Input Low-pass Filter
The first stage of the spectrum analyzer is the audio low-pass filter. It passes the frequencies lying in the audio range, i.e. 20 Hz—20 kHz, and rejects all others. This stage improves the noise performance of the analyzer by allowing only the required frequencies to pass through.
It consists of a second-order low-pass active RC filter, commonly known as a Butterworth filter, that gives a slope of -40 dB at 20 kHz. The filter gives a reasonably good response even with a single IC 741 op-amp stage. The op-amp is used to prevent loading of the input capacitor by the next stage due to its high input and low output impedance.
The transfer function equation of the filter is given below:
The above equation has a -3 dB frequency given by the equation:
In our case, the cutoff frequency is 20 kHz.