Page 4
The resistor Rz from Pin 1 to the negative supply sets up an internal reference current for the
exponential voltage-to-current converter. Its value and that of timing capacitor Ct determine the
nominal initial frequency of the VCO at zero CV applied. The equation is:
Fout = (Vref)/(Vcc x Rz x Ct)
where Vref is the voltage across pins 1 and 3, nominally 1.3 volts. The other consideration is the
current range of the V to I converter, which is optimized for a range of 300nA to 80uA. Using 80uA
as an upper limit, the timing capacitor is chosen by:
Ct = (80uA)/(Fmax1 x Vcc)
where Fmax1 is the maximum frequency at the best accuracy (80uA).
In a typical application, suppose the best accuracy is in the range of 32Hz to 8KHz. Thus Fmax1 is
8KHz, and Ct is calculated to be 2nF. The middle of this range is 500Hz which is set to Fout.
Therefore Rt calculates to 260K. Note that since the VCO input range is -4 to +4 volts with a scale
factor of 0.75 V/Octave, the VCO will sweep from 12Hz to 20KHz.
A -5V negative supply can be used if the sweep range is reduced. In this case, Ct = 4nF and
Rt = 65K.
Since the VCO was designed to be software adjusted, a simple multiplier was used with slight non-
linearity at its two extremes. Therefore, for best results, it is recommended that the scale factor
and scale linearity be auto-corrected through software means.
WAVEFORM SELECTION
The pulse width may be turned off simply by setting the Pulse Width control voltage slightly
negative to ensure a pulse width of 0%. A unique circuit on-chip keeps the average DC level of the
pulse waveform constant regardless of duty cycle, so that pulse width may be modulated without
annoying control signal feedthrough.
The relative amplitudes of the three waveforms have been set as follows to give approximately
equal loudness: The triangle is 27% larger than the sawtooth, which is about 27% larger than the
pulse wave.
EXTERNAL INPUT
The external input may accommodate any signal up to about 80mV pp; normally, a resister divider
is required to attenuated the input signal to this level. The resistance to ground should be 1k to
keep the VCA balanced, and the input signal should be capacitively coupled to minimize control
voltage feedthrough. A small adjustable offset of +-20mV may be applied to this input to further
reduce CV feedthrough.
The internal signal and external signal are nominally in equal balance- each being 6dB down from
its maximum, when the control voltage is at zero volts. The control scale is roughly audio taper:
the first 20dB of range is linear while the remaining 60dB is exponential. This external input is
ideal for adding noise, a second VCO, or both.
