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(7)
Trimming Adjustments.
Frequently a slight shift may be noted in the
function values that were first setup.
This is due to the interaction mentioned
above - setting the value at one point has a slight effect on the previously set
values.
This effect will be largest where the breakpoints are closest together,
or where the curve is steepest.
It is, for this reason, necessary to perform a trinnning adjustment of the function
values.
The best way to do this is to start at
Xo
and proceed out to
XII'
as
above.
Plot out the entire curve again.
In extreme cases, a second trimming
adjustment may be necessary, but this is seldom the case.
The best way to deter-
mine whether additional trimming is necessary is to plot the entire curve and
compare it with the desired one.
d.
Paralleling Fixed Breakpoint VDFG's
In some cases, it may be desirable to generate a function over an input range of
-10
<
X
<
+10.
A Plus VDFG and a Minus VDFG can be paralleled and the desired
function-approximated by 19 straight-line segments.
Certain points must be noted
when VDFG's are paralleled.
"The PARALLAX potentiometer of each generator can ad-
just the output at f(O).
Therefore, the two PARALLAX controls are interdependent.
Note also that there is no breakpoint at X
=
O.
The first breakpoint occurs at
X
=
+1 in the Plus VDFG and at X
=
-1 in the Minus VDFG.
The initial slope ad-
justment, labeled 1 volt, in each unit determines the slope of the output when no
diodes are conducting, that is, in the range -1
~
X
~
+1 for the paralleled units.
Therefore, the values of f(-l), f(O)2 and f(+l) must be collinear.
The segment
about -1
<
X
<
+1 is two volts long instead of the usual one volt.
The paralleled
function
gene~ators
can be set up easily by means of the following procedure.
(1)
Determine the values of f(-l) and f(+l) that give a good straight-
line fit :over the range
-I
~
X
~
+1.
(2)
Calculate £(0)
=
1/2 [£(-1) + £(+1)] •
(3)
Adjust either or both
PARALLAX
controls until £(0) measures the
calculated value.
(4)
Apply an input of -1 volt and adjust the -1 volt control on the Minus
VDFG until f(-l) is set correctly.
(5)
Apply an input of +1 volt and check f(+l).
No adjustment should be
necessary; however, if a small deviation from the desired value is found, adjust
the +1 volt control on the Plus VDFG to remove half the error.
The error should
now be divided equally between f(-l) and f(+l).----
46
(6)
(7)
Adjust f(+2), f(+3) ,
Adjust f(-2), f(-3),
...
,
f(+lO).
...
,
f(-lO) •
(8)
Recheck the entire function.
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