Sequential PROPHET-600 - Page 76

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the b « current to« 00 amp A2
usually dictates a higher value
for the reference current Al
though thi* Dm current hat
brnn temperature compensated,
it could have a worst cast
tcmpco of lOOOppm and maxi¬
mum value of 400»A Under
these conditions. a reference
current ol 10pA through .
•or instance. would have a
temoco of 40ppm It it recom
mended that, in general, the
reference current De selected m
the 3*iA to 15pA range.
Since the reference current
Din it a virtual ground summing
node, the reference current may
be sot u» with a temperature
stable resistor to Vcc, o* other
positrve stahle voltage source.
A negative current into th.s pm
will simply gate the exponen
tial generator completely off.
With the value ol Cr and
reference current now selected,
the voltage excursion at the
multiplier output, which drives
the base o* Q ( , is now deter
mined for the desired freauenev
control range. If this range were
I Ha to 20kHa, the exponential
generator current. Ijg. would
have to range from lOoA to
200ttA m the above example,
requiring the base drive volte©?.
V a . to vary from »l80mV to
-78mV. since
•eg
■ he*
# -V e fVT
The most positive voltage al
the base of Q) occurs when the
control current, l c . n aero, and
•s22Vj Rs<Rt Theiefo»e, in
the above example. R j « 22V>
1 BK18V - 5.72K. and R z «
3 OR j/22 V T • 30K nommai
Finally, since the multiplier
output current must ranoe from
*10QuA to -43uA to produce
this desired voltage excursion at
the multiplier output and on the
base of 0 |, the control input
current. Ip. ranges from 0 to
143pA A resistor from V^ to
the control input pm may be
used to set the oscillator fre¬
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1 1
quency at some initial value
with no control voltages applied
The frequency control scale
is determined by the value of
the input r wit or to the control
pm. the value of the Oi base
resistor, R s . and the multiplier
current gam. Since the multiplier
current gam, set by the ratio ol
the pin 2 current to pin 1 cur
rent, should be neat unity and
R$ should be 1 8K. the control
input resistor is the component
which should be selected for the
desired control scale For the
industry standard scale of 1
octave 1 volt, the input summing
resistors become 100h. The
recommended method for
trimming the control scale is
to tweek the multiplier current
ga n by adiustmg the value of
R? »? 0 % about the nominal
value.
Both the multiple! and the
exponential generator are com
pensated with the 4707 - OlpF
networks shown in the Bloch
Diagrams and are therefore
necessary in any application.
Since the bandwidth of the
multiplier extends beyond the
audio range. ■ ! may be desirable
to limit the bandwidth to re¬
duce possible noise at the base
of 0|, the-eby reducing FM
none and frequency jitter. This
is best accomplished by bynass-
•ng R s to ground w-th a capac
■ tor. where the corner rolloff
frequency >s given by f L p •
l;i2rR s C)
Trimming The Scale
Error
There are two basic sources
producing exponential con¬
formity error in live control
scale On* is the exponential
Current generator and the
Other it the precision multiplier
The error from the exponen
tial converter is due partly to the
bufc emitter resistance of Qj,
becoming significant at goner
ator currents greater than IOQuA,
and partly to the comparator
switching delay, becoming sig¬
nificant at frequencies greater
than 5KHr These two effects
cause the oscillator frequency
to go flat, but only at the
uppermost octaves.
Circuitry has been provided
to correct for these effects The
output of the hi fiequency
track pm (pm 7 on the 3340.
pm 8 o" the 33451 it a current
which is one fourth the gen¬
erator output current Ijc
T his current may be converted
w«th a grounded res-stor to a
vottage. a portion of which is
4