Band-reject filters

A real-world example of this is band-reject filters. A band-reject filter is a circuit designed to block the passage of signals in a specific spectrum of RF frequency while allowing other signals to pass un-attenuated. Other common names are notch filters, bandstop filters or band suppression filters. A common implementation of a band-reject filter is between a power amplifier and matching circuit prior to an antenna.

For example, in a typical application, the narrow notch filter is used to attenuate noise from heterodynes and harmonics unintentionally generated by complex, multiband, wide coverage receivers. The use of a single high-quality thin-film capacitor can essentially replace the use of six components in a twin T design due to thin film's near-ideal characteristics.

Thin-film capacitors (see Fig. 1) have an additional performance advantage not discussed earlier: a single resonant point response due to the fact that the devices use a single-layer dielectric design packaged as a multilayer ceramic capacitor (MLCC). A few of the thin-film capacitor's S21 forward transmission loss characteristic curves are shown in Fig. 2

Fig. 2. S21 forward transmission loss characteristics curve.When using a thin-film capacitor, manufacturers can reap the electrical benefits of a single-layer capacitor while being rewarded with the processing ease of an MLCC-type component. A thin-film capacitor's consistent performance impact on electrodes and oxide thickness and quality impact on dielectric K is shown in Fig. 3.

Fig. 3. A thin-film capacitor has an extremely repeatable frequency response compared to MLCCs.

It is also important to realize the limitations of thin-film capacitors used as band-reject filters. Since thin-film capacitors are typically only available in low capacitance values they are limited to relatively high-frequency band-reject filter designs. If dealing with low frequency designs, other filter methods must be utilized typically using high-Q multilayer RF capacitors.

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