AS “АLFA RPAR”

Joint Stock Company ALFA

Riga, Latvia www.alfarzpp.lv; alfa@alfarzpp.lv

4

2019 v.1

AS3394E

AS3394E

APPLICATION HINTS

POWER SUPPLY

The AS3394E was designed to operate from +5V and -6.5V supplies. The non-standard negative supply was

necessary not to compromise the VCO frequency resolution, which ranges from -4 to +4 volts, in favor of a -5V

supply (this is because there needs to be 4 diode drops for the current mirrors). Any one of the readily

available 3-terminal regulators may be used to supply the -6.5V negative supply. Since the stability and jitter of

the VCO are directly affected by noise on the positive supply, a supply as stable and clean as possible should

be use (not the +5Vdigital supply) Maximum supply allowable across the device is 25 volts.

VCO

The control scale of the VCO is temperature compensated with an internal +3300ppm tempco generator and

multiplier. Thus, as chip temperature changes, the control voltage applied to the exponential generator

changes proportional to temperature, effectively canceling the -3300ppm tempco of the control scale.

The resistor Rt 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 Rt x Ct)

where Vref is the voltage across pins 1 and 3, nominally 1,2 volts. The other consideration is the current range

of the V to I converter, which is optimized for a range of 300nA to 80

µA. Using 80µA as an upper limit, the

timing capacitor is chosen by:

Ct = (80µA)/(Fmax1 x Vcc)

where Fmax1 is the maximum frequency at the best accuracy (80µA).

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

240K. 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.

Output Saw-tooth Out (pin 9) - saw-tooth signal with an internal VCO frequency span o

f ± 50 μA. This output

can be loaded on a 1 kΩ resistor and the saw-tooth signal can be fed directly to the Ext In1 or Ext In2 inputs,

as well as used for other purposes. The maximum signal voltage at this output should not exceed ± 200 mV.

Output of a Sub-Oscillator (pin 5) - rectangular pulses with the frequency of the internal oscillator VCO divided

by 2 (square wave one octave below the fundamental frequency). The output voltage swing is ± 2 V; this

output can be loaded onto a resistor with a resistance of

at least 10 kΩ. This signal can also be mixed with

other used signals.