaler,
which
is
an
8
bit
device
(
74HC393).
The
output
of
those
8
bits
are
applied
to
one
of
the
ports
of
the
microprocessor
so
that
it
can
read
the
counter
and
also
look
at
the
count
of
its
internal
counter.
Between
the
two
readings
the
frequency
can
be
determined.
The
output
is
sent
in
serial
form
to
the
display
drivers
_
and
seven
segment
frequency
display.
5-8
5.22
CTS
TONE
GENERATION
The
CTS
tone
generator
uses
an
R-2R
ladder
network
on
the
output
of
three
of
the
microprocessor
ports.
The
microprosser
directs
a
variable
duty
cycle
wave
form
to
these
three
output
ports.
When
applied
to
the
R-2R
ladder
network,
a
stepped
approximation
of
a
sine
wave
appears
at
the
network's
output.
Active
filter
op-amp
U
10,
takes
the
digital
approximation
of
a
sine
wave,
low
pass
filters
it
and
produces
a
low
‘distortion
sine
wave
at
its
output.
That
output
is
then
available
for
modulation
within
the
Com-3
or
available
for
external
uses.
FREQUENCY
ERROR/DEVIATION
DISPLAY
5.23
The
frequency
Error
/
Deviation
Display
uses
a
20
segment
LED
bargraph.
Each
side,
plus
and
minus
is
relatively
the
same
and
uses
a
LM3914
LED
Bargraph
driver.
Internally,
the
bargraph
driver
is
nothing
more
than
10
comparators
which
have
a
resistor
divider
array
connected
to
them.
Increasing
the
voltage
that
is
applied
to
it,
increases
the
-
number
of
lit
LED
segmerits.
The
input
to
each
of
these
LED
bargraph
driver
ICs
comes
from
a
pair
of
ideal
diode
circuits
which
are
constructed
of
a
diode
and
op-amp.
An
ideal
diode
circuit
performs
as
an
ideal
diode
with
no
DC
forward
drop
at
all,
and
having
a
high
input
impedance.
Ideal
diodes
are
used
to
eliminate
the
DC
offset
problem
associated
with
regular
diodes.
Since
one
display.
has
to
indicate
in
the
negative
direction
and
one
"in
ihe
positive
direction,
signals
are
appropriately
routed
to
each
of
these
ideal
diodes
so
the
displays
read
in
the
proper
directions.
The
display
will
truly
indicate
positive
and
negative
excursions
of
the
deviation
signal
so
that
you
can
actually
read
the
amount
of
positive.
and
negative
deviation.
Another
section
of
the
op-amp
is
used
as
a
scaling
unit
which
scales
the
display"...
appropriately
for
1.5
or
7KHz
full
scale
indication.
The
gain
ot
this
op-amp
is
controlled
with
an
analog
Switch
and
a
pair
of
different
feedback
resistors.
,
This
portion
of
the
Com
3's
circuitry
also
has
some
'
blanking
circuitry
so
that
the
display
is
blanked
when
no
signal
is
received,
and
in
certain
modes
where
the
display
is
not
calibrated
or
required.