8

Use in Phase Locked Loops

Its high frequency stability makes the ICL8038 an ideal

building block for a phase locked loop as shown in Figure 9.

In this application the remaining functional blocks, the phase

detector and the amplifier, can be formed by a number of

available ICs (e.g., MC4344, NE562).

In order to match these building blocks to each other, two

steps must be taken. First, two different supply voltages are

used and the square wave output is returned to the supply of

the phase detector. This assures that the VCO input voltage

will not exceed the capabilities of the phase detector. If a

smaller VCO signal is required, a simple resistive voltage

divider is connected between pin 9 of the waveform

generator and the VCO input of the phase detector.

Second, the DC output level of the amplifier must be made

compatible to the DC level required at the FM input of the

waveform generator (pin 8, 0.8V+). The simplest solution here

amplifier has a lower output level, or to ground if its level is

higher. The divider can be made part of the low-pass filter.

This application not only provides for a free-running

frequency with very low temperature drift, but is also has the

unique feature of producing a large reconstituted sinewave

signal with a frequency identical to that at the input.

For further information, see Intersil Application Note AN013,

“Everything You Always Wanted to Know About the ICL8038”.

SINE WAVE

ICL8038

45

6

3

1

12

11

10

8

7

V-/GND

2

DUTY

CYCLE

FREQUENCY

ADJUST

ADJ.

9

SINE WAVE

TRIANGLE

OUT

SINE WAVE

ADJ.

TIMING

CAP.

FM BIAS

SQUARE

WAVE

OUT

LOW PASS

FILTER

DEMODULATED

FM

AMPLIFIER

PHASE

DETECTOR

VCO

IN

INPUT

OUT

FIGURE 9. WAVEFORM GENERATOR USED AS STABLE VCO IN A PHASE-LOCKED LOOP

3,900pF

SINE WAVE

ICL8038

456

9

2

12

11

10

8

4.7k

Ω

3

4.7k

Ω

500

Ω

10k

Ω

1N753A

DISTORTION

FUNCTION GENERATOR

100k

Ω

+

50

µF

15V

1M

Ω

(6.2V)

HIGH FREQUENCY

SYMMETRY

100k

Ω

LOW FREQUENCY

SYMMETRY

100k

Ω

+

-

741

+15V

-15V

SINE WAVE

OUTPUT

+

-

741

+15V

1k

Ω

10k

Ω

OFFSET

1,000pF

1k

Ω

FIGURE 10. LINEAR VOLTAGE CONTROLLED OSCILLATOR

ICL8038