Wednesday, April 24, 2013

A bit about tunning the daughter boards

Reference :

Is there one LO on the USRP which is being switched between these frequencies, or is there more than one LO?

The tuning is split between an LO on the daughterboard and the DDCs in the the FPGA.

In the case of the Basic Rx and LF RX, there is no LO on the daughterboard, so all the tuning is handled by the DDC. 

** Basic Rx and LF RX have LO on the daughterboard,

When using std_4rx_0tx.rbf, there are 4 DDCs available in the FPGA. 

u.tune(...) handles adjusting the LO (if any) and the DDCs transparently for the common case

Can I tune all four subdevices independently, or am I restricted to using the same frequency on a given daughterboard?  

With the Basic and LF Rx everything is independent, since there's no
LO on the daughterboard.

In the case of daughterboards with LO's, life is a bit more complicated and you'll have to explicitly control the LO on the daughterboard, and then explicitly control the 2 DDCs that are being fed from the given daughterboard.

You of course need to ensure that that two frequencies that you want within the IF passband of the daughterboard.  

To see how this is currently handled, take a look at the implementation of "tune" in gr-usrp/src/

Meanwhile :

tuning is a two step process. first we ask the front end to tune as close as the desired frequency as it can. then we use the result of that operation and out target frequency to determine the value for the DDC


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