Fragen und Antworten zur Hochfrequenztechnik

What is high-frequency technology?

High-frequency technology is a specialist field of electrical engineering. Its development can be traced back to the publication of Maxwell's equations in 1865. In high-frequency technology, the electromagnetic fields that arise in conductor structures at high frequencies are used for electrical systems such as circuits, devices, etc. High frequencies – from 3 MHz (HF, high frequency) to 300 GHz (SHF, super high frequency) or 3 THz (THF, tremendously HF) - lead to interactions between electric and magnetic fields. These are a prerequisite for the propagation of electromagnetic waves. Because of these waves, the electrodynamic processes in an electronic design can no longer be viewed as concentrated voltages and currents that are localized on the conductors and components. Instead, the electric and magnetic fields in and around the materials must also be considered, based on Maxwell's equations.

In high-frequency technology, we are interested in the sinusoidal frequencies of the field magnitudes under consideration over time. Non-sinusoidal signals can be represented as a superposition of sinusoidal curves using the Fourier transformation. Fourier analysis forms the basis for many methods of high-frequency measurement technology – for example, spectrum analysis. Today's HF circuit technology is based on complex wave amplitudes; the Vector Network Analyzer (VNA) is another suitable measuring device for this purpose. When using HF technology, the focus is usually on the digital processing of high-frequency signals in an embedded system. Examples of applications include high-frequency measurement technology, radio communication systems, radar technology and wireless navigation.

What impact does high-frequency technology have on practical applications?

The phenomenology of high-frequency technology has an impact on its practical application and on the measurement of HF signals. In an electrical circuit with high frequencies, the electrodynamic processes can no longer be considered as concentrated voltages and currents that are localized on the conductors and components. Rather, the electric and magnetic fields in and around the materials must also be considered (radiation from cables, etc.). For the practical application of HF technology, this means taking complex phenomena into account. Among others, these include:

The parasitic properties of active and passive circuit components
The coupling of subsystems through electromagnetic near fields
The connection of electrical circuits to the electromagnetic far field (antenna effect)

What impact does high-frequency technology have on measurement applications?

The phenomenology of high-frequency technology also affects the measurement of HF signals. This means that not only the parasitic properties of the circuit components have to be taken into account. The measurement of the voltage itself usually also leads to an additional load on the electrical circuit, for example due to the input impedance of the probe or measuring cable used. This also applies to high-impedance measurements. Basically, the measurement of HF signals requires special measuring feeders (transducers), including current clamps, electric and magnetic near-field probes, HF probes and antennas. Conducted measurements should be made with a fixed line impedance, typically 50 Ω.

The bandwidth of the signal spectrum of interest (useful frequency) is generally lower than the center frequency of the associated frequency band (carrier frequency). Direct measurement of high-frequency signals in the time domain is therefore subject to high noise levels due to the high bandwidth required, up to the carrier frequency range. High measurement dynamics can only be achieved with narrow-band measurements. The signal is made "visible" and measurable by converting the carrier frequency (down-conversion) and using a relatively small intermediate frequency bandwidth.

Transmission line theory describes current and voltage at any point on a line as a superposition of the outgoing and returning waves. The temporal course of the current or voltage is therefore the sum of all reflections of the wave front that have already arrived on the line at the time under consideration over the duration of the pulse length (in the past). The result corresponds to the result of a measurement using Time Domain Reflectometry (TDR).

What measuring devices are available for high-frequency technology?

Measuring devices for high-frequency technology are used for testing and debugging HF devices (cellphones, GPS satellites, etc.). They can be found in fields including development, testing, and production. The most common HF measurement devices include spectrum analyzers, (vector) network analyzers, signal generators and power sensors.

The spectrum analyzer measures which types of signals occur at which frequencies, and which levels and modulations they exhibit. Its basic function is to display power as a function of frequency. The power is specified in dB (decibels). The spectrum analyzer is also able to demodulate different signal types.

A signal generator can produce various types of HF signals that are required for the development and testing of HF devices – in particular receivers. These signals can be simple signals or complex modulated signals, as are often used in communication applications. With a signal generator, you can set all the parameters of the generated signals – for example the power, frequency and modulation.

Spectrum analyzers and signal generators are often combined in measurement technology. If the DUT (device under test) is an amplifier, for example, the signal generator is used to generate the signal that is fed into the DUT for testing. The amplified signal can then be measured using the spectrum analyzer.

A network analyzer, also known as a vector network analyzer (VNA), generates and measures HF signals, in a similar way to the combination of a signal generator and spectrum analyzer. In terms of HF technology, a "network" describes a device or system with several components (or connections) that can transmit, reflect or absorb HF signals. The components of a smartphone, for example, form a network. The network analyzer should ensure that all components are working properly and support the function of the overall system. Networks are tested by feeding an HF signal into one of the connections. This measures the HF component reflected by this connection and the HF level emerging from the other connections. It is also possible to determine how the network influences the properties of the signal fed in, for example through delay.

In most HF tests, the signals are transmitted via a cable between the device and the device under test. These conducted HF tests allow precise control of the test parameters (e.g. power level). Due to the higher frequencies, modern technologies such as 5G/6G also require so-called over-the-air tests (OTA tests via an air interface), in which the signals are transmitted wirelessly. In this case, special shielded test boxes are required to prevent other HF signals from interfering with the measurements.

There are many other types of high-frequency measuring devices, including communication testers for wireless communication devices, EMC testers for testing unwanted HF interference and oscilloscopes that are used for both high-frequency testing and other frequency ranges.

What is the voltage standing-wave ratio (VSWR)?

An electrical waveguide is, for example, a coaxial cable, a flat ribbon cable or a hollow conductor. The electrical voltage is usually regarded as the wave variable in the electrical line. However, depending on the reference, this can also be any other physical wave variable, for example the electric current or - in the case of a hollow conductor - the electric field strength.

A wave with a constant amplitude initially travels forward undisturbed on a homogeneous line. At an interference point, for example a connecting piece, there is a sudden increase in impedance so that part of the leading wave is reflected. A returning wave is created. This has a smaller amplitude than the outgoing wave. The advancing and receding waves overlap to form a standing wave.

The voltage standing-wave ratio (VSWR) describes the ratio of the outgoing to the returning wave on an approximately loss-free line. The VSWR is calculated from the maximum voltage amplitude of the standing wave divided by the minimum voltage amplitude (VSWR = Umax / Umin). The standing wave ratio is therefore a measure of the concordance between the line impedance and the impedance of a connected load (e.g. antenna). In terms of antenna technology, it indicates how much of the input power is converted into usable radio waves. If the impedance is mismatched (VSWR ≠ 1), the antenna does not emit efficiently.

Summary: If the line impedance at a transfer point deviates from the impedance of the load, the wave traveling towards it is reflected. The VSWR can be used to evaluate the efficiency of a wireless system.

What is an antenna?

The antenna is an essential component of an optimally functioning communication system. It converts the transmission and reception of wireless signals by converting electrical energy into electromagnetic energy (= transducer). The type of wireless transmission varies depending on the application and its requirements.

The physical properties of an antenna are related to the operating frequency, which also determines the wavelength. The antenna therefore only works efficiently at a specific frequency and its size varies from small, for example for WiFi routers at 2.4 GHz, to large, as with a transmission mast at a few hundred kHz. The allocation of frequency bands in the radio spectrum is usually application-related, so that similarly sized antennas are generally used for the respective frequency band.

The wavelength and frequency of a signal are inversely proportional to each other. This means: A higher frequency has a shorter wavelength (and range) and therefore requires a shorter antenna with a smaller area of spread. This aspect is particularly relevant for applications such as cellular communications, where antennas need to be as small as possible.

The following applies to high-frequency technology: If the transmission line (antenna) is short in relation to the wavelength λ up to approximately one tenth of the wavelength, no HF interference phenomena occur, i.e. there is no antenna effect (l < λ/10).

What is a dipole antenna?

Dipole and monopole antennas are well-known examples of so-called rod antennas, which play an important role in communications technology. A dipole antenna is a directional antenna consisting of straight elements of equal length that are electrically separated from each other in the middle and are fed from here. The length of a dipole antenna is directly proportional to the wavelength of the signal it is to receive or transmit. The dipole antenna emits by generating an oscillating electric field along its axis. The size and direction of the field are constantly changing, resulting in an electromagnetic wave. Dipole antennas are used, for example, for wireless systems and in WLAN devices.

What is a monopole antenna?

Monopole and dipole antennas are well-known examples of so-called rod antennas, which play an important role in communications technology. The monopole antenna consists of an emitting element that is mounted vertically on a conductive surface (ground plane). This makes it possible to send and receive electromagnetic waves in a preferred direction. By adjusting the length of the emitter, monopole antennas can be optimized for different frequency ranges, making them a flexible, cost-effective solution in the field of wireless communication. Applications for monopole antennas include cellular communications and amateur radio equipment.

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