**6. Practical implementation of radiated spurious near-field multiprobe measurement system**

To validate the system for radiated EMC measurements, two experiments have been performed. In those experiments, different post-processing techniques are used to improve the results. The same reference DUT sample is used in both cases: this consists of a PCB of 150 mm by 225 mm with a substrate thickness of 2 mm. One of the traces of the PCB is excited through an external transmitter, while the other ones are excited through coupling. The DUT represents a bad radiator, so it is a good sample of a typical EMC device.

The first experiment was done in MVG Italy by using StarLab in one of the possible EMC configurations: a reference antenna independent from the arch and SDR receiver. The DUT was also measured in a certified laboratory. In particular,

## *EMC Measurement Setup Based on Near-Field Multiprobe System DOI: http://dx.doi.org/10.5772/intechopen.99604*

the DUT was measured in CATECHOM [23] (University of Alcalá, Spain). According to the technical specifications marked by the European Standards (EN), the laboratory is certified to perform EMC measurements, providing the certification process required for the CE mark of a product. A comparison of both configurations can be seen in **Figure 6**.

The characteristics of each measurement solution are described in **Table 1**. The main difference is the measurement time, which can be drastically reduced with the multiprobe EMC solution while keeping a good accuracy, as will be seen. The processing time for the NF transformation to 3/10 meters distance is negligible.

In order to measure the multiprobe EMC system, an electric sleeve dipole was used as a reference antenna. Three different signals are compared: as a reference, the measurement in the conventional multiprobe configuration of Starlab (45 cm distance), using the vector network analyzer to feed the DUT and thus, having access to the reference signal to measure the amplitude and phase. Then, the multiprobe EMC configuration measurement is performed, where the reference antenna does the phase retrieval, and the SDR is used to reconstruct the amplitude and phase near-field pattern. Finally, the conventional EMC laboratory (CATECHOM) measurement at a 3 meters distance is done. The flowchart of the comparison procedure and post-processing steps are shown in **Figure 7**.

The results herein presented correspond to 2 GHz. In the flowchart, CST [24] was used before performing the final comparison. This was necessary to include the effect of the wooden table in the device's radiation when measured in the certified EMC laboratory. This is also an added value of the multiprobe EMC solution since the radiated emission can be computed, including different scatterers around the DUT. Moreover, source reconstruction was done by using Insight [19]. This source reconstruction is able to calculate the currents on the PCB structure, filtering out all the contributions out of the PCB itself. As shown in [2], this technique can improve the results of the measurements. This allows comparing how accurate the EMC multiprobe solution could be compared to the ideal source reconstruction that can be achieved with the vector network analyzer measurements.

#### **Figure 6.**

*EMC test-case: Compliance laboratory (CATECHOM), and multiprobe EMC system.*


#### **Table 1.**

*Measurement characteristics: Conventional EMC measurement and multiprobe EMC solution.*

The radiation pattern results at 3 m are shown in **Figure 8**. In this case, from the near-field, the sources are reconstructed, and once these currents are calculated, the electromagnetic field at 3 meters is calculated using the commercial software CST for both horizontal and vertical polarizations. It was done in this way to consider the table used to support the PCB. The results show that the difference between using a conventional VNA (most accurate system) and the SDR platform is minimal. This opens the possibility of low-cost receivers for this kind of system. Second, the differences with respect to the measurements with a conventional EMC setup are within the uncertainty of these systems. The main advantages of using near-field systems are the lower uncertainty in the measurement process, due to the easier control of the environment, and the possibility of including some external setups, using commercial electromagnetic software such as CST.

Nevertheless, the peak error for the maximum of the radiation is below 2 dBV/m. The comparison of the pattern is suggesting that the angular variation of the radiated emission is appropriately reconstructed. This would be translated into a good correlation of the currents' distribution. The source reconstruction comparison between the measurements done with the vector network analyzer and the multiprobe EMC setup can be seen in **Figure 9**.

Another experiment was performed in the MiniLab system of Microwave Vision Group in Pomezia (Italy). This system was used in a different architecture for EMC multiprobe solutions (**Figure 2**), with the on-axis top-probe used as a reference channel. In order to measure with this architecture, hardware modifications are needed since one of the connected signals to the conventional switching matrix of a multiprobe system is connected to one input of the SDR receiver. Thereby, a

**Figure 7.** *Process for comparison of the measurements of the PCB structure.*

#### **Figure 8.**

*Comparison between conventional measurement and measurement using near-field procedure: With conventional VNA and SDR platform (H component left and V component right).*

*EMC Measurement Setup Based on Near-Field Multiprobe System DOI: http://dx.doi.org/10.5772/intechopen.99604*

**Figure 9.** *Source reconstruction on top of DUT.*

calibration is needed in order to account for differences between the switching matrix RF path and the direct connection of the on-axis probe.

The goal of the experiments was to verify the performance of the multiprobe EMC solution based on the on-axis probe as a reference when it comes to modulated EMC signals. Some other experiments were performed with continuous-wave signals to validate the system. As was the case for the antenna independent from the measurement arch, good results were obtained, and some results can be found in [11].

It is well known that some DUTs could work with modulated signals. The extrapolation of radiation parameters for modulated signals and how to measure them is not that clear [25]. Some experiments have been done in order to validate EMC multiprobe measurements of modulated signals. In that sense, let us assume there is an IoT device transmitting a modulated signal. Let us also assume that the signal is LTE FDD type. Emulation of this scenario was done by exciting a known antenna with a modulated LTE FDD signal of a predefined bandwidth (see **Figure 10**).

**Figure 10.** *On-Axis architecture of multiprobe EMC system for LTE measurements.*

The SDR was optimized in order to extract the time domain near-field amplitude and phase of the transmitted signal. In the particular case of the results herein presented, an LTE FDD signal of 5 MHz bandwidth is analyzed. The parameters extraction is done in a similar way it is done for a continuous wave signal. For the power, the spectrum under interest is integrated. For the phase, linearity is assumed in such a way that the average (intermediate) phase over the whole bandwidth represents, ideally, the radiation pattern of the DUT at the central frequency. This statement is mathematically described by Eq. (7). In the equation, the measured phase at equispaced frequencies from the central one (fc) cancels out, giving the measured phase at the central frequency as a result. This is true under the assumption that the radiation pattern of the DUT is not changing over the given bandwidth, which is true for most practical cases.

$$\begin{split} \frac{1}{N+1} \sum\_{i=-N/2}^{N/2} \phi\_{meas} \left( f\_c + i \Delta\_f \right) &= \phi\_{meas} \left( f\_c - \frac{N}{2} \Delta\_f \right) + \dots + \phi\_{meas} \left( f\_c \right) + \dots \\ &+ \phi\_{meas} \left( f\_c + \frac{N}{2} \Delta\_f \right) \end{split} \tag{7}$$

The experiment was conducted at 1 GHz. First, the reference continuous wave signal radiated by the antenna was measured. Then the LTE signal of different bandwidths was measured using the EMC multiprobe architecture and the parameters at the central frequency extracted. Some comparisons for different bandwidths can be seen in **Figure 11**. The continuous wave (CW) is the reference curve. A deeper analysis of the error pattern can be seen in **Figure 12**. The near-field mean error is very low for both components, below �45 dB. This shows the low error introduced for the field reconstruction.

The correlation between the different signals is very good, demonstrating that the technique used could be suitable to characterize the radiation of EMC devices when modulation is applied. The optimized measurement corresponds to the results

**Figure 11.** *Main cut near-field reconstruction of LTE signal for different bandwidths.*

### *EMC Measurement Setup Based on Near-Field Multiprobe System DOI: http://dx.doi.org/10.5772/intechopen.99604*

obtained when optimizing the post-processing steps for retrieving the amplitude and phase information of the modulated signal. The same procedure would be applied afterward: near-field to near/far-field transformation at 3 or 10 meters to compare with EMC standards and diagnostics. In this case, the EMC measurement process is explained in **Figure 3**. This test was done on the same PCB used in the previous experiment and was used to validate the performance with a low signal-tonoise ratio, including different signal attenuations, and lower frequencies (400 MHz). Spherical modes are calculated from the acquired near-field, and some spherical mode filtering is applied. This spherical mode filtering consists of canceling those modes that cannot correspond to the signal itself.

The results are shown in **Figure 13**, where the power of the different spherical modes (under m and n index) are shown for the reference case, 30 dB and 65 dB of extra attenuation. It is observed that for 30 dB attenuation, the results are very good, and even for 65 dB attenuation, the results are acceptable. This is also reflected in the right part of **Figure 13**, where the field for different theta angles is shown. Again, the differences are within the typical uncertainty values for EMC setups, even with very low power values, and a frequency lower than the specifications of the multiprobe system for antenna measurements.

#### **Figure 12.**

*Near-field error between CW signal and EMC multiprobe measurement of 5 MHz LTE FDD signal left (Eϕ), right (Eθ).*

**Figure 13.** *Measurements at 400 MHz, using spherical near-field transformation and mode filtering for different attenuations of the received signal.*

These results for EMC multiprobe solution for modulated signals show the potential of this setup, not only for traditional EMC measurements where the spurious emissions are characterized at particular frequencies, but also represents a low cost, accurate and fast solution for addressing pre-compliance of new self-transmitting devices using modulated signals.
