Phase-shift oscillator

A phase shifter is an electronic circuit which shifts the phase of an electric wave. The degree of this shift is expressed in degrees or as a part of the full circle (360 °). Depending on the frequency, there are various possibilities for the technical realization. At low frequencies, proportions of reactances are used for technical realization, which occur at inductances and capacitances. At high frequencies, time differences are used by phasing lines.

• 4.1 phase shift generator 4.1.1 phase condition
• 4.1.2 performance condition
• 4.1.3 ring oscillator

Species

In principle, the following groups of phase shifter can be distinguished:

• Frequency-dependent phase shift. The phase shift of a specific frequency is accomplished by a time shift of the input signal. However, since different frequencies have different long period of time, the phase shift varies depending on the frequency. This can be realized by this form corresponding delay units or in the form of all-pass networks.
• Neutral frequency phase shifter. These circuits move across a spectrum uniformly by a certain angle. The resulting signal is complex- as a rule. Those phase shifters are referred to as the Hilbert transformer, and to use the so-called Hilbert transformation. In this case, each Hilbert transformer causes rotation of the spectrum by 90 °. The Hilbert transform plays in the signal processing a central role, which is, among other things in the modulation technique used.

Low frequencies

Analog technology

In electronics, an RC network (or a network of RC elements ) is used for phase shifting mostly. Here, the offset in time for the voltage on the capacitor the current flow is utilized. The current lags the voltage by 90 ° (see the phase shift at the reactance ) and thus results in a series arrangement of resistor and capacitor, a phase shift of the two voltages applied to the components to each other.

The resistance R and the reactance jX of the capacitor must be added vectorially to determine the current. The degree of phase shift between current and voltage is reduced by the series circuit with a resistor to < 90 °. Because of the reactance of the capacitor is a function of frequency, the phase shift of the RC element is frequency-dependent.

By the use of potentiometers or electronically controllable resistor, the phase shift can be made ​​adjustable. In principle, can be used instead of a capacitor and a coil with a similar result, because of the higher costs but rarely used.

An example of the phase shift with a capacitor, the generation of the auxiliary phase in the induction motor in Steinmetz connection or. A hybrid of digital and analog phase shift is the CCD principle ( BBD ).

Digital phase shifter

Digital signals can be phase shifted by

• They are pushed by a FIFO memory (shift register)
• Delaying their flanks at consistent times (only possible if the shortest pulse duration > delay ) by timers ( monostable ) and a logic circuit
• By generating a triangular wave and subsequent comparators

The latter method is used, for example, in one type of switching power supply ( phase shifters ), for feeding through a transformer to a controllable pulse duration in their RMS symmetrical square-wave voltage.

High frequencies

At high frequencies, the phase shifts are achieved by rerouting lines ( delay lines ) with defined length on a line transmission. The cable lengths required can only be achieved by winding up at larger delay times (up to about 1 microseconds ). In still longer times ( for example, to line rate 180 ° phase shift of the image signal in television receivers, i.e., 64 microseconds ) can be used ultrasonic delay lines.

For coaxial lines or waveguides, the phase shift is possible up to delay times, where the required length nor a manageable size has (about 3 ... 5 ns per meter). An upper limit frequency is determined by the ratio of the wavelength to the thickness of the cable and the potential accuracy of the positioning of the line connections in the micrometer range. At frequencies up to 100 GHz, this method is easily possible.

The graph shows a phase shifter which can switch each phase angle between 0 ° and 315 ° in 45 ° increments with three bits. The switches shown in the graph can be realized in practice by PIN diodes that can switch high power in a few nanoseconds. The picture shows a phase shifter in 22.5 ° increments phase angle between 0 ° and 337.5 ° turns with control lines with four bits wide. The length of the phasing lines is frequency dependent and also dependent on the propagation speed of the waves in the medium, that is, in a cable, a reduction factor is effective. In a waveguide, the phase velocity is larger, so this is a reduction factor greater than one effect.

Applications

Phase shift generator

For low-frequency oscillators also feedback amplifiers are used which are provided with a phase shifter chain. For vibration generating two conditions must be complied with these circuits:

Phase condition

The circuit principle in the picture above is based on an inverting amplifier stage (which acts in the present here sinusoidal signals as a phase shift of 180 ° ) with a transistor and an RC network with phase- shifting effect in the feedback path. The frequency dependence of the RC elements is used here deliberately. Each RC element must move only by 60 ° and with three members can be reached at exactly one frequency the total phase shift of 180 °. By inverting amplifier this frequency is preferably amplified and satisfies the oscillation condition. The phase shifter chain may be constructed a tripartite ( 3.60 ° ), or four members (4.45 ° ) in order to compensate for the phase inversion (180 °) of the amplifier stage. If you have a OpAmp used instead of a transistor with its high input resistance and thus lower load, it even comes with two members for each 90 ° (theoretical maximum). The Last Stand of the phase shifter chain may be equal to the input resistance of the amplifier. If the frequency generated is to be made ​​adjustable, one or more resistances of these RC elements are executed as a potentiometer (possibly in tandem version).

Another circuit principle is the Wien bridge generator; In this working in the feedback branch of a non-inverting amplifier stage, a series connection of R and C on a parallel RC circuit. , The R and C values ​​, respectively equal to a frequency obtained in exactly the phase shift is zero, so that together with the non-inverting amplifier stage, the phase condition is satisfied. Such oscillators with a Wien bridge can be realized for example as a low-frequency sine-wave generator (laboratory unit). A variable capacitor is used to change the oscillation frequency.

Performance condition

Is critical in the design of this circuit, the gain of the amplifier stage. If it is too large, increases the amplitude produced, and the amplifier is driven in an area with a working point, in which the gain is again smaller (the upper limit range). Thus, although the performance condition is satisfied again, but in this boundary region is no longer generated sinusoid, but rather a rectangle -like oscillation having many harmonics.

If the gain is too small, the load ( this includes the resistance between the collector and the operating voltage) taken too much energy and the vibration set a non- independent. If externally excited in this case the generator, so it responds with damped oscillations. Phase shifter generators are therefore usually equipped with an amplitude control. In many of such generators, this consists of a control formed as a negative feedback thermistor (small bulb ), which with increasing amplitude is heated due to the higher effective current, thereby increasing its resistance and the counter- coupling.

The signal must be decoupled as possible high impedance to allow decay by possible overload the vibrations. Often an emitter follower oscillator is therefore downstream of decoupling.

Ring oscillator

An example of a Phasenschieberozillator the ring oscillator. The frequency is determined by the phase shifts of the different transistor stages. Application is to determine the cut-off frequency of newly developed transistors in integrated circuits ( RF technology, digital technology ), because the transistors can no longer be discretely measured ( up to several 100 GHz).

High -frequency applications

• Phase shifters are used in large numbers in phased array antennas, and are controlled by a central computer for the design and for pivoting of the antenna diagram. For active antennas can be used before the amplifier and therefore need to switch only a very small capacity, which the module is smaller and more compact leaves.
• For control and measurement applications, RF - tight mechanical constructions are used, which allow feed or extraction from a cable section. The mechanical position of the coupling can be moved on the cable section. Thus, a standing wave can be detected on the line ( Lecher line ) or a supply of a measurement signal in an RF system, carried out with a defined phase position.