[hpsdr] Dumb question - HPSDR sound card

[Lyle Johnson]

> The GNU Radio and HPSDR people are using FPGA chips to
> do this. Why bother if you can buy a programmable
> decimater, mixer, A to D and so on for $15.

In a word, performance.

The Analog Devices Digital down converter chips with decimator, etc., 
were examined very closely for use in Mercury.  Unfortunately, there is 
truncation that occurs inside the chip's CIC filters.  This is fine for 
the intended application of a cellphone base station, but throws away 
some dynamic range that we really want in an HF receiver.  So, the 
decision was made to use the FPGA approach.

Which is not to say that an interesting receiver couldn't be made with 
some of these chips, just that you have dig deeply and look carefully 
before you decide one approach or the other is right, based on your 
design goals.

Enjoy!

Lyle KK7P


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