Design and characterization of printed circuit magnetic probes

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Measurement methods of PCB emissions

The electromagnetic emissions from a PCB can be classified as conducted emissions and radiated emissions or common mode and differential mode emissions. The conducted emission measurements are either a voltage-capacitive-tap type of measurement or they are a current-clamp type of measurement. On the other hand, the radiated emission measurements are unique in that they must always state “the horizontal distance from the Device-under-Test (DUT) to the receiving antenna.” This horizontal distance can be 1.5,5,10 or 30 meters[3].
TEM cell method: The radiation level of a device under test can be measured using a TEM cell as shown in Figure 1-4[4]. The DUT faces the interior of the cell while the support circuitry is maintained outside the cell. The RF voltage appearing at the input of the connected spectrum analyzer is related to the electromagnetic potential of the DUT. The measurements are repeated for at least two orientations of the DUT in order to capture all the radiations.
Surface scan method: the radiated emissions from the DUT can be measured by scanning with a probe above the DUT, as shown in Figure 1-5. The measurement result of the surface scan method provides not only the electromagnetic fields from the DUT but also the relative strength of the sources. A variety of probes such as electric, magnetic and optic probes are used for surface scanning.
Method of measurement the radiation pattern: the radiation pattern of a DUT in far field can be measured using a receiving antenna.
The DUT is mounted on a turntable and rotated through 360° to find the maximum emission direction as shown in Figure 1-6. The receiving antenna is scanned in height from 1 to 4m to find the maximum level. The DUT-to-antenna azimuth and polarization are varied through 360° during the measurement to record the radiation pattern of the DUT. The standard test procedure requires that the measurements are to be done on an open area test site or semi-anechoic chamber. A balanced dipole is used as the receiving antenna below 1 GHz, and a log-periodical antenna or a horn antenna should be used for tests above 1 GHz.
Reverberation chamber measurements: A reverberation chamber is an electrically large, highly conductive enclosed cavity equipped with one or more metallic tuners/stirrers whose dimensions are a significant fraction of the chamber dimensions. A typical measurement setup using reverberation chamber is illustrated in Figure 1-7. It is used to measure the total radiated power of a DUT. The DUT should be at least λ/4 from the chamber walls. The stirrers are rotated very slowly compared to the sweep time of the EMI receiver in order to obtain a sufficient number of samples. The mechanical tuners/stirrers can “stir” the multi-mode field in the chamber to achieve a statistically uniform and statistically isotropic electromagnetic environment. The receiving antenna measure either the maximum received power or averaged received power during a cycle of the stirrers. The recorded signals are then converted to the total radiated power and the free space field strength. The advantage of the reverberation chamber method is that it is able to measure the total field on all sides of a DUT without multiple test positions and orientations.
Direct coupling method: The simplified configuration of the 1Ω method for measuring the sum current in the common ground path is shown in Figure 1-8. The direct coupling method determines the conducted emissions from power and signal ports of a small electronic module especially an IC. RF currents developed across a standardized load is measured to allow indirect estimation of the emission level Workbench Faraday Cage (WBFC) method: IEC 61967-5 defines a method of measuring the conducted electromagnetic emissions at defined common-mode points in order to estimate the emissions radiated by connected cables. The Faraday cage is typically a metallic box of 500mm x 300mm x 150 mm. A typical workbench of Faraday cage method is shown in Figure 1-9. DUT is mounted on either an IC EMC test board or an application board if it fits inside the WBFC. With all input, output, and power connections to the test board filtered and connected to common-mode chokes, the conducted noise is measured at PCB locations specified by the standard. Magnetic probe method: IEC 61967-6 defines a method for calculating the conducted emissions from an IC pin using a magnetic field probe to measure the magnetic field associated with a connected PCB trace. The schematic of the measurement setup is shown in Figure 1-10.
A magnetic probe is used to measure the magnetic field associated with a connected PCB trace, and the RF currents inside the circuit are then calculated. The preferred test configuration is with the DUT mounted on a standard EMC test board to maximize repeatability and minimize probe coupling to other circuits. The features of all these PCB emission measurement methods are summarized in Table 1-1[4].

Near field measurements

Methods such as modulated scattering technique, electro-optic sampling, electron beam probing (modified scanning electron microscope), and perturbation method are already available for the identification and localization of sources. [5], [6] proposes source location estimation technique with the modulated scattering technique (MST) for indoor wireless environments. The uniform circular scatterer array (UCSA) that consist of five optically modulated stutterers as array elements and a dipole antenna at the center of the UCSA is employed for estimating a source location from the impinging signal, but these techniques require expensive components, complicated configuration and low signal to noise ratio.
Electromagnetic near field scanning is a general method for identifying the source of electromagnetic interference (EMI) in electronic circuits. Various methods have been developed by many authors for the calculation of far field pattern leading to the source identification from the near field measurements [7]–[11]. Thus, it is necessary to know the electromagnetic fields in their close environment.

Near field and far field

The space surrounding an antenna is divided into three (1) reactive near field (2) radiative near field and (3) far field. Reactive near field is that portion of the near field region immediately surrounding the antenna whereas in the reactive near field predominates. For most antennas, the outer boundary of this region is taken into exist as at a distance R<0.62D3/ λ.
Radiating near field region is defined as that region of the field of an antenna between the reactive near field region and the far field region where the radiative field predominates and the angular field distribution is dependent upon the distance from the antenna.
Far field region is defined as that region of the field of an antenna where the angular distribution is independent of the distance from the antenna. If D is the maximum overall dimension of the antenna, the far field region is at a distance greater than 2D2/λ, where λ is the wavelength [12].

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Why near field measurements?

The electromagnetic emissions can be measured in either near field or far field. The near field measurements have advantages in accuracy, reliability, cost and application range compared with that of the far field measurements. The effect of some uncertain factors such as weather, scattering, electromagnetic interference has less influence on the measurement because the probe and the DUT are very close to each other and hence it gives a more accurate measurement[13][14].
• The near field measurement is less dependent on test conditions. It can be conducted in normal lab environments rather than an OATS or an anechoic chamber. This makes the technique highly feasible.
• Unlike the far field measurements, which give the radiation in far field, near field measurements can be used not only to obtain electromagnetic fields, but also to provide the emission tests and source diagnostics in EMC studies of PCBs and ICs. It can be used to estimate the current on the microstrip trace of a PCB or to locate the fault in the high frequency chip[15].
• Near field-far field transformation methods and equivalent source methods enhances the application and popularity of the near field measurement method.
Depending on the applications, the near field measurements can be classified into antenna near field measurements and EMC/EMI near field measurements. Antenna near field measurements are performed in the radiating near field range, typically in the range 3λ to 5λ and are focused on the determination of far field pattern from near field. EMC near field measurements are performed in the highly reactive region (typically <λ/6) and are focused on the determination of real or equivalent radiating sources in the DUT.

Objective of the thesis

The major objective of this thesis is to predict the radiated emissions from a PCB from the near field scan measurements. The steps toward this can be divided into two main tasks.
1) Design of a high resolution and high sensitivity printed circuit magnetic probe for the near field scan measurements.
2) Development of the inverse algorithm for equivalent source representation and prediction of the electromagnetic compatibility.
The state of the art about the near field probes and source reconstruction methods is described in the next chapter (chapter 2) of this thesis.
Chapter 3 of this thesis deals with the design and characterization of printed circuit magnetic field probes for near field measurements. The main goal is the design of the low cost shielded magnetic probes in the printed circuit technology towards the achievement high spatial resolution (of the order of 100µm). The thesis describes the problems encountered while miniaturization and the solutions we put forward towards the goal. The required frequency band of operation is from 1 MHz – 1 GHz, which is set by the definitions of the project (LOCRAY). The thesis also focuses on the design and characterization of active probes in order to achieve high sensitivity. The effect of the position of the LNA on the probe and the effect of the gain of the LNA are also made under study. The performance of a 3 axis probe which can measure the 3 components of the magnetic field is also analyzed.
Chapter 4 describes the source reconstruction methods from near field scan data for the prediction of radiated emissions from the PCB. The inverse transmission line based on the inverse propagation of electromagnetic probes is used to find the field distribution at the plane of the source. The inverse algorithm is meant to use the near field scanned data measured by the designed probe. This chapter also describes a new method based on the 2D cross-correlation. This method represents the current sources on the PCB by an equivalent current. This chapter contains the validation for elementary current sources and simple PCBs on air. Finally, the radiated far field of the circuit is predicted using the equivalent current. In this, we use the time reversal property of the TLM matrix in order to reverse propagate the electromagnetic waves and reconstruct the radiating source.

Table of contents :

1. Introduction
1.1 Need of EMC in electronic components
1.2 Measurement methods of PCB emissions
1.3 Near field measurements
1.3.1 Near field and far field
1.3.2 Why near field measurements?
1.4 Objective of the thesis
1.5 References
2. State of the art
2.1 Near Field Probes
2.1.1 Electro-optical probes
2.1.2 Electromagnetic probes
2.2 Synthesis of radiating sources
2.3 Conclusion
2.4 References
3. Design and characterization of printed circuit magnetic probes
3.1 Introduction
3.2 Parameters of the probe
3.3 The near field test bench
Reference circuit
3.4 Printed magnetic field probes
3.5 Printed circuit probes type 1 (with microstrip line)
3.5.1 Simulation model
3.5.2 Simulated results
3.5.3 Effect of the structure of loop
3.5.4 Effect of the transmission line
3.5.5 Characterization of microstrip probes
3.6 Printed circuit probes type 2 (with CPW line)
3.6.1 Coplanar waveguide (CPW) design
3.6.2 Grounded coplanar waveguide (GCPW) design
3.6.3 Design of coplanar probes
3.6.4 Characterization
3.7 Response of the loop to currents and voltages on the trace
3.8 Miniaturized Shielded magnetic probe
3.8.1 Design of the shielded probe
3.8.2 Equivalent circuit model
3.8.3 Characterization
3.8.4 Effect of the length of the probe and arm of the measurement system
3.9 Design and characterization of 3 axis magnetic field probes
3.9.1 Design and characterization
3.10 Active magnetic probes
3.10.1 Design of active probe
3.10.2 Characterization
3.10.3 Positioning of the amplifier on the active probe
3.11 Conclusion
3.12 References
4. Source reconstruction methods for the prediction of EMC
4.1 Introduction
4.2 EM source reconstruction using inverse- TLM method
4.2.1 Time reversal- an overview
4.2.2 Transmission line matrix (TLM) method
4.2.3 Inverse TLM method
4.2.4 EM source synthesis with reverse TLM method- state of the art
4.3 Equivalent source from near field data using Inverse TLM Method
4.3.1 Excitation signal
4.3.2 Reconstruction of ideal sources
4.3.3 Effect of near field scanning height on source reconstruction
4.3.4 Spatial resolution
4.3.5 Application of frequency domain NFS data to time domain TLM method
4.3.6 Calculation of equivalent source
4.3.7 Limitations of source reconstruction using Inverse TLM method
4.4 Equivalent source by 2D cross-correlation method
4.4.1 2 D cross-correlation method
4.4.2 Formulation of the equations
4.4.3 Algorithm for finding the equivalent source
4.4.4 Validation by application to elementary currents
4.4.5 Application to PCBs on air
4.5 Prediction of the radiated field
4.5.1 Far field integral
4.5.2 Radiation from monopole
4.5.3 Predicted field of bend monopole
4.6 Conclusions
4.7 References
5. Conclusions and perspectives
List of publications


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