DDC in gr-sounder FPGA configuration

Dear all,

I’m reading the receiver part of gr-sounder project. As my undering, in
the FPGA configuration of this project, the received signal directly
goes to the sounder module, after being demuxed into I and Q signals by
adc_interface module. I didn’t see any module of the FPGA configuration
of this project implments DDC. In python program, however, it has been
set the tunning frequency for sounder receiver. Which part of the
receiver tunning frequency will set to in the FPGA configuration? Which
part of the FPGA configuration implements DDC?

Thank you so much in advance.