Transfer impedance

The transfer impedance , also known as the coupling resistance , is a dimension- dependent measured variable for the shielding effect of shielded electrical lines. The transfer impedance has the dimension Ω / m and is used in high-frequency technology and in electromagnetic compatibility (EMC), for example for coaxial cables . The dimensionless measured variable shielding attenuation , which is also occasionally used for cables, is used to evaluate the shielding effect of general electrical shielding . The measurement methods for transfer impedance are standardized in EN 50289-1-6 as well as in IEC 62153-4-3 and in IEC 62153-4-4.

definition

The transfer impedance describes the shielding effect of an electrically short ( ), shielded line section. It is defined as a length-related quantity and is usually specified in the unit mΩ / m (milli- ohms per meter). The transfer impedance describes the length-related ratio of the longitudinal voltage that is coupled into the inner circuit (inner side of the shield, inner conductor and terminating impedances of the line) to the current that is impressed as interference current in the outer circuit (outer side of the shield and its surroundings). The following applies to the transfer impedance: ${\displaystyle Z_{\mathrm {T} }}$ ${\displaystyle l<\lambda /10}$ ${\displaystyle U_{\text{stör}}}$ ${\displaystyle I_{\text{stör}}}$

${\displaystyle Z_{\mathrm {T} }={\frac {U_{\text{stör}}}{I_{\text{stör}}\cdot l}}}$

The transfer impedance only includes the galvanic and magnetic coupling . As a disturbance mechanism, the transfer impedance describes a disturbance voltage _source controlled by disturbance current I disturbance. The smaller the transfer impedance, the better the shielding effect. ${\displaystyle Z_{\mathrm {T} }}$

The capacitive coupling to the conductor or conductors protected by the cable shield is covered by the term transfer admittance, which, due to the comparatively weak influence of electrical fields, can usually only be considered when the cable shields are connected to ground on one side .

Typical course for cables with braided shield

Characteristic course of the transfer impedance of cables with braided shield

Typical values ​​of the transfer impedance for braided shields start at 10 mΩ / m to 20 mΩ / m for direct current. At low frequencies up to approx. 100 kHz, the value of the transfer impedance roughly corresponds to the DC resistance of the screen. This value remains constant up to approximately 1 MHz, since up to this frequency the galvanic coupling dominates the coupling of the interference voltage. From around 1 MHz, the inductive coupling to the inner conductor (s ) dominates due to the apertures in the braided shield and the transfer impedance increases linearly with the frequency. On a logarithmic scale, the increase is 20 dB per frequency decade. Only with high-quality cables is an improvement in the transfer impedance between approx. 100 kHz and 1 MHz due to the skin effect characteristic.

Influence of wavelength, test object length and frequency

Characteristic curve of the measured transfer impedance of cables with braided shield. The extinctions at the near and far end of a measuring arrangement are clearly visible. With shorter cable lengths, the cutoff frequency shifts to higher frequency ranges. Also visible for braided shields with a high degree of coverage is the onset of the skin effect, which improves the transfer impedance from approx. 100 kHz with particularly good cables

Measured drops in the transfer impedance at higher frequencies are due to the difference in wavelength between the interference current wave on the cable shield and the interference voltage wave within the shielded cable. The wavelength difference arises from the fact that the current wave impressed on the screen experiences a permittivity similar to the permittivity of the free space, while the interference voltage wave between the inner and outer conductors sees the permittivity of the insulating material. Since the wavelength within the line differs by the shortening factor from the wavelength of the interference current outside the line due to the dielectric , at higher frequencies, when the shielded line becomes electrically long, values ​​are erased when determining the transfer impedance from the measuring current and measuring voltage . The frequency at which this extinction begins depends on the length of the test object and, with the choice of the measuring point for the measuring voltage, on whether the direction of the waves between the inner and outer circuit are opposite or parallel. ${\displaystyle l}$

According to EN 50289-1-6, the coupling length L _{c is} electrically short if:

${\displaystyle L_{c}\leq {\frac {c}{10\cdot f\cdot {\sqrt {\epsilon _{\mathrm {r1} }}}}}}$

or electric long if:

${\displaystyle L_{c}\geq {\frac {c}{10\cdot f\cdot |{\sqrt {\epsilon _{\mathrm {r1} }}}-{\sqrt {\epsilon _{\mathrm {r2} }}}|}}}$

where c is the speed of light in free space. The coupling transfer function T _{n, f} represents the course of transfer impedance Z _T and shielding attenuation a _{S of} a cable shield or a shielded component or plug over the frequency. The transfer impedance is independent of the propagation conditions in the cable or component and its surroundings. According to their definition, it is not the shielding attenuation .

Above the cut-off frequencies f _{n, f} (c = cut off, n = near, f = far), the range of wave propagation or the range in which the examined objects can be regarded as electrically long begins.

Typical course with closed umbrella

Characteristic course of the transfer impedance of lines with continuously closed shield

In the case of cable shields made of self-contained shield conductor material, e.g. B. semi-rigid lines , the transfer impedance decreases with increasing frequency, because the skin effect ensures that the interference current on the inside creates an ever smaller voltage drop. The frequency at which this effect starts depends on the thickness of the outer conductor and the skin depth .

This effect is desirable because it leads to the desired decoupling between the external and internal circuit.

Measurement of the transfer impedance

The transfer impedance of a line is measured by _impressing a defined current I _disturb in the cable _shield over a defined cable length l by means of an external circuit . On the test line , the voltage U _{disturbance dropping there is} measured on the inner circuit via a terminating resistor R of the line terminated on both sides with the line impedance . The voltage measured at a terminating resistor corresponds to half of the voltage coupled into the cable shield of the cable terminated at both ends. The transfer impedance determined from the measured values ​​is then:

${\displaystyle Z_{\mathrm {T} }=2\cdot {\frac {U_{\text{stör, Messwert}}}{I_{\text{stör}}\cdot l}}}$

Triaxial measuring arrangements or arrangements with a direct feed of the interference current into the cable shield of the device under test are specified as the measuring arrangement in the literature.

Triaxial measuring method

Triaxial measurement setup for measuring the transfer impedance

The cable or component to be tested is provided with a plug at one end and a terminating resistor at the other end. The test item is built into the pipe and short-circuited to the pipe at the transmitter end. In the case of coaxial test objects, the coaxial cable and the measuring tube form a triaxial system; the cable to be tested forming the inner system and the cable shield with the pipe forming the outer system. Energy is fed into the cable to be tested or into the internal system via the transmitter.

The energy emerging from the cable to be tested or from the internal system spreads in the external system. For the wave traveling to the end near the transmitter, the short circuit creates a total reflection, so that the superposition of the outgoing and returning wave or of near and far crosstalk is measured at the receiver.

The triaxial measuring method is standardized in EN 50289-1-6 as well as in IEC 62153-4-3 and in IEC 62153-4-4.

Alternative measurement method for determining the transfer impedance

In the case of complex braided shields, the most precise method for determination is by measurement. The triaxial method is a frequently used measurement method. With this measuring method, an adapted termination of the test object is only possible with great effort. A mismatched termination produces good results in the low-frequency range, but with increasing frequency the results become increasingly inaccurate. In addition, larger plugs lead to very large pipe thicknesses, which makes it even more difficult to achieve the correct wave resistance.

There is also the standardized parallel wire method, which achieves good results with different cables. A similar measurement setup can be used for cables and cable-connector systems. With non-symmetrical plugs, this method quickly reaches its limits, because the measurement results can vary for different positions of the feed wire.

The Ground Plate Method (GPM) was developed in view of the limitations of the LIM and the triaxial method, especially for analysis in the high-frequency range. The three methods differ essentially in the type of current feed and the structure of the return conductor. The triaxial method uses a cylinder, the parallel wire method uses a feed wire, whereas the GPM uses a ground plane as a return path.

Difference between transfer impedance and shielding attenuation

Ferrite sheathed and common coaxial cable RG58

The picture shows the cross-section of two RG 58 lines. The ferrite-sheathed line type and the cable in the usual design both have the same transfer impedance, because the ferrite sheath does not change the coupling to the inner conductor when interference current is impressed on the outer conductor.

The shielding attenuation against electromagnetic fields is increased by the ferrite jacket. For the shielding attenuation, the reference signals are not current I and voltage U, but field variables E and H. In addition, the ferrite jacket acts as a common mode choke , which transforms a push-pull signal and has an inductive damping effect on a common mode signal .

Further literature

• H. Kaden: Eddy currents and shielding in communications engineering . 2nd Edition. Springer Verlag, 1959, ISBN 3-540-32569-7 (March 2006).
• Joachim Franz: EMC, fail-safe construction of electronic circuits . Teubner, Stuttgart / Leipzig / Wiesbaden 2002, ISBN 3-519-00397-X .
• IEC 62153-4-3: Metallic communication cable test methods - Part 4-3: Electromagnetic compatibility (EMC) - Surface transfer impedance - Triaxial method.
• IEC 62153-4-15: Metallic communication cable test methods- Part 4-15: Electromagnetic compatibility (EMC) - Test method for measuring transfer impedance and screening attenuation - or coupling attenuation with Triaxial Cell.
• IEC 62153-4-6: Metallic communication cable test methods- Part 4-6: Electromagnetic compatibility (EMC) - Surface transfer impedance - Line Injection Method.
• A. Mushtaq, K. Hermes, S. Frei: Alternative measurement method for determining the transfer impedance of HV cables and HV cable-connector systems for electric and hybrid vehicles . In: EMV Düsseldorf 2016.
• A. Mushtaq, S. Frei, (2016): Alternate methods for transfer impedance measurements of shielded HV cables and HV cable connector systems for EV and HEV . In: Int J RF and Microwave Comp Aid Eng. , doi: 10.1002 / mmce.20984

Individual evidence

1. ↑ IEC 62153-4-3: 2013 | IEC webstore. In: webstore.iec.ch. Retrieved March 21, 2016 . 
2. ↑ IEC 62153-4-15: 2015 | IEC webstore. In: webstore.iec.ch. Retrieved March 21, 2016 . 
3. ↑ DIN IEC 62153-4-6: 2004-07. In: beuth.de. Retrieved March 21, 2016 . 
4. ↑ Abid Mushtaq, Stephan Frei: Alternate methods for transfer impedance measurements of shielded HV cables and HV cable connector systems for EV and HEV . In: International Journal of RF and Microwave Computer-Aided Engineering . March 1, 2016, ISSN  1099-047X , doi : 10.1002 / mmce.20984 .