In many applications, the smaller wavelengths of high frequencies are beginning to approach the physical dimensions of many microwave cavities, which can lead to cavity resonance effects. The resonant frequency is the frequency where the integer half-wavelength corresponds to the physical dimension of the cavity. A wave is set up inside the enclosure whose nodes (i.e. zero amplitude) lie on the conductive walls of the enclosure. These structures behave as cavity resonators. The danger of a cavity resonance is that if a noise source has a frequency component that corresponds to a resonant point, then a large field can be generated at this frequency due to the multiplication or amplification effect by the “Q-factor”. One approach to reduce this phenomenon is to lower the “Q-factor” of the cavity by introducing losses (Q-dampening). MAST Technologies’ cavity resonance absorbers are specifically optimized to provide this dampening, reducing the cavity resonance or lowering the Q factor of the cavity.
As clock speeds and frequencies of components rise, so do emissions. Traditional metal shielding methods such as metal spring fingers and PC board shielding cans are effective at preventing emissions up to several GHz. However, as wavelengths get shorter, any aperture in a traditional shield may be a leakage point. EMI absorbers can be used in conjunction with traditional shielding solutions to effectively reduce high frequency emissions. MAST Technologies can “tune” an EMI absorber to any frequency between 1 and 40 GHz for discrete frequency absorption.
Another common practice for stopping emissions at the source is to place the absorber material directly on a radiating PCB trace. By placing EMI absorber material directly on top of a microstrip trace, the engineer can effectively reduce the fields emanating from the top side of the trace. These emissions can be a particularly troublesome coupling mechanism, if the traces are located on the bottom side of the board and laying adjacent to the bottom of the chassis enclosure. The coupling of the field to the chassis may cause currents to flow into the chassis and set up circulating currents within it. These circulating currents can then cause radiation from any slots, seams or apertures in their path. Placing absorber with pressure sensitive adhesive (PSA) on the traces reduces the field coupling to the chassis. The effect on the trace impedance is minimal since the absorber material is high impedance (> 10 kohm) and it can also be conveniently placed directly on top of the trace without any additional mechanical mounting or holding mechanisms. MAST’s surface wave absorbers are best used for this type of application.
Check out our application note describing The Use of Absorbers in Electronics for more information.