Re: [Discuss-gnuradio] Down Conversion on USRP

[Matt Ettus]

science quest wrote:
> Hi all
>             Newbie here. I am using FLEX 2400 daughterboard with USRP.
> On the exploring GNUradio web page it is mentioned under the title
> "what goes in FPGA" that there is a NCO and Decimation filter in FPGA.
> I did  a quick scan of the FLEX 2400 receive schematic and it appears
> to me that it is a direct conversion receiver. So if it is a direct
> conversion receiver with the demodulation and filtering bringing the
> signal to baseband, why do we need the NCO and Decimation filter in
> the FPGA? Please correct me if I have interpreted this incorrectly.

The RFX2400 is direct conversion, but the VCO/PLL only tunes in 4 MHz
steps (this is changeable, but leaving it this way gives the best phase
noise).  The DDC downconverts the rest of the way if necessary.

Some other daughterboards, like the TVRX, are not direct conversion.

Matt