Question about the HBF in the DDC

GNU Radio
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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.

Is there anything wrong in my
understanding? Can someone explain how the USRP2 remove the high
frequency part after the DDC mixing?

2

On Tue, Jan 4, 2011 at 1:13 PM, peng senl [email protected] wrote:

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