Re: [Discuss-gnuradio] NCO and VCO frequencies

[Marcus D. Leech]

Gnuradio controls the receiver side on the usrp inorder to receive data centered around certain center frequency.... and in doing that it sets 2 frequencies on the USRP one in the analog domain which is the center frequency of the VCO running on the daughterBoard, and the other one in the digital domain inside the fpga which is the NCO frequency which is NCO Frequency ..

We need to know how does gnuradio control or map these two frequency ? for example if we want to receive data around 913 Mhz ... what values will be loaded in the VCO frequency register and in NCO frequency register !!!!!

Objective : we need to make this desired frequency only dependant on the NCO frequency and independant on the the VCO frequency, so first we should know how gnuradio control these two values so that we could develop a method to make it only dependant on NCO frequency.

Thanks in advance.

In broad general terms, the Gnu Radio USRP "block" (whether its the original gr-usrp or the newer UHD) takes parameter values from Gnu
  Radio and "does the right thing", depending on which daughtercard is installed on the USRP motherboard.

Generally, PLL synthesizers have no standard mechanism for setting frequency, although they all use the same broad-general approach.
  I think all of the daughterboard PLL synthesizers are programmed over I2C from the USRP/USRP2/N2xx/E1xx motherboard, but the
  specific details of which registers inside the PLL are set to what values to correspond to whatever the desired center frequency is, tends
  to be highly chip-specific.

PLL synthesizers generall have a "resolution" or "minimum step size" in their frequency-setting logic.  Values between 10kHz and 25kHz are
  "typical", which means that if you specify a center frequency that isn't a multiple of the inherent step-size on the synthesizer, the NCO/DDC
  inside the FPGA does a "mop up" operation to arrange for your desired center frequency to be exactly centered at DC in your baseband
  (I and Q) signals going into the Gnu Radio flow-graph.

My suggestion would be to study the UHD source code in detail, including the FPGA/Firmware pieces for your particular setup.

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org