LFRX and the NI driver

Question

Hi!
I'm using an LFRX board with my N210 radio.  A few questions as to the operation...
1. I assumed that since the LF boards don't have a demodulator...that the "Center Frequency" input would be ignored.  However...I notice that if I change it to something other than 0 Hz....my input signal shifts around in frequency.  What's going on here?  Is this the CORDIC doing something even though there's not analog downconversion stage? 
2.  Similarly....I assumed that the "Sample Rate" input (R), which essentially defines the BW...would define the BW between 0-R Hz.  However, if I input a single tone, the spectrum I calculate goes from -R/2 -> +R/2...and my tone shows up at +f and -f.  This was at odds with my expectations.
I had initially assumed that using the LF boards essentially just turned the USRP into a digitizer for capturing real valued signals. 
However, the above observations seem to suggest that really the driver assumes that you've done a downconversion somewhere else and are just using the LF board to capture the baseband quadrature signals.  In this case, the interpretation of "center frequency" and "sample rate" (i.e. - BW) are essentially the same as if I was using the WBX.  The Center Frequency = 0 Hz because I've done the demodulation somewhere else, and the Sample Rate/BW are defined with the assumption that I'm capturing those baseband signals in quadrature.
If I want to just use each A/D channel to capture separate, un-related, real-valued signals....then the sample rate would have the interpretation of the 2*Nyquist frequency for each unrelated real-valued signal....rather than 2*Nyquist frequency of two baseband signals in quadrature, where the sample rate essentially defines the BW of a bandpass signal that was previously downconverted from some carrier.  Right?
Anyone give me a sanity check here? 
Brandon

Answer 1

Hi Evan-
I think I eventually answered my own question.  I think I just have to remember that the outputs and configuration of the LFRX isn't really all that different from the WBX....only there isn't any analog demodulation on the front end.  The shifting I was seeing was likely the CORDIC...which was essentially the same behavior if I set different LO frequencies and Center Frequencies with the WBX.
One follow-up question though.....I was hoping to use the LFRX and N210 to digitize some low frequency signals DC coupled signals.  The signals aren't in quadrature...rather, I was just trying to use it as a 'generic' digitizer.  (Yes...there are better hardware options for this...but there were some compelling arguements for using it in our system that contained other USRPs).  Anyway...I noted that the LFRX doesn't really go all the way down to DC.  Things start to roll off in the 10-100 Hz range.  Where does this bandlimiting occur?  Is it on the RF board?  Or are the ADC's AC coupled?  Or am I seeing an artifact of the DC offset removal in the FPGA?
I did notice this occasionally at some frequencies with the WBX when my input tone and USRP center frequency were very very close.  Instead of getting DC (or near DC) I and Q signals, I'd get nothing (the DC component was completely filtered out).  It was as if the ADCs were either AC coupled...or the FPGA was removing the DC component.  Never quite understood what was happening here.
Thanks for the insight!
Brandon