USRP as Spectrum

Could anyone kindly advice? Thanks!

I set the center frequency to 942MHz, believe to be the GSM
downlink(935-960MHz).

The spectrum is different from some samples and FM radio spectrums,
http://www.fungkai.com/~howie/sdr/fm3a.gif
which
the peaks are widely across several MHz instead of spikes.

Then, I set the center frequency to 902MHz, which should be the GSM
uplink
frequency. There are some spikes, but with a constant spike at around
908.5MHz.
http://www.fungkai.com/~howie/sdr/gsm_spectrum_942M_110322.png

I read from the openBTS Manual, there are spikes in the downlink
spectrum
from the operators’ BTS.
In my case, is there anything wrong in my config?
http://www.fungkai.com/~howie/sdr/gsm_spectrum_902M_110322.png

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