April 3, 2000 • ELECTRONIC DESIGN

153

ideas for design

INNOVATIVE DESIGNS FROM READERS

Ken Levine

Pacific Systems, 2838 67th Ave. SE, Mercer Island, WA 98040; [phone]; e-mail: [e-mail]

Low-Cost Function Generator

T

his low-cost function generator,
based on the Maxim MAX038
high-frequency waveform genera-

tor, produces sine, triangle, and square
waves from under 1 Hz to over 20 MHz
(see the figure). The frequency, ampli-
tude, and offset of all output wave-
forms are adjustable, as is the duty
cycle of the square wave.

Frequency adjustment is accom-

plished with a capacitor bank and a
variable resistor. Amplitude, offset, and
duty-cycle adjustments are performed
via variable resistors. Switches S2 and
S3 select the waveform type to be gener-
ated. The frequency range is chosen us-
ing one of 12 capacitors, C2-C13, via
switch S4. Resistor R16 is for fine-tun-

ing. The f

ADJ

pin of U1 is tied to ground

through a 12k resistor, so the frequency
of the waveform at U1-19 is f =
2(I

IN

/C

F

), where I

IN

= V

REF

/R, R is the

sum of R7 and R16, and V

REF

= 2.5 V.

The values of C2-C13, R16, and R7
were chosen to provide frequency-
range overlap. Table 1 shows the fre-
quency range for each capacitor value.

The output at U1-19 is 2 V p-p, cen-

tered around zero, for all waveforms.
For amplitude adjustment, R18 and R9
form a voltage divider. The portion of
U1-19 fed to the summing amplifier
U3 varies from 0.74% to 100%. The
summing amplifier multiplies this volt-
age by

−

12.2, so the signal at U3-6 will

vary from 0.18 V p-p to 24.4 V p-p.

CIRCLE 521

C

F

22 pF
82 pF

330 pF

0.001

m

F

0.0047

m

F

0.022

m

F

0.068

m

F

0.33

m

F

1

m

F

4.7

m

F

22

m

F

100

m

F

Frequency range

3.8 MHz – 22.7 MHz

1.16 MHz – 6.1 MHz

252 kHz – 1.5 MHz

83.4 kHz – 500 kHz
17.8 kHz – 106 kHz

3.8 kHz – 22.7 kHz

1.23 kHz – 7.4 kHz

252 Hz – 1.5 kHz

83.4 Hz – 500 Hz
17.8 Hz – 106 Hz

3.8 Hz – 22.7 Hz

0.8 Hz – 5 Hz

FREQUENCY RANGE

FOR SPECIFIC

CAPACITOR VALUES

The offset voltage is controlled by re-

sistor R15. This voltage is fed to the
summing amplifier, where it’s added
directly (the gain through the summing
amp is –1) to the waveform voltage. Re-
sistors R1 and R2 force the variation in
the offset voltage to be within –13.6 V

to +13.6 V to prevent the unity-gain
buffer amplifier from being overdriven.

Duty-cycle adjustment is controlled

by resistor R17. The +2.5-V reference
voltage of U1 is passed through a unity-
gain inverter to R5. R5, R17, and R6
(which is tied to the +2.5-V reference

IDEAS FOR DESIGN

154

ELECTRONIC DESIGN • April 3, 2000

voltage) form a voltage divider. The
voltage developed across R17 varies
from –2.3 V to +2.3 V. It first goes
through a unity-gain buffer, then to the
D

ADJ

pin of U1. This ensures that the

voltage at D

ADJ

falls within the operat-

ing limits of –2.3 V to +2.3 V.

+
-

+
-

+
-

+
-

S3

Triangle wave

C15

0.15

mF

U2B

LM324

C16

0.1

mF

Square or

triangle

wave

S2

Variable

duty cycle

R16 50k

R17

10k

+5 V

+5 V

+5 V

R10

1k

R5

390

R1

500

R15

10k

R2

500

R6

390

Sine wave

Square wave

R3

100k

+15 V

+15 V

+15 V

+15 V

3

2

11

4

11

4

5

6

7

11

4

1

10

9

8

8

4

1

7

3

2

6

U2A

LM324

–15 V

–15 V

–15 V

–15 V

R4

100k

R7 10k

+15 V

–15 V

R9

75

50%

duty

cycle

S1

R8

12k

U1

MAX038

R18

10k

S4

Amplitude adjust

1
2
3
4
5
6
7
8
9

10

20
19
18
17
16
15
14
13
12
11

REF
GND
A0
A1
COSC
GND
DADJ
FADJ
GND
IIN

V–

OUT

GND

V+

DV+

DGND

SYNC

PDI

PDO

GND

C6 0.0047

mF

C5 0.001

mF

C4 330 pF

C3 82 pF

C2 22 pF

Output

R14

1k

C18

0.1

mF

C17

0.1

mF

C 14

0.1

mF

C 1

0.1

mF

C19

1

mF

C20

1

mF

C13 100

mF

C12 22

mF

C11 4.7

mF

C10 1

mF

C9 0.33

mF

C8 0.068

mF

C7 0.022

mF

U3

OP37

R13 10k

R11

10k

R12

820

This MAX038-based function generator produces sine, triangle, and square waves from less than 1 Hz to over 20 MHz.