Sequential PROPHET-600 - Page 78

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The puli* output ii an open
NPN emitter. and therefore
requires a pull (town reuslor to
ground Of to any negative
voltage Any Dull (town voltage
between (pound and .5 volt
above the voltage on the negative
vuppiy pm will precisely deter-
mine the lowe' level o* the
pulse wave. For pull-down
voltages move negative than this,
the lower level will be nearly
the negative supply pm voltage.
The nominal upper level of the
pulse wave it given by: Vcc -
0.3V -1 3*C • l PlD lor l PLO >
0 .6mA, and V cc - 0.9V for
Ino*0.6mA. where l PlQ it the
pull down current. A maximum
value of 3mA fo* lp LD it recom
mended For thotc applications
which reau're a more stable. well
defined upper level, the cncuitt
shown m I igure 2 may be used
The pulse width of the pulte
output may be tet from 0 to
tOOS wi th a 0 to *5V external
voltage IV C c " *15V) applied
to the PtVM control input pm
(pm 5 on the 3340 and 33451
The fall time of the pulse wave
it slower than the rise time due
to finite comparator gain. It
may be speeded up eontiderahiy
by adding hysteresis at thown in
F igure 3 Care should be exer
cited m the layout to prevent
stray capacitive coupling be
tween the pulse output and
the PWM mput.at this can cause
comparator otciUat*on.
The square wave output Ipm
7) from the CEM 3345 aHo
requires a pull down retittor to
any negative supply greater
than -4 volts It provides an
Output swing from nominally
1 3 volts below the hard sync
reference voltage to a level
nominally the same as the hard
sync reference voltage. The
Block Diagram shows a con¬
venient wav o! generating a
full swing square wave from
this output. The current pulled
down from this output should
also be limited to a maximum
of 3mA
Frequency Synchronization
The oscillator frequency may be
hard synchronized in several
different ways. One way n to
couple positive pulses, negative
pulses, or both, into the hard
sync input pin (pm 6 on the
3340 and 3345) A positive
syne pulse will cause the triangle
wave to reverse directions only
during the using portion of the
triangle, while a negative sync
pulse will cause direction reversal
only during the falling portion
The resulting waveforms are
shown m Figure 1. and provide a
wider variety ol synchronized
sounds than possible through
conventionally synchronized
oscillators Simple capacitive
coupling as shown in the Block
Diagrams allows hard synchro*
zation on Both the rismg and
lading edoe of a roctanglc wave,
Fig^e 4 shows circuitry for
allowing only one O' the Other
of th# edges to synchronize the
oscillator The peak amplitude
of the pulses actually appearing
on the sync pin should be re
stricter) to 1 volt minimum and
Curtis t lecuomuiic Spec lies ICC Si assumes "O lesPOnsbiMv for uw ol »"v
circuitry described No circuil iicansei ai* implied CES iesei.es me right. at
in. ime w mom not-C# to chanqa said cucil'v Prime*! USD C 198“
6
3 volts mavimum tor best
operation.
Another method of hard
synchronizing the oscillator is
shown in Figure 5 Negative
Pulses only are coupled into th#
base of the **NP transistor, with
a peak amplitude of 8 to 10
volts for best results at V cc «
* 15V. Thu method will produce
the same waveforms generated
by the conventionally syn
chronized sawtooth oscillators
Finally, the oscillator may
be soft synchronized by negative
pulses applied to the threshold
voltage pm (pm 9 on the 3340
pin 10 on th* 33451. These
pulses cause the triangle upper
peak to reverse direction pre¬
maturely, causing ttae oscillation
period to be an integral multiple
of the pulse period. The peak
amplitude of these negative
pulses should be limited to 5
volts maximum and positive
pulses should Be avoided en
tirely. If this input is not used
for synchronization (Mrposes, it
is itcommended that it be by
passed with a 0 IpF capacitor
to ground to prevent synchros
zation or litter to none pulses
on the * u W*lv line.
Linear FM
The reference current input
pin may be used for linear
modulation ol the frequency.
The external input is summed
with tho refeieoce current
simply through a resistor ter¬
minating at this pm. For audio
FM, it •! recommended that a
coupling capacitor be used to
prevent frequency shift when
connecting to the external
source The value of the input
resistor should be selected so that
the maximum peak to peak input
signal produces a plus and minus
current equal to the reference
current.
G2S
curtu aKTncxnuaic aPKnnca
2900 Mauricia Ave.
Santa Clara. CA 95051
14081 247 8046