An RF band-pass filter is a circuit, which passes a specific radio frequency or a band of frequencies while attenuating all other frequencies both above or below that frequency. A band-pass filter is designed by combining the features of the low pass filter and the high pass filter. The frequency or the frequencies which the filter allows to pass through is called the pass-band. The band-pass filter consists of a combination of resistor, inductor and capacitor (RLC circuit). It has an input port as well as an output port.
An ideal band-pass filter has a flat pass-band and passes all frequencies within the pass-band to be transferred to the output and does not have any gain or attenuation. It attenuates all frequencies beyond the cutoff frequency to which it is designed. The band of frequencies that the filter allows to pass through without attenuation is called the bandwidth. It is the difference between the upper and the lower cutoff frequencies.
In practice, a band-pass filter does not totally attenuate all the frequencies outside its desired pass-band. At the edges of the pass-band, there is a small section of the band where the frequencies are attenuated, but not totally eliminated. This is called the filter roll-off and the small band of frequencies both above and below the pass-band is called the roll-off frequency. It is usually designated as dB attenuation per octave or decade of frequency.
Types of Band-pass Filter
There are two types of band-pass filter and they are as follows.
1. Narrow band-pass filter. Here, the slope of the resonance curve is sharp and narrow. The pass-band is narrow, which determines the Q of the circuit. The reciprocal of the fractional bandwidth of the circuit is the Q factor. Thus a narrow band pass filter has a high Q. A crystal ladder filter is a type of narrow band-pass filter.
2. Wide band-pass filter. Here, the pass band is wide and it allows signals from a wide band of frequencies to pass through. A wide band-pass filter has a low Q. An LC filter is a type of wide band-pass filter. Television IF filters are wide and has a wide pass-band.
There are two types of band-pass filter and they are as follows.
1. Narrow band-pass filter. Here, the slope of the resonance curve is sharp and narrow. The pass-band is narrow, which determines the Q of the circuit. The reciprocal of the fractional bandwidth of the circuit is the Q factor. Thus a narrow band pass filter has a high Q. A crystal ladder filter is a type of narrow band-pass filter.
2. Wide band-pass filter. Here, the pass band is wide and it allows signals from a wide band of frequencies to pass through. A wide band-pass filter has a low Q. An LC filter is a type of wide band-pass filter. Television IF filters are wide and has a wide pass-band.
An LC Band-pass Filter
In an RF input filter, the radio frequency signal from the antenna is fed to the input port. The LC circuit acts as the tank circuit and it resonates with the frequency to which it is tuned. The band-pass filter attenuates or cuts off the frequencies both above or below the required frequency. The selected frequency along with the signal it contains is fed to the output port without attenuation.
The resonant frequency of an LC tank circuit is given by the formula
F=1/2π✓LC
Change the value of the inductor L and the capacitor C according to the desired frequency.
In an RF input filter, the radio frequency signal from the antenna is fed to the input port. The LC circuit acts as the tank circuit and it resonates with the frequency to which it is tuned. The band-pass filter attenuates or cuts off the frequencies both above or below the required frequency. The selected frequency along with the signal it contains is fed to the output port without attenuation.
The resonant frequency of an LC tank circuit is given by the formula
F=1/2π✓LC
Change the value of the inductor L and the capacitor C according to the desired frequency.
Applications of RF Band-pass Filter:
- Narrow band-pass filters are used in the input tuned circuits of radio receivers. It helps to select specific radio signals from a radio frequency band while rejecting all other signals. Eg. RF input filter.
- Narrow band-pass filter helps to select a station from adjacent stations. It improves the tuned circuit Q and thus the signal to noise ratio (S/N) and sensitivity of the receiver. Eg. IF filter, ladder filter etc.
- Narrow band-pass radio designs help to minimize the interference and improve maximum allocation of stations within a band without adjacent station interference.
- Narrow band-pass filter is used in the output of a radio transmitter. It helps to limit the transmitted radio signals within the allowed frequency band. Eg. RF output filter.
- Wide bandwidth filter is used in the video or picture reception in television receivers. Eg. Video filters.
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