Orban 8182A Operating Manual page 8

A-2 Orban
Circuit
Description of
8182 Bargraph
LED
Bargraph
GAIN
REDUCTION
Meters
The
gain
REDUCTION meters each
consist
of
ten
comparators with
open
collector outputs.
The
ten
LEDs
in
the
bargraph
are
connected
in series,
and a current source forces
a con-
stant
current into the
LED
string.
As
the
magnitude of the input voltage to the
compara-
tors
decreases, they progressively shunt
to
the
—15V
rail
the current that
would
ordinarily
flow through the
LEDs,
turning
off
all
LEDs
closer to the
-15
V
rail.
The comparators
are
biased with equal voltage increments,
producing a meter whose
scale
is
linear.
Because
the
gain control voltage
is
linear
in
dB,
this
produces a dB-linear
meter
scale.
A
250Hz
"dither" signal
is
injected into the
reference voltage
string,
modulating
the turn-
on threshold of
each comparator and causing the
LEDs
to
appear
to
smoothly fade on and
off instead
of switching abruptly.
Component-Level
Description:
All four meters
are
electrically identical.
However,
the
LIMITING meter has a
different front-panel scale (0 to
5dB
instead
of
+
10
to
-
15dB) and
is
driven
from
a
differential
amplifier so
it
can
indicate the difference
between
the
slow
compo-
nent
and
the
peak-held
TOTAL component
of the
master gain
reduction.
We
will
describe
Meter
1
(the
TOTAL
meter).
Current source
Q
1
,
R7, R5, R6,
CR5
applies
a constant current to
the series-con-
nected diodes
in
LED
assembly
CR1.
A
string
of
ten
comparators IC1, IC2,
IC4-A, IC4-B, receives the
negative-going dB-linear gain reduction voltage.
A
voltage divider
string
biases the
comparators with
DC
reference voltages
from
-0.0746
to
-0.746VDC
in
0.0746V
steps.
In
meters
1,
2,
and 4 each
0.0746V
step
corresponds
to
2.5dB
additional
gain reduction.
In
meter
2,
each
0.0746V
step
corresponds
to
0.5dB
additional
gain reduction.
When
the gain reduction
is
less
than the
first
step
(2.5dB or
0.5dB,
depending on meter), the gain reduction
voltage
is
less than
0.0746V and
the
outputs of
all
comparators are active
negative, shunting current
from
the
series-connected
LED
string.
As
the gain
reduction increases, the
output of each comparator
in
turn
becomes
open collector
and
current
is
permitted
to
flow
in
the
LED
string until
it
encounters a
comparator
whose
output
is
still
active negative,
at
which
point the active
comparator shunts
the current
away
from the
rest
of
the
LEDs.
A250Hz
triangle-wave "dither" signal
is
injected into the
reference voltage string
through Rl. This
AC
signal
modulates
the
turn-on threshold
of each comparator,
causing
the
LEDs
to
appear
to
smoothly fade
on and off instead of switching
abruptly.