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.

What also needs attention is attenuation of the lower bands, because they are much stronger than the higher frequencies. This is to make sure we can use successfully the high dynamic range of the 16 bits, the radio is providing.

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

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 change on overloads. However the gain is set for the entire shortwave.

Band noise levels decrease with 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!