The Filter

The RX888MKII has a pretty nice input filter to cut of the frequencies above 65 MHz. This is when we use the 130MS/S sample rate.

But for our application we use only signals of the HF b and, so up to 30 MHz. Meaning that we only need a sample rate of a little higher than 60MS/S. So we have to take care of the aliasing problem ! To avoid aliasing we use an input filter which takes out the higher freq above 30 MHz.

What also is needed 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.

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

 

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