ICL8038
DUTY
CYCLE
FREQUENCY
ADJUST
7 4
5
6
3
SINE WAVE
OUT
9
ICL8038
2
V
2
+
R
1
FM BIAS
SQUARE
WAVE
OUT
TRIANGLE
OUT
V
1
+
INPUT
VCO
IN
PHASE
DETECTOR
AMPLIFIER
DEMODULATED
FM
R
2
8
10
11
12
1
SINE WAVE
ADJ.
LOW PASS
FILTER
TIMING
CAP.
SINE WAVE
ADJ.
V-/GND
FIGURE 9. WAVEFORM GENERATOR USED AS STABLE VCO IN A PHASE-LOCKED LOOP
HIGH FREQUENCY
SYMMETRY
1N753A
(6.2V)
1kΩ
1,000pF
4
+15V
5
6
9
500Ω
4.7kΩ
10kΩ
4.7kΩ
1MΩ
100kΩ
LOW FREQUENCY
SYMMETRY
SINE WAVE
OUTPUT
100kΩ
-
741
+
-V
IN
P
4
1kΩ
8
ICL8038
FUNCTION GENERATOR
3
+15V
-
+
741
+
10
10kΩ
OFFSET
3,900pF
11
12 2
50µF
100kΩ 15V
SINE WAVE
DISTORTION
-15V
FIGURE 10. LINEAR VOLTAGE CONTROLLED OSCILLATOR
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 10.
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, HA2800, HA2820).
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 genera-
tor 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
is to provide a voltage divider to V+ (R
1
, R
2
as shown) if the
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 fre-
quency 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 Harris Application Note AN013,
“Everything You Always Wanted to Know About the
ICL8038”.
8-160
HARRIS [ HARRIS CORPORATION ]
