All signals, voice, music, streaming radio, etc., consist of sinusoidal components of different frequency, amplitude and phase. Sometimes we need a filter that allows passage of the components of a frequency and prevent the passage of other components.
This project pretends design a filter that passes all frequencies except those in a stop band centered on a center frequency. Forthis objective, a Wien Bridge oscillator was designed with a frequency of oscillation of 9 kHz for the purpose to add it to an audio signal. When both signals are mixed, the audio signal cannot be heard, because the signal from the oscillator introduces noise. To return the audio signal without noise it was necessary to design a filter that passes all the frequencies except the 9kHz signal whichis generated by the oscillator. The filter that we designed was a notch filter. The amplitude response of a notch filter is flat at all frequencies except for the stop band on either side of the center frequency. That means that the voltage gain of the filter is one, therefore the signal is not amplify, only filtered. The notch filter was designed of order 4 to be sure that the transmission band isclosely to the ideal.
The project consists of a Wien bridge oscillator, a summing amplifier, and the notch filter of order 4. The implementation of the project was done in the laboratory with the purpose of learn and apply our knowledge that we acquire in the courses of Electronics I and II
Notch filter is a filter that passes all frequencies except those in a stop bandcentered on a center frequency. The amplitude response of a notch filter is flat at all frequencies except for the stop band on either side of the center frequency. The standard reference points for the roll-offs on each side of the stop band are the points where the amplitude has decreased by 3 dB, to 70.7% of its original amplitude.
The -3 dB points and -20 dB points are determined by thesize of the stop band in relation to the center frequency, in other words the Q of the filter. The -3 dB points are at about 1 kHz and 100 kHz for a Q of 0.1 and a center frequency of 10 kHz. At one tenth the bandwidth, approximately 600 Hz and 1.6 kHz, the amplitude is down 20 dB.
* The -3 dB points are at about 600 Hz and 1.6 kHz for a Q of 1 and a center frequency of 10 kHz. The -20 dBpoints, however, are not marked because they are so close to the center frequency.
* The response of the bandpass filter with a Q of ten is also shown. The -3 dB points, however, are not marked because they are so close to the center frequency - the same place as the -20 dB for a Q of 1.
* At very high Q values, the response of the circuit will begin to have overshoot and undershoot that willdestroy the integrity of the notch. The frequency that was supposed to be rejected may actually be amplified.
OP AMP NOTCH FILTER CIRCUIT
The diagram below shows a notch filter circuit using a single op amp.
Active operational amplifier notch filter circuit
The circuit is quite straightforward to build. It employs both negative and positive feedback around the operational amplifier chipand in this way it is able to provide a high degree of performance.
R = R3 = R4
C = C1 = C2
When building the circuit, high tolerance components must be used to obtain the best performance. Typically they should be 1% or better.
For this circuit, at intermediate frequencies, where the input impedance and feedback impedance are very close, the gain willbe close to 1.The notch filter is the opposite of the band pass filter: the band stop filter highly amplifies signals above and below the "notch" or middle frequencies, and doesn't amplify the intermediate frequencies. This is in contrast with the band pass that attenuated signals above and below, and also didn't amplify intermediate frequencies (gain of 1).