Re: [Discuss-gnuradio] Question about uO and FPGA

[George Nychis]

DiX wrote:
>     The problem is I have to use relatively high bit rate on these tx and rx
> paths. The demic rate should be <=16, and interp rate <=32. When I run the
> code with decim rate = 16 interp rate = 32, I keep receiving uO. My
> questions are : what's the max rate the USRP can support in this case? and
> is there any fatal fault in my graph design?

Hi Di,

uO means that the USRP is overrunning, in other words your host machine 
cannot keep up with the incoming data rate.

The USRP cannot keep up with more than 8Msps due to the USB bus 
limitation with 32-bit complex.  You will get overruns if you try 
anything more than this.  Let's take the case of a single 
daughterboard... you will not be able to use a decimation rate lower 
than 8 (64Msps/8 = 8Msps).

The first thing to do, is to check to see if your machine can keep up 
with 8Msps.  Run "./usrp/host/apps/test_usrp_standard_rx -D 8" and see 
if you get any uO.  If not, keep increasing the decimation until you get 
no uO.

Since you're using two RX chains, with a decimation rate of 8, you'd be 
generating 16Msps since each chain generates 8Msps.  Therefore, when you 
say "decim rate should be <=16" ... <=16 is not possible, only =16. 
With decim=16 you're generating 4Msps per chain, for a total of 8Msps.

- George