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(1)
Basic Setup Considerations. The diodes used in the construction of
the VDFG are not ideal slvitches.
The transition from the non-conducting to the
conducting state is not abrupt, but gradual.
As a result, instead of a sharp
"corner" in the DFG output graph, a rounded corner will result.
Since the straight-
line segment curve is usually only an approximation to a desired smooth curve this
"rounding" effect improves the accuracy of the DFG.
However, it complicates the
setup procedure for two reasons:
(~
It takes a variation of about one-fifth of a volt in the input
voltage to switch the diode off or on.
Hence the setting of the
breakpoint is complicated by the fact that the breakpoint is not
clearly defined.
(!0
If the breakpoints are close together, adjusting the value at
either one of them will affect the value at the other.
This
interaction necessitates setting the function values by succes-
sive approximation - first set the value at breakpoint 1, then
set the value at breakpoint 2, then return to breakpoint 1, and
touch it up, then return to 2, etc.
This process can be some-
what lengthy.
To avoid these difficulties, the following rules are adopted:
(,£)
The breakpoint is defined as the input voltage that produces 1.0
microamp of current through the diode.
This is approximately on
the "knee" of the diode cha,racteristic curve.
With a feedback
resistor of lOOK, this current contributes 0.10 volt to the
DFG output.
This is relatively easy to measure.
(~
To minimize interaction between the function values at different
breakpoints, adjacent breakpoints should never be placed closer
together than one-fifth of a volt.
This is not a serious re-
striction.
Since a change of about 0.2 volt in the input is re-
quired to make a diode change state, two breakpoints closer to-
gether than 0.2 volt will produce in effect only one breakpoint,
and the interaction described above will make setup difficult.
If the spacing is about 0.2 to 0.25 volt, the interaction will
be less severe, and the portion of the graph between the break-
points will be a smooth curve rather than a straight-line.
(2)
Additional Considerations
(~
Pot Loading.
The DFG input should always corne from an amplifier,
not from a pot.
The load on any pot should always be a fixed re-
sistor from the pot output terminal to ground.
If a pot is fed
into a DFG, the load on the pot will depend on the number of
diodes conducting, and a loading error will result.
(.1?)
Insufficient Slope.
The maximum slope of 1 volt per volt per
segment is not enough for all functions to be generated.
If
more slope is needed, the DFG feedback resistor can be increased,
or a pot may be placed in the feedback path.
With this arrange-
ment, the maximum obtainable slope per segment is 1 volt per
volt divided by the pot-setting (e.g., a
pot-sett~ng
of 1/2 gives
37
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