Principles

Resonance increases the coupling from the field to the circuits, decreases the immunity, and enhances the emission of system. It is often the "missing link" between system level performance and local level. Therefore, identifying resonances is an important step in understanding immunity sensitivities and emission maxima. Two orthogonal (or decoupled) probes are used to measure S21. If a resonating structure is present, the S21 value will be very high. In the absence of a resonating structure, the S21 value will be low.

Test structure of resonance scanning partically overlayed with a scan result.

Applications

With EMI Scan

First, a resonance scan is performed. For this scan the DUT is turned off. Thus, the resonance frequency, locations, and Q-factor of potentially resonating structures are identified. In the second step, the DUT is turned on and an EMI scan is performed. This determines which frequency components are present at which structure. EMI engineers know that resonating structures are much better antennas than non-resonating structures. Thus, in the third step one can identify which frequency components occur on resonating structures by comparing both scan results. If they occur on resonating structures, then a reasonable conclusion is that this resonating structure will contribute strongly to the EMI.

With Immunity Scan

A problem in immunity RF-sine wave scanning is that one cannot test every point at every frequency, as the frequency sweep at each point might take too long. For that reason, it is useful to narrow down the number of test points and test frequencies. Resonance scanning can help solve this dilemma. To start, the DUT is turned off. The resonance scan will determine the locations, resonance frequencies, and Q-factors of resonating structures. In the second step the DUT is turned on and it is observed for failures. The probes now inject RF signals (e.g. modulated RF signals via field coupling). It is reasonable to only test at locations and frequencies at which resonances have been detected. The rationale is that the external field (in a standard immunity test) would couple best to structures that are in resonance. This way one can reduce the test time of RF immunity sine wave scanning drastically.

Required Equipment

● SmartScan-RS
● VNA or SA with tracking
● Resonance probes

Resonance Probes

The probe is a key element of the resonance scan. It excites a DUT and measures the response, which will be much larger at resonance condition. The excitation and responding fields must be decoupled.