Phase shifter
A phase shifter is an electronic circuit that shifts the phase of an electrical oscillation. The degree of this shift is given in degrees or as part of the full circle (360 °). Depending on the frequency, there are various options for technical implementation. At low frequencies, components of reactances that occur in inductances and capacitances are used for technical implementation . At high frequencies, runtime differences are used by detour lines.
species
In principle, the following groups of phase shifters can be distinguished:
- Frequency-dependent phase shift. The phase shift of a specific frequency is achieved by shifting the input signal over time. However, since different frequencies have period times of different lengths, the phase rotation is different depending on the frequency. This form can be implemented using appropriate term elements or in the form of all-passes .
- Frequency-neutral phase shifters. These circuits shift uniformly over a spectrum by a certain angle. The resulting signal is usually complex . These phase shifters are also referred to as Hilbert transformers and make use of the so-called Hilbert transform . Each Hilbert transformer causes the spectrum to rotate by 90 °. The Hilbert transformation plays a central role in signal processing and is used in the field of modulation technology , among other things .
Low frequencies
Analog technology
In electronics, an RC element (or a network of RC elements) is usually used for phase shifting . The current flow at the capacitor, which is offset in time with respect to the voltage, is used here. The current leads the voltage by 90 ° (see phase shift at the reactance ) and thus effects a phase shift between the two voltages applied to the components when a capacitor and resistor are connected in series.
The resistance value 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 to <90 ° due to the series connection with a resistor. Because the reactance of the capacitor is frequency-dependent, the phase shift of the RC element is also frequency-dependent.
The phase shift can be made adjustable by using potentiometers or electronically controllable resistors. In principle, a coil with comparable results can be used instead of a capacitor, but this is rarely used because of the higher costs.
An example of phase shifting with a capacitor is the generation of the auxiliary phase in the capacitor motor or in the Steinmetz circuit . A hybrid form of digital and analog phase shift is the CCD principle ( bucket chain storage ).
Digital phase shifters
Digital signals can be phase shifted by
- they are shifted through a FIFO memory ( shift register )
- their edges are delayed by constant times (only possible if shortest pulse duration> delay)
- by timing elements ( monoflops ) and a logic circuit
- by generating a triangular wave and subsequent comparators
The latter method is used, for example, in a type of switched-mode power supply (phase shifter) to feed a transformer with a symmetrical square-wave voltage that is controllable in its effective value over the pulse duration.
High frequencies
At high frequencies, the phase shifts are achieved by means of detour lines ( delay lines ) with a defined length on a line-based transmission . In the case of longer delay times (up to approx. 1 µs), the required cable lengths can only be achieved by winding. For even longer times (e.g. 180 ° phase shift of the image signal in television receivers at the line frequency, i.e. 64 µs), ultrasonic delay lines are used.
With coaxial lines or waveguides, the phase shift is possible up to delay times where the required length is still manageable (approx. 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 possible precision of the positioning of line connections in the micrometer range. This method can be used without any problems at frequencies up to 100 GHz.
The graphic shows a phase shifter that can switch any phase angle between 0 ° and 315 ° with three bits in 45 ° steps. The switches shown in the graphic are implemented in practice using PIN diodes , which can switch high powers in a few nanoseconds. The picture shows a phase shifter that switches phase angles between 0 ° and 337.5 ° in 22.5 ° steps with control lines with a width of four bits. The length of the detour lines is frequency-dependent and also dependent on the propagation speed of the waves in the medium, that is, a shortening factor is effective in a cable . The phase velocity is greater in a waveguide , which is why a shortening factor greater than 1 is effective here.
Applications
Phase shift generator
For low-frequency oscillators , feedback amplifiers are also used, which are equipped with a phase shift chain. Two conditions must be met for these circuits to generate vibrations:
- the feedback must be in phase, that is, the phase shift of the amplifier and the feedback circuit must total 0 ° (or according to the periodicity of the sine function n · 360 ° );
- the gain of the system must be 1 , i.e. the power delivered to a load must be equal to the power generated.
Phase condition
The circuit principle in the picture above is based on an inverting amplifier stage (which acts like a phase shift of 180 ° with the sinusoidal signals present here ) with a transistor and an RC network with phase shifting effect in the feedback branch. The frequency dependency of the RC elements is specifically exploited here. Each RC element only has to shift by 60 ° and with three elements you can achieve the entire phase shift of 180 ° at exactly one frequency. This frequency is preferably amplified by the inverting amplifier and the oscillation condition is met. The phase shifter chain can be constructed with three elements ( 3 · 60 ° ) or four elements ( 4 · 45 ° ) in order to compensate for the phase reversal (180 °) of the amplifier stage. If you use an OpAmp with its high input resistance and therefore low load instead of a transistor , you can even get by with two links at 90 ° each (theoretical maximum). The last resistance of the phase shifter chain can be equal to the input resistance of the amplifier. If the generated frequency is to be made adjustable, one or more resistors of these RC elements are designed as potentiometers (possibly in tandem version).
Another circuit principle is the Wien bridge generator ; in this, a series connection of R and C to an RC parallel connection works in the feedback branch of a non-inverting amplifier stage. If the R and C values are the same, the phase shift is zero for exactly one frequency, so that the phase condition is fulfilled together with the non-inverting amplifier stage. Such oscillators with a Wien bridge are implemented, for example, as a low-frequency sine wave generator (laboratory device). A variable capacitor is used to change the oscillation frequency.
Performance condition
The design of the amplification factor of the amplifier stage is critical in this circuit. If it is too large, the generated amplitude increases until the amplifier is driven to an operating point at which the gain is smaller again (upper limit range). In this way, the performance condition is fulfilled again, but in this limitation area no more sinusoidal shape is generated, but rather a rectangular-like oscillation with many harmonics .
If the gain factor is too small, too much energy is drawn from the load (this also includes the resistance between the collector and the operating voltage), and the oscillation does not start automatically. In this case, if the generator is externally excited, it responds with dampened oscillations . Phase shift generators are therefore usually equipped with an amplitude control. In many such generators, this control consists of a PTC thermistor (small incandescent lamp ) designed as a negative feedback , which heats up as the amplitude increases due to the higher effective current and thus increases its resistance and negative feedback.
The signal must be decoupled with as high a resistance as possible in order not to let the vibrations subside due to possible overload. For this reason, an emitter follower is often connected downstream of the oscillator for decoupling .
Ring oscillator
An example of a phase shift oscillator is the ring oscillator . The frequency is determined by the phase rotations of the individual transistor stages. It is used to determine the cut-off frequency of newly developed transistors in integrated circuits (HF technology, digital technology), as the transistors can no longer be measured discretely (up to a few 100 GHz).
High frequency applications
- Phase shifters are used in large numbers in phased array antennas and are controlled by a central computer to shape and pivot the antenna diagram . With active antennas, they can be used before the power amplifier and therefore only need to switch a very low power, which makes the assembly smaller and more compact.
- For control and measurement applications, HF-tight mechanical constructions are used that allow feeding or decoupling on a cable section. The mechanical position of the coupling can be shifted on the cable section. In this way, a standing wave can be detected on the line ( Lecher line ) or a measurement signal can be fed into an HF system with a defined phase position.
Web links
- www.spicelab.de
- www.radartutorial.eu (example of high frequency application in the radar tutorial )
credentials
- Edgar Voges : High Frequency Technology , Dr. Alfred Hüthig Verlag, Heidelberg 1987, section 13 “Two-port oscillators”, ISBN 3-7785-1270-6 .
- Helmut Vogel, Physics , Springer-Verlag Berlin Heidelberg 1993, 17th edition, page 433ff
- ↑ a b Vienna - Robinson oscillator basic circuit Vienna bridge oscillator
- ↑ idw-online.de: Frequency record with Si / SiGe transistors
- ↑ heise online: Frequency record: 670 GHz amplifier