Understanding Filter Terms: Shape Factor, Selectivity, Bandwidth & Quality Factor
Filters are essential in signal processing, and understanding terms like shape factor, selectivity, bandwidth, and quality factor is crucial for effective design. These parameters define a filter's performance and help engineers optimize signal clarity and system efficiency. We will also understand frequency range, attenuation, insetion loss, isolation, passband, stopband, ripple, group delay etc.
RF Filter terminologies
Following table mentions useful RF filter terms or RF filter terminologies.
| RF Filter terminologies or terms | Description |
|---|---|
| Bandwidth | Frequency range around center frequency over which filter can be used efficiently is known as bandwidth. Frequencies above and below the center frequency at which response fall down by 3 dB with reference to center frequency indicates limits of filter bandwidth. |
| Frequency Range | The range of frequencies over which filter is useful for any application is known as frequency range. |
| Shape Factor | It indicates selectivity of the filter. It is the ratio of filter's response at 60 dB to response at 6 dB attenuation as shown. Here shape factor can be expressed as (F4-F1)/(F3-F2). Lower the value of shape factor steeper the response of the rf filter. |
| Selectivity | It is measure of capabilities of the filter to pass or to reject specific frequencies relative to center frequency. It is expressed as loss through the filter which occurs at some specific difference from center frequency of the filter. For example selectivity of cavity resonator at with center frequency of 150 MHz equals "15 dB down" at frequency of "1MHz above or below" the center frequency. |
| Attenuation | It is the amplitude or magnitude loss incurred to the signal once it passes through the RF filter. |
| Insertion loss | It is same as attenuation mentioned above. It is loss of signal power when it travels through the RF filter. |
| Isolation | It is referred as separation of one signal from the other in order to prevent any undesired interaction between them. |
| Q-factor | It is measure of selectivity of the resonant circuit. Q-factor is described as ratio of stored versus lost energy per oscillator cycle. |
| Passband | The region in frequency response of the RF filter through which signal passes unattenuated. |
| Stopband | The region in frequency response of the RF filter through which signal will be attenuated as per designed attenuation value. This is the band which stops i.e. attenuates the signal. |
| Ripple | The variation in insertion loss of the filter in passband response. The difference between maximum and minimum attenuation is taken as ripple. |
| Group Delay | It is measured in seconds. It is defined as propagation delay time of envelope of AM modulated signal when it passes through the RF filter. |
Conclusion
Mastering filter parameters such as shape factor, selectivity, bandwidth, and quality factor is key to designing effective signal processing systems, ensuring optimal performance.
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