The matc ed device u der test, DUT, whic is f ed by a generator, f orms the disturbin circ it whic may also b desig ated as the in er or the primary circ it.. The disturb d circ it, whic
General 1 0
The IEC 621 53-4-x series outlines various testing methods for assessing the screening effectiveness of communication cables, connectors, and components using a triaxial test setup An overview of these test procedures is provided in Table 1.
Table 1 – IEC 621 53, M etallic commu nication cable test methods –
Test procedures with triaxial test set-up
M etallic Communication Cabl e test methods − Electromag netic compatibility (EM C)
I EC TR 621 53-4-1 Ed 3 I ntroduction to electromagnetic (EMC) screening measurements
I EC 621 53-4-3 Ed.2 Surface transfer impedance − Triaxial method
I EC 621 53-4-4Ed 2 Shielded screening attenuation, test method for measuring of the screening attenuation a s up to and above 3 GHz
The IEC 621 53-4-7 standard outlines a shielded screening attenuation test method for assessing the transfer impedance, screening attenuation (a_s), and coupling attenuation (a_c) of RF connectors and assemblies This method, known as the tube-in-tube technique, is applicable for frequencies up to and exceeding 3 GHz.
I EC 621 53-4-9 Coupling attenuation of screened balanced cables, triaxial method
I EC 621 53-4-1 0 Shielded screening attenuation test method for measuring the screening effectiveness of feedtroughs and electromagnetic gaskets double coaxial method
I EC 621 53-4-1 5 Test method for measuring transfer impedance and screening attenuation − or coupling attenuation with triaxial cell (under consideration)
I EC 621 53-4-1 6 Technical report on the relationship between transfer impedance and screening attenuation (under consideration)
Usually RF connectors have mechanical dimensions in the longitudinal axis in the range of
20 mm to maximum 50 mm With the definition of electrical short elements we get cut off or corner frequencies for the transition between electrically short and long elements of about
1 GHz or higher for usual RF-connectors
The tube in tube procedure was developed to measure screening attenuation, particularly in the lower frequency range, as an alternative to transfer impedance This method involves extending the electrical length of the RF connector using a metallic extension tube that is RF-tight.
Figure 2 – Principle of the test set-up to measure transfer impedance and screening or cou pling attenuation of connectors with tube in tube
The tube in tube test setup utilizes a triaxial system in accordance with IEC 621 53-4-3 and IEC 621 53-4-4, comprising the device under test (DUT), a solid metallic tube, and an optional RF-tight extension tube The DUT, powered by a generator, creates a disturbing circuit known as the inner or primary circuit, while the connecting cables to the DUT are further shielded by the tube in tube configuration.
Measuring tube Connector under test
The disturbed circuit, also known as the outer or second circuit, consists of the outer conductor of the device under test (DUT) and the extension tube This circuit is connected to the connecting cable and a solid metallic tube that aligns with the axis of the DUT being tested.
Transfer impedance 1 2
The test evaluates the screening effectiveness of shielded cables by applying a specific current and voltage to the cable's screen and measuring the induced voltage in a secondary circuit to determine the surface transfer impedance This assessment focuses solely on the magnetic component of the transfer impedance For measuring the electrostatic component, or capacitance coupling impedance, the method outlined in IEC 62153-4-8 should be utilized.
The triaxial method of the measurement is in general suitable in the frequency range up to
For a sample length of 1 meter, a frequency of 30 MHz is used, while a frequency of 100 MHz is applicable for a sample length of 0.3 meters, both corresponding to an electrical length of less than 1/6 of the wavelength in the sample Detailed information can be found in Clause 9 of IEC TS 62153-4-1:2014 and IEC 62153-4-3.
Screening attenuation 1 2
The primary circuit, also known as the disturbing circuit, refers to the matched cable, assembly, or device being tested In contrast, the secondary circuit, or disturbed circuit, comprises the outer conductor of the cable or assembly, or the outermost layer in the case of multiscreen cables, along with a solid metallic housing that encases the device under test along its axis.
To measure the voltage peaks at the far end of the secondary circuit, the near end must be short-circuited, and a matched receiver is not required The expected voltage peaks are independent of the receiver's input impedance, as long as it is lower than the characteristic impedance of the secondary circuit However, minimizing mismatch is beneficial, which can be achieved by selecting appropriately sized housings For further details, refer to Clause 10 of IEC TS 62153-4-1:2014 and IEC 62153-4-4.
Coupling attenuation 1 2
Balanced cables and devices operating in differential mode can emit a small amount of input power due to symmetry irregularities In unscreened configurations, this radiation correlates with the unbalance attenuation, denoted as \( a_u \) For screened balanced systems, unbalance generates a current in the screen, which is coupled into the outer circuit through transfer and capacitive coupling impedances The component's screen mitigates this radiation, which is associated with the screening attenuation, \( a_s \).
The effectiveness of shielded balanced cables, connectors, or assemblies against electromagnetic disturbances is determined by the sum of the unbalance attenuation (\$a_u\$) of the pair and the screening attenuation (\$a_s\$) of the screen Both values are typically expressed in a logarithmic ratio, allowing them to be easily combined to calculate the coupling attenuation (\$a_c\$) using the formula: \$a_c = a_u + a_s\$.
Coupling attenuation \( a_c \) is calculated using the logarithmic ratio of the feeding power \( P_1 \) to the periodic maximum values of the radiated power \( P_{r, \text{max}} \), which are influenced by the voltage peaks \( U_2 \) in the outer circuit.
Env is the minimum envelope curve of the measured values in dB
The relationship of the radiated power P r to the measured power P 2 received on the input impedance R is: s max
The voltage \$U_2\$ at the far end will vary due to electromagnetic coupling through the screen and the superposition of partial waves This variation is influenced by the surface transfer impedance \$Z_T\$, the capacitive coupling impedance \$Z_F\$ (which affects both the far and near ends), and the waves that are totally reflected from the near end.
To effectively feed the balanced device under test, a differential mode signal is required, which can be generated using a two-port network analyzer along with a balun or a multiport network analyzer The methodology for measuring coupling attenuation with a multiport network analyzer is currently being evaluated.
General 1 3
Measurements will be conducted at a temperature of (23 ± 3) °C The testing method assesses the transfer impedance, screening attenuation, or coupling attenuation of a Device Under Test (DUT) using a triaxial test setup, in accordance with IEC 62153-4-3 and IEC 62153-4-4 standards.
Tube in tube procedure 1 3
Usually RF connectors have mechanical dimensions in the longitudinal axis in the range of
Electrically short elements range from 20 mm to a maximum of 50 mm, with cutoff or corner frequencies typically around 1 GHz or higher for standard RF connectors, marking the transition between electrically short and long elements.
In the frequency range up to the cut-off frequency, the transfer impedance of the device under test (DUT) can be accurately measured when it is electrically short Conversely, for frequencies exceeding the cut-off frequency, the DUT becomes electrically long, allowing for the measurement of screening attenuation.
Extending the electrical length of an RF connector with a metallic extension tube effectively lowers the cut-off frequency, allowing for measurements of screening attenuation in the lower frequency range This approach also facilitates the calculation of effective transfer impedance for electrically short devices.
The test setup features a triaxial system that includes the device under test (DUT), a solid metallic tube, and an RF-tight extension tube The DUT, powered by a generator, creates a disturbing circuit, also referred to as the inner or primary circuit.
The disturbed circuit, also known as the outer or second circuit, consists of the outer conductor of the device under test (DUT), which is linked to an extension tube and a solid metallic tube that houses the DUT along its axis.
The test setup, illustrated in Figures 2 and 3, is designed to measure both transfer impedance and screening or coupling attenuation, with the flexibility to adjust the lengths of the inner and outer tubes.
Figure 3 – Principle of the test set-up to measure transfer impedance and screen ing attenuation of a cable assembly
The voltage ratio between the near end (U₁) of the inner circuit (generator) and the far end (U₂) of the secondary circuit (receiver) is determined by measuring U₁/U₂, with the near end of the secondary circuit being short-circuited.
The results of the tested combination, which includes the device under test (DUT) and the extension tube, can be represented in different ways based on their electrical length These representations may include transfer impedance, effective transfer impedance, or screening attenuation (also known as coupling attenuation).
A matched receiver is not required for this measurement, as the voltage peaks at the far end are independent of the receiver's input impedance, as long as it is lower than the characteristic impedance of the secondary circuit However, minimizing mismatch is beneficial, which can be achieved by choosing a variety of tube diameters for different sizes of coaxial cables.
Test equipment 1 4
The principle of the test set-up is shown in Figure 2 and 3 and consists of:
The apparatus features a triple coaxial design with an adequate length to facilitate the superimposition of waves within narrow frequency bands, allowing for the construction of an envelope curve It includes tubes of variable lengths, which can be adjusted through different sections of the tubes or by utilizing a movable tube within a tube For larger connectors or components, the triaxial tubes can be substituted with a triaxial cell in accordance with IEC 621 53-1 5.
The RF-tight extension tube, designed as a tube-in-tube structure, should be variable in length and have a diameter that ensures a characteristic impedance of 50 Ω, matching the nominal characteristic wave impedance of the network analyzer, generator, or receiver Constructed from non-ferromagnetic and highly conductive materials like copper or brass, the tube must have a thickness of at least 1 mm to ensure that its transfer impedance is negligible compared to that of the device under test Additionally, a signal generator and receiver equipped with a calibrated step attenuator, along with a power amplifier if needed for high screening attenuation, are essential; these components may also be integrated within a network analyzer.
A balun is essential for matching the impedance of an unbalanced generator output signal to the characteristic wave impedance of balanced cables, which is crucial for measuring coupling attenuation The requirements for the balun are specified in IEC 62153-4-9:2008, section 6.2 Alternatively, a Vector Network Analyzer (VNA) with a mixed mode option can be utilized, with procedures for using mixed mode VNAs currently under consideration.
Connector interface Assembly under test Matching resistor
– time domain reflectometer (TDR) with a rise time of less than 200 ps or network analyser with maximum frequency up to 5 GHz and time domain capability.
Calibration procedure 1 5
Calibration must be conducted at the same frequency points as the measurements, utilizing a logarithmic frequency sweep across the entire specified range for transfer impedance.
To ensure accurate measurements with a vector network analyzer and S-parameter test set, it is essential to perform a complete two-port calibration that includes the connecting cables linking the test setup to the test equipment The calibration reference planes are defined at the connector interfaces of these cables.
When operating a vector network analyzer without an S-parameter test set, it is essential to perform a THRU calibration that includes the test leads connecting the setup to the test equipment.
When utilizing a separate signal generator and receiver, it is essential to measure the composite loss of the test leads and save the calibration data to enable accurate result correction.
P 1 is the power fed during calibration procedure;
P 2 is the power at the receiver during calibration procedure
If amplifiers are used, their gain shall be measured over the above-mentioned frequency range and the data shall be saved
To ensure accurate measurements, an impedance matching adapter should be utilized, with attenuation assessed across the specified frequency range and recorded accordingly This can be accomplished by linking two impedance matching adapters from the same manufacturer and of identical type.
“back to back” together and measure:
Connection between extension tube and device under test 1 5
To ensure accurate testing, the connection between the extension tube and the device's attached cables must maintain negligible contact resistance Annex D provides a potential connection technique and discusses the impact of contact resistances.
Dynamic range respectively noise floor 1 5
With the verification test, the residual transfer impedance respectively the noise floor due to the connection of the feeding cable to the extension tube shall be determined
To ensure accurate testing, the feeding cable must be matched to its characteristic impedance and connected to the test head Next, the extension tube should be attached to the feeding cable without the Device Under Test (DUT), utilizing the same connection method as used during the test It is crucial that the cable segment between the connection points is kept as short as possible.
Figure 4 – Principle set-up for verification test The voltage ratio U 1 /U 2 shall be measured with the VNA
The noise floor a n of the connection of the extension tube to the feeding cable is then given by:
The noise floor shall be at least 1 0 dB better than the measured value
The residual transfer impedance of the connection of the extension tube to the feeding cable is given by:
Impedance matching 1 6
To determine the nominal characteristic impedance of a (quasi-)coaxial system, one can utilize a TDR with a maximum rise time of 200 ps or follow the method outlined in Annex A It is advisable to avoid using an impedance matching adapter to connect the generator's impedance with that of the (quasi-)coaxial system, as this can diminish the dynamic range of the test setup Additionally, self-made adapters may only provide adequate matching (return loss) up to 100 MHz for impedances other than 60 Ω or 75 Ω, as discussed in Annex B.
Influence of Adapters 1 6
To accurately measure transfer impedance and screening or coupling attenuation on connectors or cable assemblies, it is essential to use test adapters when mating connectors for the device under test (DUT) are not available.
Test adapters and/or mating connectors may limit the sensitivity of the test set up and may influence the measurement
Measuring tube Short cable piece,
High screened cable, e.g semi rigid cableExtension tube
The type and/or the design of the test adapter shall be stated in the test report
A more detailed description on the design and the influence of test adapters is under consideration
Coaxial connector or device 1 7
A feeding cable must be installed to the connector being tested and its corresponding mating part, following the manufacturer's specifications One end of the cable should connect to the test head, ensuring that it matches the nominal characteristic impedance of the device under test.
It may be short circuited, when measuring the transfer impedance with method C: (Mismatched)-short-short without damping resistor according to I EC 621 53-4-3
To connect the generator, pass the other end of the connecting cable through the extension tube Ensure that the screen of the feeding cable is connected to the extension tube with low contact resistance on the device under test side (refer to 6.2 and Annex B) However, on the generator side, do not connect the screen of the feeding cable to the extension tube.
Balanced or multiconductor device 1 7
A balanced or multiconductor cable should be connected to both the connector under test and its mating part, following the manufacturer's specifications.
When assessing transfer impedance or screening attenuation, screened balanced or multiconductor cables are regarded as a quasi-coaxial system It is essential to connect all conductors of every pair together at the open ends of the feeding cable Additionally, all screens, including those of individually screened pairs or quads, must be interconnected at both ends, ensuring that all screens are connected around the entire circumference.
The test head is connected to one end of the feeding cable, ensuring it matches the characteristic impedance through screening attenuation and transfer impedance using a short/matched procedure, or alternatively, with a short circuit for the transfer impedance using a short/short procedure.
To ensure accurate testing, one end of the connecting cable must be linked to the test head, aligning with the characteristic impedance of the Device Under Test (DUT) This involves the principle preparation of balanced or multi-conductor connectors for various conditions: a) transfer impedance in short/short configurations, b) transfer impedance in short/matched configurations along with screening attenuation, and c) coupling attenuation.
Figure 5 – Preparation of balan ced or multiconductor connectors
Mated connector under test Screen
Mated connector under test Screen
Mated connector under test Screen
Cable assembly 1 9
If the cable assembly fits into the tube, it shall be measured according to Figure 3 Longer cable assemblies can be cut and each site measured separately
General 1 9
IEC 621 53-4-3 describes three different triaxial test procedures:
– Test method A: Matched inner circuit with damping resistor in outer circuit
– Test method B: Inner circuit with load resistor and outer circuit without damping resistor – Test method C: (Mismatched)-short-short without damping resistor
The procedure outlined is fundamentally similar to test method B of IEC 62153-4-3, utilizing a matched inner circuit without an impedance matching adapter or the damping resistor R2 This method offers a greater dynamic range compared to test method A of IEC 62153-4-3.
The load resistor R1 can either match the impedance of the inner circuit or the generator impedance The latter scenario is particularly relevant when utilizing a network analyzer with a power splitter instead of an S-parameter test set.
NOTE Other procedures of 621 53-4-3 may be applied accordingly if required.
Principle block diagram of transfer impedance 1 9
A block diagram of the test set-up to measure transfer impedance according to test method B of IEC 621 53-4-3 is shown in Figure 6
Figu re 6 – Test set-up (principle) for transfer impedance measurement accordin g to test method B of IEC 621 53-4-3
Measuring procedure – Influence of connecting cables 1 9
When measuring a connector or a component without tube in tube, the transfer impedance of the connecting cables inside the tube to connect the DUT shall be measured
The transfer impedance of the connecting cables linking the Device Under Test (DUT) must be measured in accordance with IEC 62153-4-3 This measurement should account for the length of the connecting cables within the test setup, resulting in the transfer impedance of the connecting cables, denoted as \$Z_{con}\$.
Callibrated receiver or network analyzer
Measuring
The DUT shall be connected to the generator and the outer circuit (tube) to the receiver
Attenuation should ideally be measured using a logarithmic frequency sweep across the entire specified frequency range for transfer impedance, ensuring that measurements are taken at the same frequency points used during the calibration process.
P 1 is the power fed to inner circuit;
P 2 is the power in the outer circuit.
Evaluation of test results
The conversion from the measured attenuation to the transfer impedance is given by following formula:
= (1 7) when using the tube in tube method where
The system impedance, denoted as \$Z_0\$, is typically 50 Ω The term \$a_{meas}\$ refers to the attenuation measured during the testing process, while \$a_{cal}\$ represents the attenuation of the connection cables, which may not be accounted for if the calibration procedure of the test equipment does not eliminate it.
R 1 is the terminating resistor in inner circuit (either equal to the impedance of the inner circuit or the impedance of the generator);
Z con is the transfer impedance of connecting cables;
Z Tr is the residual transfer impedance, see 6.6
NOTE Contrary to the measurement of the transfer impedance of cable screens, the transfer impedance of connectors or assemblies is not related to length.
Test report
The test report shall record the test results and shall conclude if requirements of the relevant detail specification are met
The use and the design of test adapters (if any) shall be described
General
This method is in principle the same as described in IEC 621 53-4-4.
Impedance matching
General
Measuring of screening attenuation can be achieved with or without impedance matching
To determine the nominal characteristic impedance of a quasi-coaxial system when the impedance of the Device Under Test (DUT) is unknown, one can utilize a Time Domain Reflectometer (TDR) with a maximum rise time of 200 ps or apply the measurement method outlined in Annex A of IEC 62153-4-4.
Using an impedance matching adapter to align the generator's impedance with that of the system device under test is not advisable, as it can diminish the dynamic range of the test setup Self-made adapters may only provide adequate matching (return loss) up to 100 MHz for impedances other than 50 Ω or 75 Ω, as detailed in Annex B of IEC 62153-4-4.
Figure 7 – M easuring the screening attenu ation with tu be in tube with impedance matching device
The DUT, along with the connected extension tube, must be installed within the measuring tube, with the extension tube short-circuited to the measuring tube at the generator's near end Additionally, the feeding cable should be linked to the generator, potentially through an impedance matching device, while the measuring tube's output is to be connected to the receiver.
The scattering parameter S 21 shall be measured
Only the peak values of the obtained screening attenuation graph are used to determine the envelope curve
Tube in tube Connecting cable IEC
Impedance matching adapter Tube Connector under test
Evaluation of test results with matched conditions
The screening attenuation a s shall be calculated with the arbitrary determined normalised value Z S = 1 50 Ω 1 a d
(1 9) where as is the screening attenuation related to the radiating impedance of 1 50 Ω in dB; a imd Is the attenuation of the impedance matching device (if appropriate);
Env is the minimum envelope curve of the measured values in dB;
S 21 is the scattering parameter S21 (complex quantity) of the set-up where the primary side of the two port is the DUT and the secondary side is the tube;
Z 1 is the characteristic impedance of the cable under test, in Ω
At frequencies lower than the limit of the electrically long coupling length, the measurement will be similar to that for surface transfer impedance.
Measuring with mismatch
The DUT shall be connected to port 1 and the test head of the set-up shall be connected to port 2 of the vector network analyser
If not known, the characteristic impedance Z 1 of the DUT shall be measured (see 9.2)
The scattering parameter S 21 shall be measured
Only the peak values of the obtained screening attenuation graph are used to determine the envelope curve.
Evaluation of test results
The screening attenuation a s which is comparable to the results of the absorbing clamp method shall be calculated with the arbitrary determined normalised value Z S = 1 50 Ω 1
The normalized value of the characteristic impedance, denoted as \$Z_s\$, represents the typical cable installation's environment and is independent of the outer circuit's impedance in the test setup.
IEC 621 53-4-7:201 5 © IEC 201 5 – 23 – a S is the screening attenuation related to the radiating impedance of 1 50 Ω in dB;
Env is the minimum envelope curve of the measured values in dB;
The S21 parameter represents the complex scattering coefficient in a two-port network, where the device under test (DUT) is connected to the primary side and a tube is connected to the secondary side Additionally, the reflection coefficient is denoted by r.
Z 0 is the characteristic impedance of system in Ω, (usually 50 Ω);
Test report
The test report shall record the test results and shall conclude if requirements of the relevant detail specification are met
The use and the design of test adapters (if any) shall be described
The Device Under Test (DUT) is connected to the cables as per the manufacturer's instructions, with differential and common mode terminations at the far end, as illustrated in Figure 5c The sample is centered in the tube and excited in differential mode using a generator through a balun, as shown in Figure 8 Alternatively, the DUT can be powered by a multiport Vector Network Analyzer (VNA), as depicted in Figure 10, which is currently under consideration.
The voltage ratio between the output of the outer circuit and the input of the cable is measured using either a network analyzer or a calibrated step attenuator, assuming the receiver's input impedance matches the signal generator's output impedance (R = Z₁) This step attenuator serves as an alternative to the triaxial apparatus.
To determine the envelope curve, it is essential to measure and record only the peak values of the maximum voltage ratio or the minimum attenuation as a function of frequency.
Attenuation introduced by the inclusion of adapters, instead of direct connection, must be taken into account when calibrating the triaxial apparatus
The measured voltage ratio remains unaffected by the diameter of the outer tube in the triaxial test setup or the characteristic impedance \( Z_2 \) of the outer system, as long as \( Z_2 \) exceeds the input impedance of the receiver.
NOTE The procedure to measure with a VNA with mixed mode option instead of using a balun is under consideration
Figu re 8 – M easuring the coupling atten uation with tube in tube and balu n
The attenuation of the balun shall be subtracted from the measuring results The coupling attenuation a c shall be calculated with the normalised value Z S = 1 50 Ω:
The coupling attenuation, denoted as \( a_C \), is associated with a normalized radiating impedance of 150 Ω and is measured in dB The minimum envelope curve of the measured values is represented by \( a_{m,min} \), also in dB Additionally, \( a_z \) refers to the extra attenuation introduced by an inserted balun, unless it has been removed through calibration, and is expressed in dB.
U 1 is the input voltage of the primary circuit formed by the cable in V;
U 2 is the output voltage of the secondary circuit in V;
Z 1 is the (differential mode) characteristic impedance of the cable under test in Ω
The test report shall indicate whether the results of minimum coupling attenuation comply with the value indicated in the relevant cable specification
For a cable system operating at a specified power level, the difference between the power level and the defined limit of radiating power must not exceed the coupling attenuation of the cable designed for that system.
The use and the design of test adapters (if any) shall be described
Tube in tube Connecting cable
A typical measurement graph of a connector is given in Figure 9
Figu re 9 – Typical measurement of a connector of 0,04 m length with 1 m exten sion tu be
To accurately measure coupling and unbalance attenuation, a differential signal is essential This differential signal can be produced using a balun, which transforms the unbalanced output from a 50 Ω network analyzer into a balanced signal However, it is important to note that commercially available baluns are typically limited to frequencies up to 1.2 GHz.
Alternatively, a balanced signal may be obtained with a network analyser having two generators with a phase shift of 1 80° Another alternative is to measure with a multi-port VNA (virtual balun)
The balunless test procedure is under consideration
Figu re 1 0 – M easuring the coupling attenuation with multiport VN A
(balunless procedu re is under con sideration)
Tube (CoMeT 40) Balanced/ unbalanced load
–60 –50 –40 –30 –20 –1 0 0 Connector, tube in tube 1 m/4 cm
Annex A (normative) Determination of the impedance of the inner circuit
To determine the impedance \( Z_1 \) of the inner circuit when it is unknown, one can utilize a Time Domain Reflectometer (TDR) with a maximum rise time of 200 ps or employ a vector network analyzer (VNA) for measurement.
The prepared sample is connected to the Vector Network Analyzer (VNA), calibrated for impedance measurements at the connector interface reference plane The test frequency is set to approximately the frequency corresponding to a sample length of \( \frac{1}{8} \lambda \), where \( \lambda \) represents the wavelength.
≈ ⋅ (A.1 ) where f test is the test frequency; c is the speed of light 3 × 1 0 8 m/s;
L sample is the length of sample
The sample is short-circuited at the far end The impedance Z short is measured
The sample is left open at the same point where it was shorted The impedance Z open is measured
Z 1 is calculated as: open short
Annex B (informative) Example of a self-made impedance matching adapter
Figures B.1 and B.2 illustrate the attenuation and return loss characteristics of a 50 Ω to 5 Ω impedance matching adapter An impedance of 5 Ω is commonly encountered when testing multipair cables with individually screened pairs or when assessing high voltage cables used in electric vehicles.
The attenuation and return loss were obtained from an open/short measurement One can obtain that the matching adapter only works up to 1 0 MHz
Figure B.1 – Attenu ation and return loss of a 50 Ω to 5 Ω impedance matching adapter, log scale
Self made 50/5 Ω impedance matching adapter open/short measurement
Figu re B.2 – Attenuation and retu rn loss of a 50 Ω to 5 Ω impedance matching adapter, lin scale
Self made 50/5 Ω impedance matching adapter open/short measurement
Measurements of the screening effectiveness of connectors and cable assemblies
General
The rise in the use of electric and electronic devices has led to an increase in electromagnetic pollution To mitigate this issue, it is essential to screen all system components, particularly connecting cables Standardized measurement procedures are necessary to evaluate the screening effectiveness of various designs, focusing on key parameters such as transfer impedance (\$Z_T\$) and screening or coupling attenuation (\$a_S\$ or \$a_C\$) While the triaxial and line injection methods can measure the transfer impedance of cables and connectors, a simple and cost-effective method for assessing screening or coupling attenuation in connectors and cable assemblies is currently lacking.
This article introduces a novel method that addresses a significant gap in the field It utilizes the recently developed shielded screening attenuation (long triaxial) test method to measure the screening or coupling attenuation of cables.
Physical basics
General coupling equation
To measure coupling effectively, it is beneficial to utilize the concept of operational attenuation based on the square root of power waves, as outlined in the definition of scattering parameters Consequently, the general coupling transfer function is defined accordingly.
The electromagnetic influence between the sample under test and the surrounding is in principle the crosstalk between two lines and is caused by capacitive and magnetic coupling
At the near end of the sample, magnetic and capacitive coupling increases, while it decreases at the far end The overall coupling along the sample length is determined by integrating the infinitesimal coupling distribution with the appropriate phase The phase effect in this integration is represented by the summing function S When sample attenuation is disregarded, S can be expressed using an equation that incorporates the phase velocities of the primary and secondary circuits, denoted as β₁ and β₂, respectively, along with the coupling length l The indices n and f refer to the near and far ends of the sample, respectively.
The equivalent circuit for two coupled lines is given in Figure C.1
2 Figures in square brackets refer to the bibliography
P0 square root of the feeding power n
P2 square root of the coupled power, near end f
P2 square root of the coupled power, far end
Z nm matching resistors, 1 = primary circuit, 2 = secondary circuit, n = near end, f = far end
Z n characteristic impedance, 1 = primary circuit, 2 = secondary circuit ε rn dielectric constant, 1 = primary circuit, 2 = secondary circuit v n velocity of propagation, 1 = primary circuit, 2 = secondary circuit
Figu re C.1 – Equivalent circuit of coupled transmission lines
Figure C.2 shows the summing function which is in principle a sin (x)/x function For high frequencies, the asymptotic value becomes:
And for low frequencies the summing function be-comes:
The point of intersection between the asymptotic values for low and high frequencies is the so called cut-off frequency f c This frequency gives the condition for electrical long samples:
0 r f n r c, e e l c f ⋅ ≥ π ⋅ ± (C.5) where e r1 is the relative dielectric permittivity of the inner system; e r2 is the relative dielectric permittivity of the outer system; l s the cable length.
Coupling transfer function
The key screening parameters of a screen include the surface transfer impedance \( Z_T \), capacitive coupling impedance \( Z_F \), and effective transfer impedance \( Z_{TE} \) In the case of homogeneous screens, such as those used in connectors or cables, these impedances can be considered constant along their length, simplifying the integration process The coupling between the sample and its surroundings can be represented by a specific coupling transfer function, particularly for matched lines.
At low frequencies with S = 1, the coupling transfer function reflects the frequency behavior of surface transfer impedance and capacitive coupling impedance Following an increase of 20 dB per decade, the coupling transfer function exhibits distinct cut-off frequencies, denoted as \( f_{cn,f} \), for both the near and far ends Beyond these cut-off frequencies, the samples are regarded as electrically long.
The calculated coupling transfer function of a coaxial cable is given in Figure C.3 The principle set-up of the triaxial test procedure is given in Figure C.4
IEC log scale S log ( l × f ) ( l × f ) cn ( l × f ) cf
Figure C.3 – Calcu lated coupling transfer function ( l = 1 m; e r1 = 2,3; e r2 = 1 ; Z F = 0)
Below the cut-off frequencies, the surface transfer impedance Z T is the measure of the screening effectiveness The value of the transfer impedance Z T increases with the sample length
Above the cut-off frequencies for wave propagation, where samples are electrically long, the screening attenuation \( a_S \) serves as a key measure of screening effectiveness Notably, screening attenuation is a length-independent parameter.
Cable assemblies consist of a cable and connectors at both ends It's essential to consider not only the quality of the cable and connectors but also the transition between them Simply pairing a high-quality connector with a good cable does not guarantee an effective assembly, as the connection between the cable and the connector can significantly impact performance.
Each component of the system, including connector A, transitions, cable, and connector B, requires integration in distinct sections Initially, it can be assumed that the velocity is uniform across each section The coupling transfer function for matched lines is subsequently defined by this assumption.
(C.8) where γ 1 ,2 is the complex wave propagation constant of inner, respectively outer circuit;
L c is the whole coupling length (sum of the segment lengths);
L i is the length of segment i; n is the number of segments (for cable assemblies, 3);
T n,f is the coupling transfer function at the near respectively far end;
1 0 kHz 1 00 kHz 1 MHz 1 0 MHz 1 GHz 1 0 GHz
Z 1 ,2 is the characteristic impedance of inner, respectively outer circuit;
Z F is the capacitive coupling impedance;
Z T is the surface transfer impedance; γ is the propagation constant
= ( α+jβ ), where αis the attenuation constant and β is the phase constant
C.2.2.3 Couplin g in the triaxial set-up
The coupling transfer functions are applicable only when the primary and secondary circuits are matched In a triaxial setup, however, the secondary system, or outer circuit, is mismatched This mismatch occurs at the near end due to a short circuit between the sample screen, and at the far end, it arises from the impedance difference between the outer circuit and the receiver input impedance, leading to a reflection coefficient denoted as \( r_{2,f} \) Consequently, the coupling transfer function at the receiver end is derived from these mismatched conditions.
= (C.9) where γ 2 is the complex wave propagation constant of outer circuit;
L c is the whole coupling length (sum of the segment lengths); r 2, f is the reflection coefficient;
T n,f is the coupling transfer function at the near respectively far end.
Triaxial test set-up
General
The triaxial test setup is a traditional method for measuring transfer impedance, recently adapted to assess the screening attenuation of cable screens Detailed in IEC 62153-4-3 and IEC 62153-4-4, this setup features a brass or aluminum tube with an inner diameter of approximately 40 mm.
Figu re C.4 – Triaxial set-up for the measuremen t of the screening attenuation a S and the tran sfer impedance Z T
To measure transfer impedance with an electrically short coupling length, the tube length should be between 0.5 m and 1 m In contrast, for screening attenuation measurements requiring an electrically long coupling length, the measuring tube must be extended to a length of 2 m to 3 m.
In the outer circuit, the screen under test is connected to the measuring tube at the near end, creating a short circuit This setup allows electrical waves to be coupled throughout the entire length of the cable.
The R1 = Z1 system transmits signals into the outer system, propagating in both directions towards the near and far ends At the short-circuited end, these signals are completely reflected, allowing the measuring receiver to capture the superposition of near and far end coupling as the disturbance voltage ratio.
U 2 /U 1 The screening attenuation as a power ratio is then related to a standardised characteristic impedance of the outer system Z s = 1 50 Ω
Z 1 is the characteristic impedance of the sample under test and Z s is 1 50 Ω.
Measurement of cable assemblies
When measuring cable assemblies in a triaxial test setup, variations in lengths can pose challenges, particularly when they differ from the standard measuring tubes of 2 m or 3 m Research indicates that for assemblies exceeding the length of the measuring tube, it is sufficient to measure only the accessible ends Conversely, for shorter assemblies, the tube-in-tube method can be employed, which involves extending the assembly with a well-screened cable housed within a closed copper tube.
In cable assembly screening attenuation measurements, the results are determined by the weakest component, whether it be the cable, connector, or the transition between them For assemblies longer than the measuring tube, measuring from both ends is adequate, assuming a homogeneous cable screen The worst-case results from these measurements represent the overall screening attenuation of the assembly, as illustrated by the simulated graphs in Figures C.5 and C.6.
The simulation parameters include a cable screen with a length of 500 cm, a direct current (d.c.) resistance of 1.3 mΩ/m, magnetic coupling of 0.04 mH/m, and capacitive coupling of 0.02 pF/m Additionally, the connector screen, which encompasses the transition from cable to connector, has a length of 5 cm, a d.c resistance of 2 mΩ/m, magnetic coupling of 0.002 mH, and no capacitive coupling The outer circuit, representing the secondary system, has an impedance of 150 Ω and a dielectric permittivity of 1.1, while the inner circuit, or primary system, has an impedance of 50 Ω.
Figure C.5 – Simulation of a cable assembly
(logarithmic scale) Figure C.6 – Simulation of a cable assembly
The blue line represents the complete cable assembly, consisting of a 500 cm cable and two connectors, while the red line illustrates the results for a partial assembly with a 195 cm cable and one connector In the lower frequency range, the results vary with length due to the samples being electrically short Conversely, in the higher frequency range, where the samples are electrically long, both assemblies achieve the same minimum screening attenuation of 47 dB.
C.3.2.3 Assembly shorter than the measuring tube
When the assembly is shorter than the measuring tube, it can be extended using a well-screened connecting cable housed within a closed copper tube, known as the tube-in-tube method.
The extension tube functions as a resonator, a principle similarly applied in the measurement of connectors For more information, please refer to the detailed explanation regarding connector measurements.
Measurement of connectors
Usual RF connectors have mechanical dimensions in the longitudinal axis in the range of
Electrical long elements are defined within the range of 1 mm to 50 mm, resulting in cut-off frequencies of approximately 3 GHz or higher for standard RF connectors Frequencies exceeding this threshold are classified as electrically long.
Screening attenuation is only applicable in the frequency range above the cut-off frequency, where the components are electrically long Therefore, the screening attenuation of an RF connector can only be accurately measured at frequencies exceeding 3 GHz.
By using a RF-tight closed metallic tube to extend the RF-connector, an electrically long cable assembly is created, which lowers the cut-off frequency for measuring screening attenuation Connecting this extension tube directly to the connector under test allows for the measurement of the connector's screening attenuation along with its mated adapter Alternatively, if the extension tube is connected to the cable near the connector, the measurement reflects the screening attenuation of both the connector (and its mated adapter) and the transition between the cable and connector.
The connector remains electrically short; however, when combined with the extension tube, it demonstrates the screening attenuation characteristics of the connector when linked to a well-shielded cable.
47 47 assy cmpl assy part min
0, 1 1 1 0 1 00 1 000 assy cmpl assy part which has a screening effectiveness better than the one of the connector (or the transition between cable and connector) See also the explanation in C 3.3.2
Figure C.7 – Triaxial set-up with extension tube for short cable assemblies
Figu re C.8 – Triaxial set-up with exten sion tube for connectors
The measurement of RF connectors utilizes a triaxial setup in accordance with IEC 62153-4-3 and IEC 62153-4-4, enhanced by a RF-tight closed metallic tube This extension tube can be linked either to the connector being tested or to the screen of its connecting cable At the opposite end, the connector under test is attached to the screening cap of the triaxial test setup through its mated adapter.
The measurement of the screening attenuation itself is the same as the measurement of cable screens according to I EC 621 53-4-4
In this initial approach, short pieces of coaxial cable with a 75 Ω impedance and foam PE dielectric were measured instead of using a connector, eliminating the influence of mating adapters and transitions The cable features a single braid screen that is under-braided Simulations were conducted using equations (C.7), (C.8), and (C.9) with two sections, where the first section consists of the connecting cable along with the RF-tight extension tube.
The transfer impedance and capacitive coupling impedance of the section are disregarded The cable under test has the following parameters: a direct current resistance of 8 mΩ/m, magnetic coupling of 0.6 mH/m, capacitive coupling of 0.02 pF/m, and an impedance of 75 Ω.
The comparison between simulation results and measurement data demonstrates a strong correlation, particularly in the lower frequency range where samples are electrically short In contrast, the higher frequency range reveals the impact of the extension tube The 10 cm sample remains electrically short across the entire frequency spectrum, with a cut-off frequency of 5.9 GHz, leading to an increase in coupled power as frequency rises However, the quasi cable assembly, consisting of the connector and extension tube, becomes electrically long above 590 MHz, resulting in a constant maximum coupled power Notably, the maximum coupled power remains unaffected by sample length, as illustrated by the simulated results of a 4 cm sample in both 1 m and 2 m tubes, which show identical envelope curves.
Figu re C.9 – Simul ation , logarith mic freq u en cy scale Figu re C.1 0 – M easu rement, logarithmic frequ ency scale
Figure C.1 1 – Si mul ation , linear frequen cy scale Figure C.1 2 – M easu remen t, linear frequ ency scale
SB 1 0 cm in 1 0 cm tube
SB 1 0 cm in 1 0 cm tube
SB 1 0 cm in 1 0 cm tube
SB 1 0 cm in 1 0 cm tube
Figure C.1 3 – Si mul ation , logarith mic frequ en cy scale Figure C.1 4 – simu lati on, linear frequen cy scale
Conclusion
Customers increasingly prefer screening effectiveness values in decibels (dB) over transfer impedance values in mW or mW/m for RF cables, cable assemblies, and connectors The tube-in-tube method addresses this demand by providing a straightforward and reliable way to measure the screening attenuation in dB for connectors and cable assemblies This method extends the shielded screening attenuation (long triaxial) test setup as outlined in IEC 62153-4-4.
The comparison of the measured and the calculated curves show good concordance
The advantages of the tube in tube method for connectors and assemblies are the same as for the measurement of the screening attenuation of cable screens in the tube:
• simple and easy test set-up;
• insensitive against electromagnetic disturbances from outside;
Annex D (informative) Influence of contact resistances
Contact resistances between the feeding cable and the extension tube or screening case in the test head can significantly affect test results It is essential to prepare contacts with low resistance or low impedance, ensuring they are established around the entire circumference of the screen Critical contacts are illustrated in Figure D.1.
Figure D.1 – Con tact resistances of the test set-up
The equivalent circuit of the complete test setup, which includes contact resistances, is illustrated in Figure D.2 It is essential to design the test setup so that the contact resistances of the extension tube are in series with the input impedance of the receiver, while the contact resistance of the screening case, along with the matching load of the Device Under Test (DUT), is in series with the generator.
R 1 , R 2 and R 3 contact resistances depicted in Figure D 1
Z câbl e characteristic impedance of the connecting cable (see Figure B.1 )
Z DU T transfer impedance of the DUT
Figure D.2 – Equivalent circuit of the test set-up
In this case, contact resistances of a few mΩ in series with the 50 Ω input resistance of the generator or the receiver are negligible
To ensure accurate measurements, the test setup must be configured to prevent contact resistances from being in series with the transfer impedance of the Device Under Test (DUT) If contact resistances are allowed to be in series, they can significantly affect the results.
[1 ] IEC 621 53-4-8, Metallic communication cable test methods – Part 4-8: Electromagnetic compatibility (EMC) – Capacitive coupling admittance
[2] IEC 621 53-4-9, Metallic communication cable test methods – Part 4-8: Electromagnetic compatibility (EMC) -Coupling attenuation of screened balanced cables, triaxial method
[3] IEC 621 53-4-1 0, Metallic communication cable test methods – Part 4-10: Shielded screening attenuation test method for measuring the Screening Effectiveness of Feedtroughs and Electromagnetic Gaskets
[4] IEC TR 621 53-4-1 6, Metallic communication cable test methods – Part 4-16: Technical report on the relationship between transfer impedance and screening attenuation (under consideration)
Breitenbach, Hähner, and Mund (1998) presented a study on the screening effectiveness of cables in the MHz to GHz frequency range, highlighting an extended application of a straightforward measuring method This research was discussed at a colloquium focused on screening effectiveness measurements held at Savoy Place, London, on May 6, 1998.
[6] HÄHNER T., MUND B., "Test methods for screening and balance of communication cables", 1 3th international Zurich EMC Symposium, February 1 6-1 8 1 999
The article by Halme and Kytönen, presented at the Colloquium on Screening Effectiveness Measurements in London on May 6, 1998, discusses the fundamental aspects of electromagnetic (EM) screening behaviors and the measurement techniques for coaxial and symmetrical cables, as well as cable assemblies and connectors.
[8] HALME L., SZENTKUTI, B, "The background for electromagnetic screening measurements of cylindrical screens", Tech Rep PTT(1 988) Nr 3
[9] KLEIN W., "Die Theorie des Nebensprechens auf Leitungen", (German), Springer
[1 0] MUND B., “Measuring the EMC on RF-connectors and connecting hardware, Tube in tube test procedure”, Proceedings of the 53rd IWCS/Focus 2004, Philadelphia, USA