Switchable Broadband-to-Tunable Narrowband Magnetic Probe for Near-Field Measurements

Abstract

This paper presents a printed magnetic probe that can switch from broadband to tunable narrowband for near-field measurement. In the early design stage, we created a printed loop gap resonator as a magnetic reference sensor for the pre-compliance test in a band up to 6 GHz. Consequently, the results showed a good response in terms of the S11 and S21 parameters of the proposed probe compared with the commercial magnetic sensor XF-R 3-1. The source noise might spread among different frequency bands, making the broadband magnetic probe the closest choice for estimating the magnetic field in the near-field region. Unfortunately, broadband magnetic probes have lower sensitivity than narrowband ones. One of the solutions to get high sensitivity is to connect the LNA to the output of the passive magnetic sensor. This work proposes a novel method to solve this issue using a PIN diode to change the broadband status into a high sensitivity narrowband status and then tune this narrowband across the most critical applications such as 3.5° GHz, 3.75° GHz, 4.8° GHz, and 5.2° GHz with the help of a varactor diode. Compared to the broadband status, an improvement of more than 10 dB has been obtained across all these wireless bands. Furthermore, the proposed structure’s isolation between the electrical and magnetic fields is about 13 dB.

A Two-Turn Shielded-Loop Magnetic Near-Field PCB Probe for Frequencies up to 3 GHz

“…The NFP2 probe which uses a folded line [ 1 ] to increase the probe self-inductance has a smooth response up to 2 GHz, after which the sensitivity increases considerably due to the resonance at 2.7 GHz and decreases above that frequency. When compared with resonant probes in [ 8 , 9 , 10 , 11 , 12 ] which have a high sensitivity in a narrow band around the resonant frequency, this probe also offers a higher sensitivity due to resonance but is suitable for broadband measurements up to the resonant frequency. It is important to note that because the NFP3 probe maintains the same outer loop dimensions as NFP2 and the conventional probe, the spatial resolution is not significantly affected by this design technique.…”

“…The initial design was optimized to increase the probe sensitivity in the frequency range from 10 MHz to 3 GHz. The probe initial loop area was selected according to similar probes in the literature [ 1 , 2 , 12 ] for the frequency range of interest. The probe sensitivity dependence on loop area is simulated using a parametric sweep to find the best fitting values for increased sensitivity in the analyzed frequency range.…”

“…Multiple probe designs were evaluated in [ 3 ] with the goal of increasing electric field suppression ratio of the developed probes with operating frequencies up to 17.8 GHz. A commonly used method to increase the probe sensitivity is by using the resonance phenomena as shown in [ 8 , 9 , 10 , 11 , 12 ]. Resonant probes incorporate a LC resonant circuit into their design by adding additional capacitance into the loop structure.…”