[Discuss-gnuradio] Question about the HBF in the DDC

[peng senl]

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?