6

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Neither time nor frequency are dependent on supply voltage,

even though none of the voltages are regulated inside the

integrated circuit. This is due to the fact that both currents

and thresholds are direct, linear functions of the supply

voltage and thus their effects cancel.

Reducing Distortion

To minimize sine wave distortion the 82k

Ω resistor between

pins 11 and 12 is best made variable. With this arrangement

distortion of less than 1% is achievable. To reduce this even

further, two potentiometers can be connected as shown in

Figure 4; this configuration allows a typical reduction of sine

wave distortion close to 0.5%.

For any given output frequency, there is a wide range of RC

combinations that will work, however certain constraints are

placed upon the magnitude of the charging current for

optimum performance. At the low end, currents of less than

1

µA are undesirable because circuit leakages will contribute

significant errors at high temperatures. At higher currents

(I > 5mA), transistor betas and saturation voltages will

contribute increasingly larger errors. Optimum performance

will, therefore, be obtained with charging currents of 10

µAto

1mA. If pins 7 and 8 are shorted together, the magnitude of

The capacitor value should be chosen at the upper end of its

possible range.

Waveform Out Level Control and Power Supplies

The waveform generator can be operated either from a

single power supply (10V to 30V) or a dual power supply

(

±5V to ±15V). With a single power supply the average levels

of the triangle and sine wave are at exactly one-half of the

supply voltage, while the square wave alternates between

V+ and ground. A split power supply has the advantage that

all waveforms move symmetrically about ground.

The square wave output is not committed. A load resistor

can be connected to a different power supply, as long as the

applied voltage remains within the breakdown capability of

the waveform generator (30V). In this way, the square wave

output can be made TTL compatible (load resistor

connected to +5V) while the waveform generator itself is

powered from a much higher voltage.

Frequency Modulation and Sweeping

The frequency of the waveform generator is a direct function

of the DC voltage at Terminal 8 (measured from V+). By

altering this voltage, frequency modulation is performed. For

small deviations (e.g.

±10%) the modulating signal can be

applied directly to pin 8, merely providing DC decoupling

with a capacitor as shown in Figure 5A. An external resistor

between pins 7 and 8 is not necessary, but it can be used to

increase input impedance from about 8k

Ω (pins 7 and 8

connected together), to about (R + 8k

Ω).

For larger FM deviations or for frequency sweeping, the

modulating signal is applied between the positive supply

voltage and pin 8 (Figure 5B). In this way the entire bias for

the current sources is created by the modulating signal, and

a very large (e.g. 1000:1) sweep range is created (f = 0 at

supply voltage; in this configuration the charge current is no

longer a function of the supply voltage (yet the trigger

thresholds still are) and thus the frequency becomes

dependent on the supply voltage. The potential on Pin 8 may

ICL8038

456

9

2

12

11

10

8

7

C

100k

Ω

V- OR GND

3

V+

1k

Ω

1

10k

Ω

100k

Ω

10k

Ω

FIGURE 4. CONNECTION TO ACHIEVE MINIMUM SINE WAVE

DISTORTION

I

R

1

V+

V-

–

()

×

R

1

R

2

+

()

----------------------------------------

1

R

A

--------

×

0.22 V+

V-

–

()

R

A

------------------------------------

==

ICL8038