SOTABEAMS make a Digital Variable Audio Filter Module that could usefully be added to your uBITx.
IK8YFW Giuseppe has used a cheap US$2 processor to create a DSP audio processing unit that works with any radio (including the µBITx).
The project was aimed at achieving an economic and simple DSP unit, based on the ARM Cortex STM32f103 processor module. Guiseppe implemented two narrow CW filters of about 300 and 700 hz and two SSB filters with a bandwidth of less than 2200 Hz and less than 3300 Hz. He also included a 6-level noise reduction algorithm. The project is a very cheap solution suitable for embedding in any and every QRP project. The project code can be found on Github. It is not perfect, as Guiseppe is still experimenting with the code. The project can be found here:
Some test here:
Nik VK4PLN has now received his 700Hz CW audio filter boards and built up the board as shown below:
He put these on the spectrum analyser. The spectrum before the filter is included in circuit:
Seems to work! To Nik’s ears an LM324 gives a better result than the TLC274 op amp. You mileage may vary.
Nik VK4PLN has yet another board design – this time for an audio CW filter similar to the QCX and HiPerMite. Nik says “he is loving the cheap and easy EasyEDA/JLPCB boards and simple web designer”.
He “takes no credit for the design, just copied it and added a switching relay. He has ordered 10 boards at $10 posted”.
Details of the filter
Center Frequency: 700 Hz
Bandwidth: 200 Hz
DC Power: 5VDC
GAIN: Upto 20dB via R11 and R12 + trimpot.
Resistors 1206 SMD:
Capacitors 1206 SMD:
C1 0.1u 104
C2 47n 473
C3 36n 363 (33n + 3n3 mount on side together)
C4 1n 102
C5 39n 393 (33n + 5n8 mount on side together)
C6 1n 102
C7 2n2 222
C8 2n2 222
C11 0.1u 104
C14 220u Electrolytic
Relay OMRON G6S-2-DC5
R11 and R12 are optional to parallel the 200k trimpot to make it a logarithmic 100k. Or just use a linear 100k pot.
Use a switch connected to 5V to turn on the filter.
This powers the opamp and switches the relay feeding the audio into the circuit. When off the relay just passes the audio straight through unfiltered.