Re: [Discuss-gnuradio] USRP2 and Data Rate

[Matt Ettus]

On 07/07/2010 11:43 AM, Garrett Wenger wrote:
> I am not quite sure what data rate I am getting.  I've been using a
> signal analyzer to receive and demod what I'm transmitting and it can
> generally pick up the bit sequence that I am now sending out, but I get
> an error rate the fluctuates from about 5% all the way up to 70% or higher.
> My cpfsk mod index is set to 1 and the distance between the peaks is
> about 10kHz, so it really seems like my data rate is somewhere around
> what I want it to be, but the error rate is just way to high.

The problem is likely to be frequency offset.  I don't know your carrier 
frequency, but at 1 GHz, 10 ppm is 10 kHz.  So your 10 kHz wide signal 
at 1 GHz could be completely out of the passband if the oscillators are 
off by 10 ppm or more.  The USRP1 oscillator is typically 5-10 ppm but 
the spec is 20 ppm, and the USRP2 is typically 10-20 ppm when not locked.

> Also, I went ahead and added in the interpolating filter like you
> suggested, but just for future reference, what is bad about having a
> high samples per symbol value?

In theory, nothing.  In practice, a couple of things.  The higher the 
samples per symbol the longer your filters need to be.  Also, the 
filters in there are chosen for good timing recovery properties, not for 
out of band rejection.  By having so many samples per symbol, you get a 
lot more out of band energy which needs to be rejected.

Matt