Line adaptation

from Wikipedia, the free encyclopedia

Among line matching refers to the correct load of an electrical line with a terminating resistor . The aim is to avoid disturbing reflections from waves or pulses. The output resistance (also known as the source resistance ) of the transmitter has no influence on this.

This must not be confused with power adjustment, which is about drawing maximum power from a source with a specified internal resistance.

General

If electrical energy or signals in the form of waves or impulses are sent to consumers via electrical lines ( cables ), two essential features are decisive:

  • The attenuation weakens the signal; In the case of direct current and low frequencies, it is determined by the ohmic resistance of the wires; in the case of high frequencies, there are also heat losses in the insulation material.
  • The impedance Z is largely independent of frequency at frequencies above a few kHz and real-valued and dampens not the transfer, but is solely a characteristic value for the design of the optimum load resistance.

If the wave impedance Z does not match the terminating resistor R , unwanted and mostly disruptive impulse or wave reflections occur. The correct value of R is determined by time domain reflectometry or standing wave meters. When transmitting a narrow frequency band, the value of R can be corrected by a resonance transformer . There is no possibility of correction if the signal bandwidth is large .

The wave impedance is identical to the terminating resistor

All signals from the transmitter are absorbed without reflection when they first arrive at the load resistor R. From the sender's point of view, it is not possible to distinguish whether R is connected directly or via a cable. The internal resistance (source resistance) of the transmitter has no influence, but its value depends on whether the power is matched or not. This is even important in energy technology with the comparatively very low network frequency, because the cable does not produce any reactive power with natural power . If - as in power engineering or with powerful transmitter systems - an efficiency of over 50% is required, power adjustment must be avoided as far as possible.

Z not equal to R

The inevitable signal reflections at the end of the cable cause pulses (or waves) to travel back to the transmitter. However, the corresponding energy does not have to be “lost”; under favorable circumstances, the load can still benefit. That depends on some boundary conditions:

Transmission of waves of fixed frequency

  • If the length of the cable an integral multiple of half the wavelength ( n · λ / 2) has the value of the characteristic impedance even with large variations of R little impact because the cable then no transforming properties (see line theory ). From the "point of view" of the transmitter, the load is connected directly.
  • If the length of the cable is an odd multiple of λ / 4, the cable transforms the resistance values ​​according to the following formula

This effect can be very desirable in circuits used in high frequency technology (see also resonance transformer ).

Transmission of pulses

The pulses reflected at the end of the cable should be absorbed by the source resistance of the transmitter so that they cannot reach the load again. Given the low power used in data transmission, this cannot damage the transmitter. The fact that the efficiency has to be less than 50% is also irrelevant in these applications.

literature

  • Andres Keller: Broadband cables and access networks. Technical principles and standards. Springer-Verlag, Berlin / Heidelberg 2011, ISBN 978-3-642-17631-9 .
  • Kristian Kroschel: data transfer. An introduction. Springer-Verlag, Berlin / Heidelberg 1991, ISBN 3-540-53746-5 .
  • Ekbert Hering, Klaus Bressler, Jürgen Gutekunst: Electronics for engineers and natural scientists . Springer Verlag, Berlin / Heidelberg 2014, ISBN 978-3-642-05499-0 .
  • Dieter Ebner: Technical basics of computer science. Electronics - data processing and process control for scientists and engineers. Springer-Verlag, Berlin / Heidelberg 2013, ISBN 978-3-642-93371-4 .
  • Werner Groß: Digital circuit technology . Friedrich Vieweg & Sohn Verlagsgesellschaft, Wiesbaden 1994, ISBN 3-528-03373-8 .
  • Leonhard Stiny: Passive electronic components. 2nd Edition. Springer Fachmedien, Wiesbaden 2015, ISBN 978-3-658-08651-0 .

See also

Web links