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Behind The Mystery of Electromagnetic Compatibility Design 6 Instead of attenuating noise, the resonance peak will cause the noise to amplify. Resistor and ferrite beads can be used to damp a system to reduce the amount of resonant peaking; since a ferrite bead still has a high Q factor, it acts as an inductor in the low frequency region (Section 2). The resistive part of a ferrite bead does not become dominant until reaching the megahertz range. Therefore, in general, the WE-CBF HF has higher inductance compared to WE-CBF even in low-frequency regions. Because of their construction, a low cut-off frequency is achievable (Figure 3). The following circuit design with a WE-CBF HF will produce a cut off frequency slope twice the amount to 40dB/decade and a filter drop of more than -3dB (Figure 4). This suppresses the resonance peak at the corner frequency so that a smoother transition can occur. The advantages of using an additional chip bead ferrite: ■ 40dB/decade ramp down frequency response ■ Suppresses the resonance peaking ■ Takes less space and fewer components than an op-amp circuit design ■ Overall filter design has lower cost Terminating stub for log periodic dipole antenna (LPDA) Log periodic dipole antennas (LPDA) are now used in applications operating over a wide band of frequencies. LPDA consists of several dipole elements gradually increasing in length where the elements are spaced at intervals following a logarithmic function of the frequency. Printed LPDA layout is an alternative way of applying this antenna array using microstrip printed technology. Designing an antenna on a PCB has different considerations to take into account. For example, an accurate calculation of the length and distances of the element and the compatibility of the microstrip line tracks to the PCB is just as important. In the design phase, there may also be PCB size or material budget constraints. The antenna feeder should include a high impedance termination stub when a higher front-to-back ratio at the lowest frequency is desired (Figure 5). This shortened stub acts as a reflector to ensure a match is provided to the antenna feeder. This length Z is a quarter of the longest element. Therefore, depending on the operating frequency and size, a suitable length is calculated. Figure 3: Gain response of a low pass filter for an inductor and ferrite beads. (Source: Würth Elektronik) Figure 4: LT Spice model of an anti-aliasing filter with WE-CBF HF (742861160). (Source: Würth Elektronik) Figure 5: Diagram of an LPDA terminating with a high impedance. (Source: Würth Elektronik) The WE-CBF is still the component of choice when operating at lower frequencies. "