Another addition to my uBITX project is a variable audio bandpass filter that I built from a constructional article by Eric GW8LJJ (Practical Wireless, May 2018).
The uBITX is a wonderful all band transceiver, but it has no filtering like the popular DNR, DNF and DBF filters that are found in commercial transceivers. To get around this a simple audio low pass and high pass filter connected in the audio output would make a big difference in reducing noise and separating CW stations on the bands. This design uses a very effective variable ‘brick wall filter’ The low pass filter is very sharp which is variable between 200Hz – 4Mhz.
On the back is a power socket, audio input / audio output and speaker connections. At the front is power, high pass filter and low pass filter controls.
Results with the uBITX are excellent, I can now filter out most of the background noise and separate stations, particularly CW stations, easily and effectively by adjusting the brick wall LPF. The HPF is less effective, but this only works between 40Hz – 200Hz, which is designed to filter out hums and other low frequency noise.
I decided to build the filter in an external box so that I can use it with other receivers, but this could easily be integrated into a uBITX itself.