High-frequency (HF) technology – what is it?
The term high-frequency is a fairly general one. The question which frequency ranges it refers to hinges on various factors, starting with the technological application concerned, in addition to country-specific stipulations and sometimes even the year in which it happens to be a subject of discussion.
The following characterization lends itself well to defining HF technology when it comes to the production of electrical conductors:
at a given point in time, a conductor does not feature the same signal level at all its points.
Extract from the Transmission theory
A signal diffuses throughout space or via a conductor at finite speed. In the event of the signal sinusoidally changing its polarity during a cycle, this cycle translates into a distance, which is determined by its velocity. This distance is also called wave length. E.g.: inside a conductor, the signal travels at about 200.000 km/s equating to 2/3 of light speed. Thus, a signal boasting a frequency of 2.5 MHz has a wave length (λ) of 80 m (=200.000.000 m/(2.500.000 Hz *s)). With signal levels widely differing at both ends of a conductor as short as 10 m (λ/8) at a given point in time, this already calls for the Transmission theory to be applied. The same holds true for a similar conductor at a length of 10 cm and a frequency of 250 MHz and, conversely, for a conductor featuring an audible signal of 2.5 kHz and a length of 10 km.
In cables and conductors, the transmission theory is made use of when it comes to transmitting information such as files, data, music, videos, language etc., as opposed to the transmission of energy (e.g. in engine drives or heating applications).
Requirements regarding HF cables
Transmission losses as low as possible
When travelling through a cable, the signal is partially lost. The ratio between output signal and input signal is called “insertion loss”.
This loss is caused by:
- The dwindling portion of metal usable as a conductor with frequencies rising due to the skin effect.
- The interaction of the electric field with the dipoles of the dielectric leads to energy losses as frequencies increase.
- Other drivers such as radiation and loss through parasitic conductivity of the dielectric.
The correlation between loss and frequency can be expressed by means of the parameters αD and αM, which explain the magnitude of the loss in the dielectric and the metal in the following equation: α = αM × √f + αD × f
In most cases, this ratio is indicated in dB/ length.
As little radiation / crosstalk as possible
HF cables transport power from a sender to a receiver. During the transportation process electric and magnetic fields emerge, either around the inner conductor of a coaxial cable or between and around the pair of conductors in a symmetric transmission. The outer limits to the field are provided using a shield, e.g. the outer conductor of the coax or else a pair shield or an overall shield. Construction-related gaps and thin-layered foils result in fields detaching and energy being lost through radiation, which a lack of symmetry prompted by tolerances always contributes to as well. Energy wasted this way does not reach its destined receiver and will also be found in measurable quantity in unintended places. The radiation also impacts the so-called EMC (electromagnetic compatibility), which may entail the transgression of statutory limits. In multiple-pair cables, this effect can be immediately verified inside the cable by measuring the crosstalk (XT) level. XT is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. In unfavorable circumstances, the interference can go to the point of deteriorating or even preventing a transmission. Suitable pair-related lay lengths and – if necessary – a pair-shield are counteracting elements of construction.
Attuning the Conductor to the System / Wave Impedance / Return Loss
Wave impedance is essential to transmitting signals via conductors. It depends upon construction, gauges and materials used. Each time the wave impedance changes, reflections arise. Their undesirability originates from the fact that reflected signal energy is unusable at the end of the transmission path. It possibly adds even more heat to the sender, which may reach the point of considerably interfering with the useful signal in the event of multiple reflections occurring. Therefore, it is important for the sender, the transmission path (conductor) and the receiver to retain the same wave impedance throughout.
Over and above this, LEONI as a wire manufacturer has to make sure that gauges and material properties are virtually barred from varying.
In symmetric conductors a pair always consists of at least two wires, with each wire carrying half of the signal (differential signal transmission).
- Lower insertion loss
- A good symmetry minimizes the amount of signal energy absorbed by the shield and vice versa (see (1): shield loss, crosstalk)
In order to obtain a sufficient degree of symmetry, the gauges of both wires and their respective positions under the shield must be as identical as possible. Likewise, the electrical properties of the materials of both wires must be absolutely identical since they affect the speed at which the signal spreads out. Since the useful signal is split in two halves, neither half of the bit must reach the receiver at the end of the line before the other half (=> Intra Pair Skew).
Particular challenges in HF Technology
Alternating currents are pushed to the surface of a conductor as a consequence of eddy currents arising in its core. This phenomenon is called skin effect.
- Current flow is limited to a thin layer at the surface of the conductor
- Resistance of conductor increasing due to reduced cross-section effectively used
Additional demands placed on in-line and off-line metrology
The manufacturing of HF cables requires additional measuring equipment both in-line and for final inspection.
In-line measurement devices:
- Continual measurement of diameter and centricity
- Continual scanning for knots and high-frequency errors
- Nowadays standard solution for “normal” conductors, too
- Additional testing and controlling of the foaming ratio with the help of capacitance measurement devices in HF applications
- Assurance of a very good, consistent quality across all stages of manufacturing ensured by extremely narrow tolerances and an optimized process control
- FFT analysis
Intermediate inspection / Final inspection:
- Network analyzer used for frequency-related tests
- TDR/T oscilloscope und BER testing device for time-related tests
Cables suitable for high frequencies require a very homogeneous structure. It is mainly periodically recurring interferences that adversely affect its electrical properties.
Signal analysis via Fast-Fourier-Transformation (FFT) stands for a mathematical method of parsing recorded data, which determines the consistency of a signal.
Thus, various manufacturing parameters can be monitored. Periodic faults, caused by malfunctioning machinery for example, are detected during the process, which allows for them to be remedied.
The mathematical basis for this possibility is the so-called Fourier series. It decomposes periodic functions or periodic signals into the sum of a (possibly infinite) set of simple oscillating functions, namely sines and cosines. The necessary calculations are executed by a computer, which receives the periodic signals and takes care of the spectral decomposition, translating the data into a histogram.
Insulation materials used for HF applications
Only those insulation materials are suitable for HF applications that have both a low dielectric constant εr and a low dielectric loss factor tanδ.
At LEONI, we use the following materials, depending on what is required from a mechanical, thermal and electrical perspective:
- PE and cellular PE
- PP and cellular PP
- FEP and cellular FEP
- PTFE foil and E-PTFE foils
Software for complex field simulations
Highly complex mathematical calculations and simulations constitute a precondition to meeting all electrical requirements right after producing a new conductor, connector or entire assemblies for the first time. LEONI possesses both – various programs by different providers and the know-how required to apply them.
Whether you need wires or strands to manufacture an HF conductor, whether you need an HF conductor to do your own assembling, or whether you seek full-blown assemblies or cable harnesses, you will find that LEONI offers an excellent solution for your purposes:
Wires and strands suited for HF technology
Our products are geared towards HF applications such as coaxial cables or ParaLink (high-speed parallel pair cables instead of one centered conductor).
Our product range covers:
- Traditional high-frequency strands made of enameled single wire
The enamel insulates the individual wires. An increased cross-section effectively transporting current yields better transmission properties. The latter is impaired by the skin and proximity effect (see above) when the single wires fail to be insulated.
- High-precision single wires used as conductors inside coaxial cables
These wires are produced on a machine specifically designed for this purpose. During the manufacturing process (Inline), the surface undergoes cleansing as well as inspection for recurring weak spots by means of an FFT analysis (see above). This allows for an estimation of the suitability of the HF wire for the applications/ frequencies envisaged.
- Strands made up of single wires where the coating (e.g. silver) boasts a better conductivity than the base metal
As for the optimization of conductivity, cf. skin and proximity effect.
- Strands with very smooth (compacted) surfaces
Featuring enhanced cross-section within skin area (cf. skin effect) as well as reduced loss resp. optimized wave impedance (see above).
- Strands with variable lay length
Geared so as to avoid recurring weak spots relative to lay length. May be combined with using highly conductive precious metals as coating materials.
- Miniaturized cables and cable assemblies
- Flexible conductors for moving applications
- Tailor-made high-grade special cables
- Comprehensive selection of insulation materials
- Small lot sizes
- Coaxial transmission lines, e.g. for mobile communications
- Symmetric 100 ohm data cables as per Cat5, Cat6, Cat7, Cat7A up to 1GHz und higher
- Symmetric star quads
- Symmetric ParaLink cables up to 25 GHz und higher
- High Speed Assemblies