On Tue, Jan 4, 2011 at 1:13 PM, peng senl <[email protected]> wrote:
> Hello, > > I get a question about the half band filter in the DDC. I found that the LO > (local oscillator) in my daughter board is set at 0.2MHz less than the > carrier center frequency, which means that the IF of the output signal is > center at 0.2 MHz. > > > I am assuming the NCO in the FPGA is generating a sinusoid at 0.2MHz to > mix the IF signal to base band. Then we would get a base band signal and a > high order frequency part at 0.4MHz. But the half band filter after the CIC > filter has a pass band of 0.6MHz from the reference materials what I have > read. This means that the high order part won't be filtered by the HBF. > It's generating a _complex_ sinusoid. It's not modulating the signal like an analog LO but performing complex multiplication. Tom
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