[USRP-users] FM demodulator. Lots of dropped packages if USRP2 decimation is less than 512

Jorge Miguel jmiggal at gmail.com
Tue Nov 2 11:41:58 EDT 2010

Dear all,

I am getting started with the USRP2 blocks from Matlab 2010b along with the
USRP2 basic receiver.

Although the basic Rx cannot be tunned I tried to build a FM demodulator

The simulink setup is: USRP2 Source 256dec- Mixer 100KHz- LPF 75KHz- FM Dem
- Resampler 48KHz- Audio sink (file attached)
(This setup tries to demodulate a signal at 99,9 MHz which is band pass
sampled at 100KHz due to the 100MHz frequency sample of the ADC)
If I set a decimation less than 512 the 'Lost packets' output reports a lot
of dropped packages. Since I have a quite new quad-core laptop I think it
should be enough to run this setup. Do I have to set-up some optimum
configuration parameters elsewhere in order not to loose packets? By the
way, how is the lost packet output triggered? I mean how the block figures
out that more data arrived to the UDP buffer and it couldn't be stored. I
would like to know the inside of the block (besides the USRP2_RX code that
doesn't say a lot).

If I close the FFT scope form the workplace the number of lost packets
decreases. Does simulink need a lot of CPU capability? I expected to see it
running with different decimation values and the spectrum scope at the same
When running my set up at 256 decimation I get more or less 60% of packets
which should be fair to identify my FM modulated signal but I only hear
noise at the output.

Did anybody tried the same?

Many thanks in advance,
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