Non-Compliance Introduced by Conventional EMC Sniffer Probes Solved with Active Microphotonic Sensor Technology

Introduction

Figure 1: Numerical test setup to evaluate the influence of different probe types on the near-field distribution. Shown is the magnetic vector field on the top of a resonant structure: a) in the absence of a probe, b) in the presence of a simplified, isolated (photonic) near-field-probe, c) in the presence of a conventional EMC probe.

Handheld electromagnetic compatibility (EMC) and signal integrity (SI) sniffing has been the debugging tool of choice of EMC engineers for decades [1]. Originally developed for use at relatively low frequencies, traditional sniffing probes are used today to analyze the entire radiofrequency (RF) domain. Significant research has been invested in the design optimization of EMC probes to cover the microwave portion of the spectrum with micrometer resolution [2].

Figure 2: Resonance frequency of the DUT (Figure 1) a) in the absence of a probe, b) with an isolated probe, c) with a conventional EMC probe.

EMC Sniffer Probe Technology

Conventional Probes

Figure 3: Shown are different designs of conventional EMC probes. a) simple unshielded loop probe, b) loop probe with asymmetric shield, c) loop probe with symmetric shield, d) shielded loop probe with common-mode rejection. All probe designs shown are based on coaxial transmission lines, i.e., are conductive.

Figure 3 shows the designs of several commonly used conventional near-field EMC sniffer probes. As outlined in the introduction, the main optimization criteria in the past were in the areas of the frequency range, miniaturization, E-field, and common mode sensitivity suppression. Suppression of unintended reception, for instance, is achieved by the presence of a shielded loop (Figure 3b), ideally with a symmetrical shield (Figure 3c), and common-mode rejection, for instance by means of a bifilar wound choke (Figure 3d).

Figure 4: Schematic drawing of a conventional EMC probe used with an EMC receiver for near-field sniffing on a printed circuit board (PCB). Metallic components of the probe are in close proximity to the board under test.

However, in any existing probe design, the probe remains a conductor and hence becomes an arbitrary EM boundary condition. A typical test setup with conventional EMC sniffer probes is shown in Figure 4. It is obvious that, with this technology, a strong EM boundary condition is always introduced close to the DUT.

Active Microphotonic Probes

Figure 5: Block diagram of the active microphotonic sensor platform, consisting of a miniature sensor head that is linked exclusively via fiber optics to a remote unit

Figure 6: Schematic diagram of SPEAGs microphotonic TDS-SNI near-field sniffer probe used for EMC magnetic near-field sniffing (top). Schematic diagram of the TDS active microphotonic sensor technology.

RF EM Transparency of Conventional and Photonic EMC Probes

Figure 7: Experimental setup to evaluate the RF EM field perturbation induced by a conventional EMC probe. Shown is a SPEAG H1TDSx-SNI H- field probe used as a reference probe mounted on a support bracket, a 900 MHz wireless transmitter device, and a spectrum analyzer.

The experimental setup for the evaluation of the RF EM transparency of near-field EMC probes is shown in Figure 7. An H1TDSx-SNI H-field probe mounted on fixed support is used as a reference probe to detect the 900 MHz carrier transmitted by the DUT. The signal measured with the TDS probe is continuously monitored on a spectrum analyzer. In Figure 8, the DUT is approached with a medium size (10 mm)  conventional EMC loop probe, and the spectral field variation detected by the reference probe is continuously recorded.

Figure 8: A 10 mm conventional EMC probe positioned close to the PLL circuit of the DUT. The conductive probe elements -reroute the RF energy over the PCB into the PLL circuitry, causing additional harmonics that are detected with the H1TDSx-SNI probe on the tripod.

 When the conventional EMC probe is in the vicinity of the phase-locked-loop (PLL) circuitry of the DUT, the PLL is unlocked and the DUT produces additional harmonic content. This harmonic content, however, is not present in absence of the probe. In this case, the use of the conventional probe may lead to unnecessary re-design of the DUT circuit. The same experiment is performed with the H1TDSx-SNI probe in Figure 9. On the spectrum analyzer screen, only the fundamental DUT output signal is visible. The EM-transparent TDS probe does not differentially modify the local field distribution or introduce spurious emission artifacts as seen previously with the conventional probe.

Conclusions

Figure 9: Placement of the H1TDSx-SNI probe at the same position close to the DUT PLL. The dielectric EMC probe does not disturb the PLL, and no additional harmonic content is observed on the spectrum analyzer.

Our results show that the use of conventional EMC probes can lead to significant misinterpretation of the sniffing results when, for instance, the probe introduces PLL unlocking and hence unrealistic spurious emissions, while TDS SNI probes detect the true DUT signal without perturbation. Such false positives introduced due to improper near-field probes have the potential to prompt unnecessary, costly, and time-consuming device re-design.

Bibliography