Sine oscillator

A sine oscillator is an electronic circuit for generating a mostly undamped sine oscillation . The sine generator can be subsumed under the signal generators .

In the low frequency range , sine oscillators are used for. B. as measuring and testing devices. There you will also find the term sinus generator , which means devices or assemblies that work on the basis of sinus oscillators for generating sinusoidal signals. Sinus oscillators are also used in analog musical instruments.

In the high frequency range , sine oscillators are used for. B. in transmission systems to generate the carrier oscillation required for signal transmission and in superimposition receivers for frequency conversion in the mixer .

Mode of action

As with other oscillators , the functional principle is based on positive feedback . For this purpose, an amplifier stage or arrangement is connected in such a way that the output signal is fed back to the input of the same circuit via a frequency filter. So that an undamped electrical oscillation can occur, the feedback must be in the correct phase.

If this arrangement is to generate a sinusoidal signal itself without subsequent signal processing, the gain effective for the feedback must ideally be one. If it is smaller than one, there is no continuous oscillation. When excited by an electrical pulse, only a damped oscillation can develop. On the other hand, if the gain is greater than one, distortion occurs. The signal then deviates more or less from the sinusoidal shape and thus contains harmonics .

In practice, the gain is chosen to be slightly above one. This ensures that the vibration cannot stop as a result of external influences or changes in component properties as a result of aging or temperature influences. The more independent the amplification is made by means of a suitable circuit concept of influences such as changes in the supply voltage and the ambient temperature in particular and the more stable components are used, the closer the amplification to one and thus the output signal to the ideal sinusoidal shape without clinking . For circuits with discrete components ( tubes , transistors ), a control circuit was often used in the past to solve this problem, in that a DC control voltage regulated the stage gain through rectification and subsequent integration based on the output signal. Circuits were also used in which the inertia of a hot conductor or PTC thermistor ( incandescent lamp ) was used.

Types of sine wave oscillators

LC oscillator

With the LC oscillator , the signal is fed back via an electrical oscillating circuit which determines the frequency of the signal generated. The arrangements according to Meißner and Hartley are or were most commonly used for generating sinusoidal signals . In the case of oscillators for the low-frequency range, the required high number of turns in the coil or the high AL value of the coil core is disadvantageous. If the frequency is to be variable over a larger range, the required large electrical capacitance of the required variable capacitor comes up against design limits. In power electronics today, LC sine wave oscillators are preferably used in the form of the Royer oscillator. It produces a relatively undistorted sinusoidal oscillation with a more or less fixed oscillation frequency, which is mostly in the range of 20 ... 200 kHz. The sinusoidal shape enables low interference radiation and low switching losses in the power transistors. The main area of application is power supplies (inverters) for gas discharge lamps, but e.g. B. Induction ovens. The disadvantage is the electricity requirement.

Phase shifter oscillator

The phase shifter oscillator uses an amplifier stage that inverts the signal, i.e. one with tubes that is designed as a cathode base circuit and with transistors that is designed as an emitter circuit. The output signal is fed back to the input via a group of low-pass or high-pass filters used for phase shifting. In practice almost only RC circuits are used. Low-pass filters are preferred because they make it easier to achieve a good sinusoidal shape.

The gain just has to compensate for the voltage loss in the phase shifter chain. In transistor circuits, the small input resistance is no longer negligible. The phase shift oscillator oscillates at the frequency at which the phase shift is exactly 180 degrees, i.e. three times 60 degrees with three RC elements.

With the phase shifter oscillator, frequency variation is in principle possible using adjustable resistors (e.g. with a potentiometer ). However, it is disadvantageous that three resistance values must be made changeable at the same time. If only one or not all of the resistance values involved in the phase shift are changed, this not only affects the frequency but also the waveform of the signal.

Wienbrücken oscillator

Here there is a Vienna-Robinson bridge in the feedback path . With this there is exactly one frequency at which the phase shift is zero degrees. This is the oscillation frequency of the Wienbrücke oscillator. A non-inverting amplifier arrangement is therefore required, which at the same time not only has to bring about a current gain but also a voltage gain that compensates for the losses. In circuits with tubes or transistors, it must therefore be at least two-stage. The advantage of the Wien Bridge oscillator is that only two resistance values have to be made synchronously variable to vary the frequency. For example, the generated frequency can be made variable over a relatively wide range using a conventional double potentiometer.

All-pass oscillator

The mode of operation of the all-pass oscillator is comparable to that of the phase shifter oscillator. Instead of the phase shifter chain, however, there are at least two all-passes . In practice, exactly two all-passes are used, as a higher number would increase the effort without any advantages.

Since an all-pass theoretically has a gain of 1, the coupled stage should have a gain only slightly above 1. As with the Wien bridge oscillator, the frequency of the allpass oscillator can be varied with two synchronously variable resistors over relatively wide limits. For each generated frequency, each of the two all-pass filters has a phase shift of 90 degrees.

The signal obtained in this way is fed to a functional network consisting of diodes and resistors or a chain of differential amplifiers, which distort the triangular signal in such a way that a sinusoidal signal with low distortion factors is produced.

The advantage of such arrangements is that the frequency of the sinusoidal signal generated in this way can be made variable within wide limits with a single, frequency-determining RC element. This concept is particularly suitable for implementation in integrated circuits , in which the comparatively large amount of circuitry is hardly significant. The trigger / integrator combination can be designed without great additional effort so that not only can the original triangular signal be generated, but also a sawtooth signal and a square-wave signal with a possibly variable pulse width. This is how most of the function generators used for measurement purposes work . Such a working chip is e.g. B. the 8038.

Digital oscillators

The oscillation of the oscillator is mathematically simulated in digital circuits. It is possible to realize the system of equations of the 2nd order in the form of a virtual oscillating circuit as well as the direct generation of the sinusoidal oscillation using the method of direct digital synthesis , in which a counter is used to run through a table containing the stored sinusoidal values. In both cases, the digital values are output with a digital-to-analog converter and converted into an electrical voltage. With the help of a CORDIC algorithm implementation, the required sine values can also be calculated in real time in software, e.g. B. with an FPGA , in a DSP or a microcontroller.

Web links

Sine generators and the SC sine generator (the terms generator and oscillator fit here)

Individual evidence

↑ Patrick Schnabel: Oscillators. In: elektronik-kompendium.de. 2020, accessed on August 17, 2020 .
↑ Prof Krucker: Oscillators - LC oscillator. In: krucker.ch. 2016, accessed August 2020 .
^ Detlef Mietke: Vienna-Robinson-Oscillator. In: elektroniktutor.de. 2018, accessed August 17, 2020 .
↑ U. Tietze - Ch. Schenk, semiconductor circuit technology, 10th edition, 1993, Springer-Verlag, Berlin, Heidelberg, New York
↑ ICL 8038. uni-muenchen.de, accessed on August 17, 2020 .
↑ J. Schuhmacher: Digital sine function - Mikrocontroller.net. In: mikrocontroller.net. A. Schwarz, August 3, 2011, accessed August 17, 2020 .
↑ Gjlayde: AVR arithmetic / sine and cosine (CORDIC) - Mikrocontroller.net. In: mikrocontroller.net. A. Schwarz, July 6, 2009, accessed August 17, 2020 .
↑ various authors: AVR-CORDIC. In: Mikrocontroller.net. 2018, accessed August 17, 2020 .

[1] Patrick Schnabel: Oscillators. In: elektronik-kompendium.de. 2020, accessed on August 17, 2020 .

[2] Prof Krucker: Oscillators - LC oscillator. In: krucker.ch. 2016, accessed August 2020 .

[3] Detlef Mietke: Vienna-Robinson-Oscillator. In: elektroniktutor.de. 2018, accessed August 17, 2020 .

[4] U. Tietze - Ch. Schenk, semiconductor circuit technology, 10th edition, 1993, Springer-Verlag, Berlin, Heidelberg, New York

[5] ICL 8038. uni-muenchen.de, accessed on August 17, 2020 .

[6] J. Schuhmacher: Digital sine function - Mikrocontroller.net. In: mikrocontroller.net. A. Schwarz, August 3, 2011, accessed August 17, 2020 .

[7] Gjlayde: AVR arithmetic / sine and cosine (CORDIC) - Mikrocontroller.net. In: mikrocontroller.net. A. Schwarz, July 6, 2009, accessed August 17, 2020 .

[8] various authors: AVR-CORDIC. In: Mikrocontroller.net. 2018, accessed August 17, 2020 .