If there is any stray resistance (especially likely in the inductor), this will diminish the filter’s ability to finely discriminate frequencies, as well as introduce antiresonant effects that will skew the peak/notch frequency.Ī word of caution to those designing low-pass and high-pass filters is in order at this point. In all these resonant filter designs, the selectivity depends greatly upon the “purity” of the inductance and capacitance used. In other words, this is a very “selective” filter. The amplitude at the notch frequency, on the other hand, is very low. Once again, notice how the absence of a series resistor makes for minimum attenuation for all the desired (passed) signals. Parallel resonant band-stop filter: Notch frequency = LC resonant frequency (159.15 Hz). (Figure below) parallel resonant bandstop filter The parallel LC components present a high impedance at resonant frequency, thereby blocking the signal from the load at that frequency.Ĭonversely, it passes signals to the load at any other frequencies. The variable capacitor and air-core inductor shown in Figure above photograph of a simple radio comprise the main elements in the tank circuit filter used to discriminate one radio station’s signal from another. Variable capacitor tunes radio receiver tank circuit to select one out of many broadcast stations. In most analog radio tuner circuits, the rotating dial for station selection moves a variable capacitor in a tank circuit. It should be noted that this form of band-pass filter circuit is very popular in analog radio tuning circuitry, for selecting a particular radio frequency from the multitudes of frequencies available from the antenna. That series resistance will always be dropping some amount of voltage so long as there is a load resistance connected to the output of the filter. Just like the low-pass and high-pass filter designs relying on a series resistance and a parallel “shorting” component to attenuate unwanted frequencies, this resonant circuit can never provide full input (source) voltage to the load. Parallel resonant filter: voltage peaks a resonant frequency of 159.15 Hz. (Figure below) parallel resonant bandpass filter Under or over resonant frequency, however, the tank circuit will have a low impedance, shorting out the signal and dropping most of it across series resistor R 1. The tank circuit will have a lot of impedance at resonance, allowing the signal to get to the load with minimal attenuation. The other basic style of resonant band-pass filters employs a tank circuit (parallel LC combination) to short out signals too high or too low in frequency from getting to the load: (Figure below) Parallel Resonant Band-pass Filter However, different values for the load resistor will change the “steepness” of the Bode plot (the “selectivity” of the filter). Series resonant band-pass filter: voltage peaks at resonant frequency of 159.15 Hz.Ī couple of points to note: see how there is virtually no signal attenuation within the “pass band” (the range of frequencies near the load voltage peak), unlike the band-pass filters made from capacitors or inductors alone.Īlso, since this filter works on the principle of series LC resonance, the resonant frequency of which is unaffected by circuit resistance, the value of the load resistor will not skew the peak frequency. (Figure below) series resonant bandpass filter Series LC components pass signal at resonance, and block signals of any other frequencies from getting to the load. The two schemes will be contrasted and simulated here: Series Resonant Band-pass Filter Knowing this, we have two basic strategies for designing either band-pass or band-stop filters.įor band-pass filters, the two basic resonant strategies are this: series LC to pass a signal (Figure below), or parallel LC (Figure below) to short a signal. Series LC circuits give minimum impedance at resonance, while parallel LC (“tank”) circuits give maximum impedance at their resonant frequency. We should know by now that combinations of L and C will tend to resonate, and this property can be exploited in designing band-pass and band-stop filter circuits. So far, the filter designs we’ve concentrated on have employed either capacitors or inductors, but never both at the same time.
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