The output of the mixer goes to the input of the IF filter-amplifier, which is tuned to a center frequency of 40 kHz. The circuit consists of an IC 741 op-amp together with an arrangement of capacitors and resistors working as an active-RC filter to provide a sharp band-pass response at 40 kHz.
The governing equation for the IF center frequency is:
fIF = fVCO – fBPF = 40 kHz
Rearranging the above equation, we have:
fBPF = fVCO – 40 kHz
From this equation, and as described earlier, we note that when VCO frequency is 40 kHz, the frequencies around 0 Hz are selected, and when it is 60 kHz, the 20 kHz frequency is passed.
The equations governing the working of the filter are:
The values of R1, R2, R3 and C are chosen so as to yield a center frequency of 40 kHz and a bandwidth of about 2 kHz.