Another byproduct of the Operational Amplifier presentation I did for a recent MN-ARTS meeting was to produce a design and PCB for a Op Amp based audio filter. The first prototype I did was a 4th order Chebyshev Bandpass, but as I ran the designs in the TI FilterPro Desktop software, I settled on the smoother Butterworth response to account for CW QSOs slightly off of a 250 Hz boundary.
The schematic below is the design normalized for common resistor values. the TI FilterPro software has the nice feature that it pins the C values to something common, then varies the R values accordingly. Various options are supported to make the BOM and implementation less idealized. The design point was 650 Hz center frequency, 100 Hz 3dB bandwidth, and a 10 dB voltage gain.
The two-layer PCB that resulted was simple and designed around 100% through-hole parts. I sent this to OSHPark like all my other prototype designs. The TL972 or MCP6022 Op Amps are excellent, inexpensive choices for an audio range active filter as both are low noise/low distortion parts. The PCB was designed for flexibility in it’s use and that is why the audio and power connections are oriented to wires rather than jacks.
The designed gain achieved 8.9dB as a real circuit. Using the FilterPro software, any 4th order active filter, single-ended Bandpass topology will work, regardless of response type and the desired voltage gain. The intent of this design was to provide an add-on for homebrew or purchased rigs with insertion into either the audio chain, or directly driven at low volume levels when used with an amplified external speaker.
The Bode plot of the gain and phase below was produced using a Analog Discovery module by Digilent. The WaveForms software used with the Analog Discovery includes a Network Analyzer virtual instrument (similar to NI LabView). The actual response tracked closely to what was predicted by FilterPro.
There’s enough of a roll-off with just a 4th order active filter to produce excellent results. With the current design point, any kind of receiver (DC, Super Het, SDR) architecture works once the signal has been demodulated to audio.