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5.3.1.2
Circuit Description
5.
3.
1.2.1
Refer
to
the
schematic shown on page
7-10.
When
relay
RLC
is
energized, the input
impedance
seen
at
SK5 (INPUT
A
)
is
50£2
,
given by resistors
R78
and
R79
in
parallel.
5. 3.
1.2. 2
When
energized,
RLA
gives
DC
coupling
of the input
signal.
With
RLA
deenergized, the
signal
is
AC
coupled
via
C65.
R165
limits
the current
surge
which
occurs
if
DC
coupling
is
selected while
C65
is
in
the
charged
state.
5. 3.
1.2.3
The xl/xlO
attenuator
is
formed by R82,
R83,
R87
and
RLF.
With
RLF
deenergized,
the
attenuator has a
series
element,
R82, and
a shunt
element
formed
by
R83 and R87
in
parallel.
The
attenuation
is
20
dB
(nominal).
With
RLF
energized
R82
is
short-circuited, giving
0
dB
attenuation.
5. 3.
1.2.
4
The
attenuator output
is
fed- to
the
high-frequency channel
buffer,
Q15
and Q17,
via
R160
and C73.
The
gate
of
Q15
is
protected
against excessive
negative
voltage
swings by D5.
The
gain
from
the
attenuator output
to
the
emitter of
Q17
is
approximately
0.94.
5.
3.1.
2.5
The
buffer of the
low frequency
channel,
IC34 and Q25,
receives
its
input
from
the potential divider
R87.
The
gain
from
R87
pin
1
to
the
emitter
of
Q25
is
approximately
0.94.
Any
offset
in
the
system can be
nulled
by
adjusting
R192,
5.
3.1.2.
When
RLE
is
deenergized (Channel
A
filter
not
selected),
the
signals
from
the
two
buffers are
combined
at
the
base
of
Q21
by
the
network
C79
and R107. These
components
act
as
a
low-pass
filter
to
the output of the
low frequency
buffer,
and
as
a
high-pass
filter
to the
output
of the high
frequency
buffer.
The
crossover
frequency
is
5
kHz.
5.3.1.2.7
The
signal at
Q21
emitter
is
fed
to
the
Schmitt
trigger
IC36a
via
the diode
bridge
formed
by D18, D19, D20, and D21.
This
protects the
input of
IC36a by
limiting
the signal
swing
to
approximately ±1V.
5. 3.
1.2.8
The
differential
output
of
IC36
forms
the input
to the
measuring
block.
The
hysteresis of
IC36,
and
therefore
the
channel
sensitivity,
can be
set
by
adjusting
R149.
5.
3.1.
2.
9
The
trigger level
is
set
by
the
DAC,
H2,
shown
on page
7-11
and
is
fed to
IC34-2
via
R202
and one
section of
R89.
Feedback, taken from
the
emitter
of
Q21
to
IC34-2
via
R89
pins
5
and
3,
makes R89
pin
3
a
virtual
ground
point,
and the gain
from
the
R136/R202
junction
to
the
emitter
of
Q21
is
-0.94.
A
1
VDC
level at
the
Channel
A
input
and a
IV
trigger
level,
therefore,
combine
to give
0V
at
Q21
t
s
emitter.
Thus, the
selected trigger point
on
the
input
signal
is
always brought
to
0V
at
Q21's emitter.
5.3.1.2.10
When
Channel
A's
low-pass
filter
is
selected,
RLE
is
energized.
This
opens
the
high-frequency channel
circuitry
and connects
C87
across the
low frequency
channel.
The low-frequency
channel bandwidth
is
then nominally
50
kHz.
5.3.1.2.11
The
circuit of
Channel B
is
similar to that
of
Channel
A, but
is
not
provided with a low-pass
filter.
Energizing
RLH
connects
the signal
applied
at the
Channel
A
input to
both channel
amplifiers.
5.3.1.2.12
The
relays are controlled by the
microprocessor.
The
voltage
levels
on
the control
lines
are latched
in
IC24
as
shown
on page
7-11.
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