Magnetic Near-Field Probes With High-Pass and Notch Filters for Electric Field Suppression

Several new types of low-cost and robust magnetic near-field probes manufactured in low-temperature co-fired ceramics (LTCC) are presented in this paper. Parallel C-shaped strips and their variations are inserted into the loop area in the front end of probes to achieve common-mode high-pass and notch filters for electric-field noise suppression. These probes with this kind of filter have excellent wideband electric field suppression. They are called high electric field suppression probes type A ~ D. The size of loop aperture in all probes is 100 μm long and 400 μm wide. The signal received from the loop is routed to a measurement apparatus through a semi-rigid coaxial cable with an outer diameter of 0.047 in. The flip-chip junction with low loss and good shielding is used between the probe head in LTCC and the semi-rigid coaxial cable. We take the probes over a 2000-μm-wide microstrip line as device-under-test to measure the probe characteristics. The isolation between electric and magnetic fields for a reference probe based on an old design using the same LTCC process is better than 30 dB from 0.05 to 12.65 GHz. The type A probe has two parallel C-shaped strips, it has better isolation of 35 dB from 0.1 to 11.05 GHz. Type C has one end of its strip shorted to ground, its 30-dB isolation frequency range can be extended to 0.05 ~ 17.8 GHz. With additional layout variation in type D, isolation can be improved to 40 dB up to 10.9 GHz. The spatial resolution for these probes is 140 μm when the distance between the metal surface of the microstrip line and the nearest edge of the loop is held at 120 μm. The calibration factors of the proposed probes are only slightly increased as compared with reference probe.

I. Introduction

Complicated electromagnetic interference (EMI) [1], [2] in high-speed, versatile, and small-sized electronic products is often caused by the fast switching operation of large scale integration (LSI) circuits. The rapid switching current usually disturbs other victims by coupling to an unpremeditated path or transmitting along an unideal ground loop. These mechanisms will increase the difficulty in directly detecting undesired Fig. 1.

(a) Coaxial cable shielded-loop probe. (b) Thin-film shielded-loop probe.

noise interference. In order to comply with the electromagnetic (EM) regulations, pinpointing the undesired emission sources is always the most efficient and low-cost solution during the development of a product. Thus, magnetic near-field loop probes [3]–[6] are developed to locate the EMI sources because of their capability to provide field mapping information associated with the current density in physical circuits. Fig. 1(a) shows a conventional coaxial cable shielded-loop probe with a gap on the shielding layer. This gap discontinuity should be located at the middle of the loop, and then the common-mode currents on the external surface of the shield, which are caused from the electric field in the -axis direction, can be cancelled based on the symmetric structure when the probe is placed perpendicular to the -plane. This kind of probe can be homemade inexpensively, but its size is usually too large for a chip-level circuit. Magnetic shielded-loop coils [7]–[11] are formed and manufactured in the thin-film, integration circuits, and printed circuit board (PCB) processes. A thin-film shielded-loop probe comprising an inner conductor and two shielded-ground plates is shown in Fig. 1(b). Since this tri-plates structure is applicable to modern electronic processes, the size of the loop aperture could be reduced to less than 100 , and thus the thin-film magnetic probe can achieve high spatial resolution to accurately pinpoint the interference sources. Some probes [12], [13] are proposed to reduce electric field coupling for high and wideband operation. The basis of these probes is to form a common-mode stop-band filter by adjusting the structure of the ground layer. Nevertheless, more electric field will directly couple into these probes through the unshielded loop of large area. If the very intense electric field is asymmetric for the probe with good shield, electric field with different magnitude relative to the middle gap may introduce asymmetric current distribution on the outside of shield [14]. The undesired common-mode voltage will still be induced from the unequal current. These kinds of probes with unshielded loop are not appropriate for the complicated system, or the larger sized probe with high ratio of loop area to signal metal width is needed to increase the electric and magnetic field isolation, but increasing the loop area will result in poor spatial resolution.