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TDS Technology

The TDS technology can be summarized as a fusion of power-over-fiber and radio-frequency (RF) over fiber technologies. SPEAG has advanced these two technologies for integration into miniaturized fully isolated active optical probes for near electromagnetic field measurements.

The detailed functional principle of the TDS technology is summarized in the block diagram. Every TDS probe contains a sensor head and a sensor ID. The sensor head is located at the very tip of the probe and contains the actual electric- or magnetic-field sensor while the sensor ID is located in the rear of the probe body. The function of the sensor ID is to uniquely identify each probe to the remote unit and to provide a redundant optical link, which is continuously monitored for LASER safety. 

The remote unit acts as the optical power supply in the power-over-fiber forward link. At the sensor head, the photonic energy is converted into electrical energy from which the active elements in the sensor head are supplied. The sensor head uses electrically small transducers to pick up the electric (E-) or magnetic (H-)fields. The RF signal from the transducer is amplified by a low-noise amplifier (LNA) and modulates the optical output of a high-speed vertical cavity surface emitting LASER (VCSEL). The optical signal from the VCSEL is then transmitted to the remote unit over an optical fiber. At the remote unit, the optical signal is demodulated by means of a high-speed photodiode (PD), amplified by a trans-impedance amplifier (TIA), and made available over a standard 50 Ω output to connect a standard measurement receiver such as an oscilloscope or spectrum analyzer.

Basically, the TDS system can be regarded as a miniature, broad-band, optically isolated antenna. The antenna factor, the frequency-dependent transfer function that converts the output from the remote unit in dBm to an H-field (in dBA/m) or an E-field (in dBV/m) is made available with the calibration certificate of each TDS probe.

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tdssystemblockdiagram.png

 

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.

 

However, the basic principle has for decades remained the same, which means that the presence of the probe when brought very close to the device under test (DUT) introduces an arbitrary EM boundary condition. In this application note, we assess the adverse effects of traditional (metallic) EMC probes on the DUT in comparison to near-field sniffing performed with SPEAG’s EM-transparent microphotonic TDS SNI probes.

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
Schmid & Partner Engineering AG (SPEAG (Switzerland)), in cooperation with the IT’IS Foundation(Switzerland), have developed an active microphotonic near-field probe system that uses direct laser modulation for signal transmission and electrically small transducers to pick up the electric (E) or magnetic (H) fields. The system consists of a sensor head and a remote unit (Figure 5), as described in [3]. The actual sensor head is located at the very tip of the probe and contains miniaturized electric or magnetic field transducers. The remote unit acts as the photonic power supply via a power-over-fiber forward link. At the sensor head, the photonic energy is converted into electrical energy, from which the active elements in the sensor head are supplied. The sensor head uses electrically small transducers to pick up the EM fields.

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.
The RF signal from the transducer is amplified by a low noise amplifier (LNA) and modulates the photonic output of a high-speed vertical cavity surface emitting LASER (VCSEL). The signal from the VCSEL is then transmitted to the remote unit via fiber optics. At the remote unit, the photonic signal is demodulated by means of a high-speed photodiode (PD), amplified by a transimpedance amplifier (TIA), and made available over a standard 50 Ω output to connect the EMC receiver. This sensor concept allows highly sensitive, fully electrically isolated miniature near-field sensors with a large bandwidth of 10 MHz – >10 GHz to be designed. Because of the large bandwidth and optimized flat frequency domain response, i.e., that presents a non-dispersive system for wide-band signals, we call our sensor implementation time-domain sensor – TDS – probes. The typical test setup for use of the TDS EMC sniffer probes is shown in Figure 6. It is obvious that, with TDS probe technology, no metallic parts are introduced close to the DUT.

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

H. Whiteside and R. King, "The loop antenna as a probe," IEEE Trans. Antennas and Propagation, vol. 12, pp. 291-297, 1964.

N. Ando, N. Masuda, N. Tarnaki, T. Kuriyama, S. Saito, K. Kato, K. Ohashi, M. Saito and M. Yarnaguchi, "Miniaturized thin-film magnetic field probe with high spatial resolution for LSI chip measurement," in InternationalSymposium on Electromagnetic Compatibility, Santa Clara, 2004.

A. Kramer, P. Muller, U. Lott, N. Kuster and F. Bomholt, "Electro-optic fiber sensor for amplitude and phase detection of radio frequency electromagnetic fields," Optics letters, vol. 31, no. 16, p. 2402–2404, 2006.

 
 
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