The Low pass shelving filter

The RX888MKII has a pretty nice input low pass filter to cut of the frequencies above 65 MHz. This can be used when sampling at 130Ms/s sample rate.

But for our application we use only signals of the HF band, so up to 30 MHz. We only need a sample rate of a little higher than 60Ms/s. To avoid aliasing we use an extra low pass input filter to cut off  frequencies above 30 MHz.


Even the RX888 has a limited dynamic range (distance between quantization noise level and overload level). The front end gain in the RX888 has to be set to receive just enough band noise. Any extra gain increases the chance on overloads. However the gain is set for the entire shortwave.

Band noise levels decrease with increasing frequency. As a result we have much higher noise levels on the lower bands. To minimize the chance of overloads, we have to adjust the signal levels for each band/frequency so the receiver receives just enough band noise on each band. A higher level only increases the noise level at the cost of potential overloading problems.

We use an active small broadband loop antenna with an almost constant antenna factor over frequency. That simplifies adjusting the band signal levels. We can use the Turn Island Systems shelfing filter for attenuating the lower bands. This equalizes the noise level over all bands.

Note: received noise levels on each band depend on location and antenna factor of the antenna!


A little pcb which is doing both can be bought at Turn Island Systems. Look here.

We have built in this filter in our system (between antenna and RX888MKII), and have excellent results with it.


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