OFDM output in spectrum analyzer



I am testing the OFDM trunk code 9798 in openSUSE 10.3 (i586). I run
the benchmark_ofdm_tx.py script while having connected the USRP to a
spectrum analyzer and to my surprise, using the default parameters of
the script (those are: Modulation Type: bpsk, FFT length: 512,
Occupied Tones: 200, CP length: 128, Tx amplitude: 200)

The spectrum shape that I get corresponds more to a single carrier
modulation than to an OFDM signal (I see just 4 peaks spaced around
4MHz). I cannot see where all the subcarriers are gone!

There is not any error message prompted or apparently any reason so
that this is not working. I would really appreciate some help with
this issue.
Thanks in advance.

Natalia Olano