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three-phase

sine wave generator

027

A Burr-Brown application

The diagram shows how a three-phase sine wave oscillator can be

built with a single Type UAF42 state-variable filter and some resis-

tors and diodes. Three output nodes are available: high-pass out,

band-pass out, and low-pass out. The signal at the band-pass and

low-pass out nodes is 90

15V

R1

C1

1N914

10 µ

25V

out of phase, respectively, with

that at the high-pass node. An on-chip auxiliary op amp is avail-

able for use as a buffer or amplifier stage.

The frequency of oscillation is set with resistors R F 1 and R F 2

according to

°

and 180

°

Highpass Out

A sin(

Lowpass Out

ω

R2

t)

A sin(

t + 180 °

)

R

150k

C

EXT

R

150k

F2

C EXT

R

10M

C3

FB

Bandpass Out

t + 90

100n

f OSC =1/2

RC ,

A sin(

)

IC1

12

13

8

7

14

1 5

10

where

50k

and

R=R F 1 =R F 2

C1

C2

C=C 1 =C 2 =1000 pF

50k

1n

1n

6

2

The maximum frequency of oscillation obtainable with the UAF42

state-variable filter is 100 kHz. Distortion becomes a factor,

though, for frequencies above 10 kHz. For frequencies of oscilla-

tion below 100 Hz, the use of external capacitors is recommended.

These should be placed in parallel with the internal capacitors C 1

and C 2 . This will reduce the requisite values of R F 1 and R F 2 . The

external capacitors should preferably be NP0 ceramic or mica

types.

To obtain the requisite output levels, resistors R 1 –R 4 should

meet the following requirement:

50k

UAF42

50k

3

8

7

11

4

9

R4

C4

1N914

100n

R3

C2

10

25V

R 1 /R 2 =R 3 /R 4 =(V O +V S )/(V O –0.15)–1

994049 - 11

15V

The values indicated in the diagram apply to a frequency of 1 kHz.

Elektor Electronics

7-8/99

57

F1

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At this frequency,. the external capacitors may be omitted, since

the internal ones are sufficient.

The actual output level may differ slightly from the calculated

one owing to non-ideal operation of diodes and op amps. It may

therefore be necessary to adapt the ratios R 1 /R 2 and R 3 /R 4 to

some extent.

Positive feedback necessary for the onset of oscillation is pro-

vided by coupling the output of the band-pass section to the input

of the summing amplifier via resistor R FB . Suitable values for this

resistor are 10 MΩ

for f >1 kHz, 5 MΩ

for f =10–1000 Hz, and

for f <10 Hz. Smaller values result in an increase in the

output level and, consequently, distortion.

[994049]

750 k

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