Finally, I decided to use the Raspberry Pi Wobbulator to test the frequency response characteristics of the G6LBQ multiband bandpass filter which I had built as part of a homebrew transceiver project. The fully assembled filter board is shown below (it is part of a Bitx Transceiver project I’m working on) and it covers the whole HF spectrum in 9 bands, all the way from the 160m band (1.8 – 2.0 Mhz) to the 10m band (28 – 30 MHz). Each bandpass filter uses a 3rd order Butterworth design and built using the “redeveloped” component kit from Spectrum Communications. Please note that there is a revised and improved version of the G6LBQ multiband bandpass filter due out very shortly – see the DX KITS web site for details.
Prior to testing with the Raspberry Pi Wobbulator, each bandpasss filter was adjusted to give peak responses at the following frequencies on each band by connecting an RF signal generator to the input of the filter and measuring the output on an Oscillioscope (which is the “normal” way of adjusting such filters).
- 160m band – adjusted for peak response at 1.9 MHz
- 80m band – adjusted for peak response at 3.6 MHz
- 40m band – adjusted for peak response at 7.1 MHz
- 30m band – adjusted for peak response at 10.1 MHz
- 20m band – adjusted for peak response at 14.2 MHz
- 17m band – adjusted for peak response at 18.1 MHz
- 15m band – adjusted for peak response at 21.2 MHz
- 12m band – adjusted for peak response at 24.9 MHz
- 10m band – adjusted for peak response at 29.0 MHz
A screen shot of the results obtained from testing the 160m bandpass filter is shown below. Channel 1 on the ADC Pi module was used and the PGA gain was set to the maximum value of 8x. The wobbulator performed a sweep from 1.8 MHz to 2.0 MHz in increments of 1000 Hz (1 KHz) and the response shows a double peak centered around 1.9 MHz.
A screenshot of the results obtained from testing the 80m bandpass filter is shown below, using channel 1 on the ADC Pi module with PGA gain of 8x. The wobbulator performed a sweep from 3 MHz to 4 MHz in increments of 10 KHz, and the response shows a nice peak around 3.6 MHz.
A screenshot of the results obtained from testing the 40m bandpass filter is shown below, using channel 1 on the ADC Pi module with PGA gain of 8x. The wobbulator performed a sweep from 6.6 MHz to 7.6 MHz in increments of 10 KHz. Although the response of the filter peaks around 7.1 MHz the response curve would suggest that the various filter elements are not well aligned.
