AN-263.PDF

Sine Wave Generation

Techniques

National Semiconductor

Application Note 263

March 1981

Producing and manipulating the sine wave function is a

common problem encountered by circuit designers. Sine

wave circuits pose a significant design challenge because

they represent a constantly controlled linear oscillator. Sine

wave circuitry is required in a number of diverse areas, in-

cluding audio testing, calibration equipment, transducer

drives, power conditioning and automatic test equipment

(ATE). Control of frequency, amplitude or distortion level is

often required and all three parameters must be simulta-

neously controlled in many applications.

A number of techniques utilizing both analog and digital ap-

proaches are available for a variety of applications. Each

individual circuit approach has inherent strengths and weak-

nesses which must be matched against any given applica-

tion (see table).

PHASE SHIFT OSCILLATOR

A simple inexpensive amplitude stabilized phase shift sine

wave oscillator which requires one IC package, three tran-

sistors and runs off a single supply appears in Figure1. Q2,

in combination with the RC network comprises a phase

shift configuration and oscillates at about 12 kHz. The re-

maining circuitry provides amplitude stability. The high im-

pedance output at Q2's collector is fed to the input of the

LM386 via the 10 m F-1M series network. The 1M resistor in

combination with the internal 50 k X unit in the LM386 di-

vides Q2's output by 20. This is necessary because the

LM386 has a fixed gain of 20. In this manner the amplifier

functions as a unity gain current buffer which will drive an

8 X load. The positive peaks at the amplifier output are recti-

fied and stored in the 5 m F capacitor. This potential is fed to

the base of Q3. Q3's collector current will vary with the dif-

ference between its base and emitter voltages. Since the

emitter voltage is fixed by the LM313 1.2V reference, Q3

performs a comparison function and its collector current

modulates Q1's base voltage. Q1, an emitter follower, pro-

vides servo controlled drive to the Q2 oscillator. If the emit-

ter of Q2 is opened up and driven by a control voltage, the

amplitude of the circuit output may be varied. The LM386

output will drive 5V (1.75 Vrms) peak-to-peak into 8 X with

about 2% distortion. A g 3V power supply variation causes

less than g 0.1 dB amplitude shift at the output.

TL/H/7483±1

FIGURE 1. Phase-shift sine wave oscillators combine simplicity with versatility.

This 12 kHz design can deliver 5 Vp-p to the 8 X load with about 2% distortion.

C 1995 National Semiconductor Corporation

TL/H/7483

RRD-B30M115/Printed in U. S. A.

Sine-Wave-Generation Techniques

Typical

Amplitude

Type

Frequency

Distortion

Comments

Stability

(%)

Range

(%)

Phase Shift

10 Hz±1 MHz

1±3

3 (Tighter

Simple, inexpensive technique. Easily amplitude servo

with Servo

controlled. Resistively tunable over 2:1 range with

Control)

little trouble. Good choice for cost-sensitive, moderate-

performance applications. Quick starting and settling.

Wein Bridge

1 Hz±1 MHz

0.01

Extremely low distortion. Excellent for high-grade

instrumentation and audio applications. Relatively

difficult to tuneÐrequires dual variable resistor with

good tracking. Take considerable time to settle after

a step change in frequency or amplitude.

1 kHz±10 MHz

1±3

Difficult to tune over wide ranges. Higher Q than RC

Negative

types. Quick starting and easy to operate in high

Resistance

frequency ranges.

Tuning Fork

60 Hz±3 kHz

0.25

0.01

Frequency-stable over wide ranges of temperature and

supply voltage. Relatively unaffected by severe shock

or vibration. Basically untunable.

Crystal

30 kHz±200 MHz

0.1

Highest frequency stability. Only slight (ppm) tuning

possible. Fragile.

Triangle-

k 1 Hz±500 kHz

1±2

Wide tuning range possible with quick settling to new

Driven Break-

frequency or amplitude.

Point Shaper

Triangle-

k 1 Hz±500 kHz

0.3

0.25

Wide tuning range possible with quick settling to new

Driven

frequency or amplitude. Triangle and square wave also

Logarithmic

available. Excellent choice for general-purpose

Shaper

requirements needing frequency-sweep capability with

low-distortion output.

DAC-Driven

k 1 Hz±500 kHz

0.3

0.25

Similar to above but DAC-generated triangle wave

Logarithmic

generally easier to amplitude-stabilize or vary. Also,

Shaper

DAC can be addressed by counters synchronized to a

master system clock.

ROM-Driven

1 Hz±20 MHz

0.1

0.01

Powerful digital technique that yields fast amplitude

DAC

and frequency slewing with little dynamic error. Chief

detriments are requirements for high-speed clock (e.g.,

8-bit DAC requires a clock that is 256 c output sine

wave frequency) and DAC glitching and settling, which

will introduce significant distortion as output

frequency increases.

LOW DISTORTION OSCILLATION

In many applications the distortion levels of a phase shift

oscillator are unacceptable. Very low distortion levels are

provided by Wein bridge techniques. In a Wein bridge stable

oscillation can only occur if the loop gain is maintained at

unity at the oscillation frequency. In Figure 2a this is

achieved by using the positive temperature coefficient of a

small lamp to regulate gain as the output attempts to vary.

This is a classic technique and has been used by numerous

circuit designers* to achieve low distortion. The smooth lim-

iting action of the positive temperature coefficient bulb in

combination with the near ideal characteristics of the Wein

network allow very high performance. The photo of Figure3

shows the output of the circuit of Figure2a. The upper trace

is the oscillator output. The middle trace is the downward

slope of the waveform shown greatly expanded. The slight

aberration is due to crossover distortion in the FET-input

LF155. This crossover distortion is almost totally responsi-

ble for the sum of the measured 0.01% distortion in this

*Including William Hewlett and David Packard who built a few of these type circuits in a Palo Alto garage about forty years ago.

oscillator. The output of the distortion analyzer is shown in

the bottom trace. In the circuit of Figure2b, an electronic

equivalent of the light bulb is used to control loop gain. The

zener diode determines the output amplitude and the loop

time constant is set by the 1M-2.2 m F combination.

The 2N3819 FET, biased by the voltage across the 2.2 m F

capacitor, is used to control the AC loop gain by shunting

the feedback path. This circuit is more complex than Figure

2a but offers a way to control the loop time constant while

maintaining distortion performance almost as good as in

Figure2a.

HIGH VOLTAGE AC CALIBRATOR

Another dimension in sine wave oscillator design is stable

control of amplitude. In this circuit, not only is the amplitude

stabilized by servo control but voltage gain is included within

the servo loop.

A 100 Vrms output stabilized to 0.025% is achieved by the

circuit of Figure4. Although complex in appearance this cir-

cuit requires just 3 IC packages. Here, a transformer is used

to provide voltage gain within a tightly controlled servo

loop. The LM3900 Norton amplifiers comprise a 1 kHz am-

plitude controllable oscillator. The LH0002 buffer provides

low impedance drive to the LS-52 audio transformer. A volt-

age gain of 100 is achieved by driving the secondary of the

transformer and taking the output from the primary. A cur-

rent-sensitive negative absolute value amplifier composed

of two amplifiers of an LF347 quad generates a negative

rectified feedback signal. This is compared to the LM329

DC reference at the third LF347 which amplifies the differ-

ence at a gain of 100. The 10 m F feedback capacitor is used

to set the frequency response of the loop. The output of this

amplifier controls the amplitude of the LM3900 oscillator

thereby closing the loop. As shown the circuit oscillates at 1

kHz with under 0.1% distortion for a 100 Vrms (285 Vp-p)

output. If the summing resistors from the LM329 are re-

placed with a potentiometer the loop is stable for output

settings ranging from 3 Vrms to 190 Vrms (542 Vp-p!) with

no change in frequency. If the DAC1280 D/A converter

shown in dashed lines replaces the LM329 reference, the

AC output voltage can be controlled by the digital code input

with 3 digit calibrated accuracy.

TL/H/7483±2

TL/H/7483±3

(b)

FIGURE 2. A basic Wein bridge design (a) employs a lamp's positive temperature coefficient

to achieve amplitude stability. A more complex version (b) provides

the same feature with the additional advantage of loop time-constant control.

(a)

Trace Vertical Horizontal

Top 10V/DIV 10 ms/DIV

Middle 1V/DIV 500 ns/DIV

Bottom 0.5V/DIV 500 ns/DIV

TL/H/7483±4

FIGURE 3. Low-distortion output (top trace) is a Wein bridge oscillator feature. The very

low crossover distortion level (middle) results from the LF155's output stage. A distortion

analyzer's output signal (bottom) indicates this design's 0.01% distortion level.

A1±A3 e (/4

LM3900

A4 e LH0002

A5±A7 e (/4

LF347

T1 e UTC LS-52

All diodes e 1N914

* e low-TC, metal-film types

TL/H/7483±5

FIGURE 4. Generate high-voltage sine waves using IC-based circuits by driving a transformer in a step-up mode. You

can realize digital amplitude control by replacing the LM329 voltage reference with the DAC1287.

matched pair accomplish a voltage-to-current conversion

that decreases Q3's base current when its collector voltage

rises. This negative resistance characteristic permits oscilla-

tion. The frequency of operation is determined by the LC in

the Q3-Q5 collector line. The LF353 FET amplifier provides

gain and buffering. Power supply dependence is eliminated

by the zener diode and the LF353 unity gain follower. This

circuit starts quickly and distortion is inside 1.5%.

NEGATIVE RESISTANCE OSCILLATOR

All of the preceding circuits rely on RC time constants to

achieve resonance. LC combinations can also be used and

offer good frequency stability, high Q and fast starting.

In Figure 5 a negative resistance configuration is used to

generate the sine wave. The Q1-Q2 pair provides a 15 m A

current source. Q2's collector current sets Q3's peak collec-

tor current. The 300 k X resistor and the Q4-Q5 LM394

TL/H/7483±6

FIGURE 5. LC sine wave sources offer high stability and reasonable distortion levels. Transistors Q1 through Q5

implement a negative-resistance amplifier. The LM329, LF353 combination eliminates power-supply dependence.

RESONANT ELEMENT OSCILLATORÐTUNING FORK

All of the above oscillators rely on combinations of passive

components to achieve resonance at the oscillation fre-

quency. Some circuits utilize inherently resonant elements

to achieve very high frequency stability. In Figure6a tuning

fork is used in a feedback loop to achieve a stable 1 kHz

output. Tuning fork oscillators will generate stable low fre-

quency sine outputs under high mechanical shock condi-

tions which would fracture a quartz crystal.

Because of their excellent frequency stability, small size and

low power requirements, they have been used in airborne

applications, remote instrumentation and even watches.

The low frequencies achievable with tuning forks are not

available from crystals. In Figure6, a 1 kHz fork is used in a

feedback configuration with Q2, one transistor of an

LM3045 array. Q1 provides zener drive to the oscillator cir-

cuit. The need for amplitude stabilization is eliminated by

allowing the oscillator to go into limit. This is a conventional

technique in fork oscillator design. Q3 and Q4 provide edge

speed-up and a 5V output for TTL compatibility. Emitter fol-

lower Q5 is used to drive an LC filter which provides a sine

wave output. Figure 6a, trace A shows the square wave

output while trace B depicts the sine wave output. The 0.7%

distortion in the sine wave output is shown in trace C, which

is the output of a distortion analyzer.

Q1±Q5 e LM3045 array

Y1 e 1 kHz tuning fork,

Fork Standards Inc.

All capacitors in m F

TL/H/7483±7

FIGURE 6. Tuning fork based oscillators don't inherently produce sinusoidal outputs. But when you do use

them for this purpose, you achieve maximum stability when the oscillator stage (Q1, Q2) limits.

Q3 and Q4 provide a TTL compatible signal, which Q5 then converts to a sine wave.

Trace Vertical Horizontal

Top 5V/DIV

Middle 50V/DIV 500 m s/DIV

Bottom 0.2V/DIV

TL/H/7483±8

FIGURE 6a. Various output levels are provided by the tuning fork oscillator shown inFigure6.

This design easily produces a TTL compatible signal (top trace) because the oscillator is allowed

to limit. Low-pass filtering this square wave generates a sine wave (middle). The oscillator's

0.7% distortion level is indicated (bottom) by an analyzer's output.

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