Sequential Pro One Technical Manual page 83

the bias current for op amp A 2
usually dictates a higher value
for the reference current. Al-
though this bias current has
been temperature compensated,
i t could have
a
worst cast
tempco of 1000ppm and maxi-
mum value of 400nA.
Under
these
conditions, a reference
current of 1
OpA through
Q 1 ,
for instance, would have a
tempco of 40ppm.
I t
Is
recom-
mended that, in general,
the
reference current be selected in
the
3flA
TO
75pA
range.
Since the reference current
pin is a virtual ground summing
node, the reference current may
be set up with a temperature
stable resistor t o VCC, or other
positive stable voltage source.
A
negative current into this pin
will simply gate the exponen-
tial generator cornpZetely
aff.
With the value of CF and
reference current now selected,
the voltage excursion a t the
multiplier output, which drives
the base of Q 1 , is now detea-
mined for the desired frequency
control range. If this range were
1Hz
to 20kW2, the exponential
generator current, IEG, would
have to range from 10nA to
200pA in the above
example.
requiring the base drive voltage,
VB, t o vary from +180mV t o
-78rnV,
since
The most positive voltage
the base of
Ql
occurs when the
~ 0 n t r 0 l current, IC. i s zero, and
i s
22vT
RSJRT. Therefore, ~n
the above example, R T
=
2 2 V ~
1.8KJ.18V
=
5.72K,
and
R z
=
3.0RT/22VT
=
3OK nominal.
Finally, since
the
multiplier
output current must range from
4100pA to -43yA to produce
this desired voltage excursion at
the multiplier output and on the
base of
Q 1 , the control input
current, IC, ranges from
0
to
143pA. A resistor from Vcc t o
the controt input pin may be
used to set the oscillator f re-
quency a t some initial value
with no control voltages applied.
The frequency control scale
i s determined by the value of
the input resistor to the control
pin, the value of the
Q1
base
resistor,
RS,
and the multiplier
current gain. Since the multiplier
current gain, set by the ratio of
the pin 2 current to pin
1
cur-
rent, should b e near unity and
R S
should be 1.8K, the control
input resistor is the co,nponent
which should
be
selected for the
desired contro! scale. For the
industry standard scale of 1
octavelvolt, the input summing
resistors become 100k. The
recommended method for
trimming the control scale is
-to tweek the multiplier current
gain by adjusting the valuenf
R z
k20% about the nominal
value.
Both the multiplier and the
exponential generator are com-
pensated with
the 470R
-
.01pF
networks shown in the Block
Diagrams and are therefore
necessary In any application.
Since the bandwidth of the
mu Itiplier extends beyond the
audio range,
it
may be desirable
to limit the bandwidth to re-
duce possible noise at the base
of
QT,
thereby reducing FM
noise and frequency jitter. Thrs
1
POSITIVE H A R M SYNC
H C C A l f V E H A R O SYNC
\
* w
+5v
- I -
CI
=
z
k
5
- 4 =
ov
POSITIUE A H 0 MEGATIUE HnRO S Y I C
I
t l 0 V
+
+
m
2
Q
-
+
-
? $
z
*
ov
is best accomplished by bypass-
ing R S to ground with a capac-
itor, where the corner rolloff
frequency is given by: fLp
=
1/(2nRSC).
Trimming The Scale
Error
+ l l U -
7
-
-
There are two basic sources
producing exponential con-
formity error in the control
scale: One is the exponential
current generator and the
: :
5
E
F I G U R E 1 OUTPUT WAVEFORMS
a
i
2 ;
0v
-
other is the precision multiplier.
The
error from the exponen-
tial converter i s due partly to the
buk emitter resistance of Q2,
becoming sign~ficant at gener-
ator currents greater than 1 OQpA,
and partly to the compara-r
switching delay, becom~ng sig-
nificant
a t
frequencies greater
than 5KHz. These two effects
cause the oscillator frequency
toga f l a t , but only at the
uppermost octaves.
Circuitry has been provided
to correct for these effects. The
output of the hi-frequency
track pin (pin 7 on the 3340,
pin 8 on the 3345) i s a current
which i s one fourth the
gen-
erator output
current,
I
EG.
This current may
be
converted
with
a
grounded resistor t o
a
voltage,
a portion of which is
k
I
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