Newbie question on USRP2, synchronization done by the FPGA?


Sorry if this is a simple question. I’m trying to figure out what
synchronization is done by the FPGA on the USRP2. Does it perform both
phase and freq synch? If I want to implement a simple digital modulation
tx-rx, do I just need to do timing synchronization? Is there a simple
example of basic digital modulation that is a good reference?



P.S. Sorry if this is a re-post, I can’t tell if the other email went

On 09/01/2010 06:47 PM, Sam K. wrote:

I’m not sure what you’re asking for, but I’ll take a stab at guessing

I’ll describe what the USRP2 does on the receiver side:

o Complex samples the input (which is complex, that is I + Q,
baseband, typically) at 100Msps

o Filters and decimates that complex sample stream down to whatever
sample rate/bandwidth you wish to appear across the
1GiGe interface (and hence into your Gnu Radio application).
Your Gnu Radio application then does whatever it does with
than complex-sampled stream. That stream is a time-series with
fixed and uniform timing, so any discrete-time-series “math”
you want to do on that stream will “work”.

o Both the sample clock and synthesizer clocks can be synchronized to
an external source, via the 10MHz SMA inputs, or via
the so-called “mimo bus”.

o The FPGA “assists” in the programming of the various daughter-cards
programmable elements, like the PLL synthesizers and
variable-gain elements in the gain chain.

The transmit side is the logical reverse, with the D/A sampling at
200Msps, and the FPGA interpolating your application data stream
as appropriate, and presenting it as a complex sampled baseband
stream to the D/A. From there, it is presented to complex mixers
on the daughtercard to produce the desired final RF signal.

I don’t know if this even comes close to answering your question.