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Installation
T.
B
5111
1331A-A-1
1
Figure
2-2.
Ventilation
Tolerances
from the
BNC
shield to
ground
must be less than 1
kilohm. Inputs not in use should be shorted. This
can
be done with
a
BNC shorting plug
(HP
Part
No.
1250-0774).
2-19. AMPLIFIER INPUT MODIFICATIONS.
2-20.
Several modifications
can
be
made
to the
X-,
Y-,
and
Z-axis amplifier input
circuits.
These
circuits
can
be
connected
for
either
differential or single-
ended operation. The input termination resistance
can
be
changed.
The input
attenuation
ratio
can
be
changed.
The modifications
are
described in the
following paragraphs.
2-21.
Deflection
Amplifier
Inputs.
The
inputs
to the
x
-
and y-axis amplifiers are through floating
BNC
connectors,
with the shield and center
con-
ductor connected to the two inputs of
a
differential
amplifier. Figure
2-3
is a simplified schematic
showing the configuration of one
deflection
amplifier
input.
As shipped from the factory,
Ra
=
Rb
=
2.7n,
Re= 100 kn,
and
Rd
=
47n. Ca; Cb, Cc, and Cd
are
not installed
but pads
are
provided on the circuit
board
(see figure 2-4) for
customer
installation. The
inputs
can be
modified
to be
fully
differential
and
balanced,
both
in
impedance and sensitivity,
com-
pletely
single ended with shield grounded
(A
and
B on figure 2-4), or anything in between.
2-22. Single-ended
Operations.
When
coaxial
cable
is used, large
spurious currents
may be
gene-
2-2
Model 1331A
rated in the
cable shield through capacitive or
inductive
coupling and
none,
or
very little, in the
center conductor.
This
is especially
true
where
long
cables are
used
or
where
the cable is routed near
units drawing
or generating
large
amounts
of
current.
One method of reducing the interference
is to
ground
the
shield at
the input. Sometimes,
however,
a common
mode rejection capability is
needed to reject noise
common
to both the
shield
and
the
center conductor.
At
other
times,
ground
loops may develop when the shield
is
directly
grounded.
In these
cases, a
satisfactory
compromise
is
accomplished
by
substituting a
low value
of
resist-
ance
for Rd. This is the input
configuration (Rd= 47Q)
of
Model 1331A
as
shipped from the factory.
2-23.
Differential
Operation.
If
the deflection
input is driven by
a
balanced cable, it
is
usually
better to maintain balanced impedances at the input;
that is, Re= Rd. As shipped from the factory,
sub-
stituting a 100 kn resistor for Rd restores full differ-
ential
balance.
2-24. X.
and
Y-axis
Amplifier
Attenuation.
As shipped
from the
factory,
full
scale
deflection
(8
divisions
vertical
and
10 divisions horizontal) is
accomplished with 1 volt
input
to
X
INPUT
or
Y
INPUT. The
attenuation
ratio
can
be
changed
by
changing
the values
of
the
attenuator components
in
the
input
circuits.
Figure
2-3
is
a simplified schematic
showing the
input
circuit components and figure 2-4
shows their
physical locations. Table
2-1
lists
the
components
required
for four common attenuation
ratios.
Table
2-2 contains a
description
of
the
com-
ponents and
their Hewlett-Packard part
numbers.
NOTE
The 2. 7 ohm resistor shown in figure
2-3
must be removed when Ra
is
used.
2-25.
Resistor
values
for
attenuation
ratios not
listed in
table
2-1
can
be determined from the
fol-
lowing
formula:
Ra+
Re
Vfsd =
Re
Resistor
spacing
on main
assembly
A5 is
for
1
/iW,
1%,
metal film resistors. If
Ra
+
Re is less than
5
kilohms, the
maximum input
voltage
must be re-
duced because of resistor power dissi-
pation limitations.
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