Near-Field Planar Characterisation Applied to the Design of Digital and Radiofrequency Systems

Near-field planar characterisation is a very attractive tool for the circuit designer dealing with digital and radiofrequency systems. The characterisation method is firstly introduced, and its key points are underlined. The power of nearfield mapping is illustrated by the means of two examples coming from the digital as much as from the radiofrequency areas. Keywords-characterisation; electromagnetic compatibility; mapping; near-field; radiofrequency systems. I. NEAR-FIELD PLANAR CHARACTERISATION The observation of the electromagnetic field on the surface of a device allows the collecting of a quantity of information relating to the complex phenomena arising within a system when operating under its normal conditions. The planar mapping of the electromagnetic field over a given surface shows to have several advantages over other more classical characterisation methods used in radiofrequency and microwaves: the vectorial network analysis is for instance based on the measurement of global entities, needing the introduction and the calibration of ports with well defined measurement planes. This procedure become rapidly cumbersome when the number of input/output pins increases as it is the case of digital circuits on a printed circuit board. Meanwhile, the proposed characterisation, as it operates without contact, by using a collecting probe sensitive to a given field component, makes it possible to analyse directly the device under its working conditions. Moreover, the fact that the characterisation is performed in near-field zone permits the rendering of complex phenomena, as the interaction of the different modes, may they be of propagating or evanescent type, and makes it possible to model the radiating of electrical and magnetic sources. More specifically, it is also thinkable to determine with this kind of measurement method the radiated emission of the electromagnetic fields over a printed circuit board and to solve the problems dealing with electromagnetic compatibility [1-2]. The wealth of information comes from the point that it is of geographical nature, localised within a plane, thus it helps to associate the observed phenomenon with the different components or the geometry of the studied system. This method of near-field characterisation can be based on measurements using a near-field scanner (see Fig. 1), or on 3D field simulation with the availability of modern and powerful CAD tools, as their methodology and goals are the same: the planar mapping of the electromagnetic phenomena. Figure 1. Measurement setup of the planar near-field scanner with positioner, spectrum analyser and controlling computer The near-field probe is an essential element within the measurement system [3-4]. This probe can be of current loop type or of voltage monopole/dipole type, and will be sensitive to a specific component of the electromagnetic field: Fig. 2 shows some examples of realised probes at IETR. Two essential parameters which are characterising a near-field probe are the resolution and the sensitivity. If the sensitivity is proportional to the size of the probe, as the electromagnetic field is integrated along the structure, the resolution is better when the probe is smaller. When designing a near-field probe, a compromise has to be found between these two parameters, the resolution and the sensitivity. The realised probes have a length of 10mm, the width of the rectangular probe is of 6mm. The singularity introduced at the end of the triangular probe permits to increase somehow its resolution. The measured return loss of the built probes is presented in Fig. 3 within the frequency band of 0 GHz to 8 GHz. They all show a good behavior up to 4 GHz. At higher frequencies however, there are some resonance effects which are appearing, due to the size of the probes. Figure 2. Near-field probes of current loop type (Hxy) and voltage monopole type (Ez) Figure 3. Return loss of the realised near-field probes The near-field probe used for the electromagnetic field mapping is connected to the measurement device which can be, depending on the case, a spectrum analyser or a vectorial network analyser. The spectrum analyser is used for the characterisation of independent systems producing by themselves the radiating sources. The vectorial network analyser is very interesting as it is possible to normalise the amplitude and the phase of the measure, by an injection of a reference in the device under test. II. APPLICATION TO THE DESIGN OF DIGITAL AND RADIOFREQUENCY SYSTEMS The near-field characterisation method is illustrated by means of two examples coming from different areas: the measurement of radiated emission of the electromagnetic field over a digital printed circuit board, thus dealing with a electromagnetic compatibility problem, and the characterisation of the physical phenomena occurring in a microwave filter, in order to asses the circuit designer. A. Digital printed circuit board The near-field characterisation is applied to a printed circuit board (Fig. 4), built around an integrated circuit of FPGA type (ALTERA EPM7032), a voltage regulator, various decoupling capacitors for the biasing, and a RAM memory. The circuit is fed by a clock signal of 10 MHz with the help of an external signal generator. Figure 4. Digital printed circuit board built around an FPGA from ALTERA, a voltage regulator, decoupling capacitors, and a RAM memory The measurement of the electromagnetic field is made in a plane located at a height of 5mm above the level of the board, on the side of the components, for ease of identification with the different parts; however it is also possible to perform the measurement on the side of the backplane. As the frequency is of 10 MHz and the indicated height of 5mm, the measurement is really done in the near-field reactive zone. As the components have some relief, the chosen height cannot be smaller than a certain amount. The BNC entry of the coaxial cable for the clock signal also restricts the measurement area which can be scanned by the probe. The near-field is sampled on a surface of 60mm * 100mm with a step of 1mm in both directions. The used probe is of rectangular current loop type and is oriented in the x-direction. The measurement is made with means of a spectrum analyser, and the measured power is proportional to the Hy component of the electromagnetic field. The field mapping is done at the fundamental frequency of the clock signal (10 MHz), as much as at the fifth harmonic (50 MHz), and their respective maps are shown in Fig. 5 and 6 for both frequencies. Figure 5. Near field measured on the surface of the digital printed circuit board for the fundamental frequency of the clock signal (10 MHz) Figure 6. Near field measured on the surface of the digital printed circuit board for the fifth harmonic frequency of the clock signal (50 MHz) For the fundamental frequency of 10 MHz, the signal dynamic is of 30 dB and the maximum of power is of –85 dBm. For the fifth harmonic, the signal dynamic is of 40 dB and the maximum of power is of –66 dBm, almost 20 dB greater than at the fundamental frequency. The electromagnetic field concentrates over the principal devices (FPGA, voltage regulator, memory, decoupling capacitors,.), with some variations on the highest measured levels depending on the considered frequency. The limitation of the resolution is due to the size of the probe (its width is of 6mm), however for our working frequency (10 MHz), this is largely enough for sizing all the variations of the electromagnetic field. The sampling step has voluntarily been chosen small compared to the size of the probe, thus corresponding to a spatial oversampling of the measurement.