Spectrum analysis shows strange carrier

Hi folks. We have a Toy-Spectrum-Analyser here, so i have measured the output of our BS-11 when initalized with -f 123 http://www.root.runningserver.com/pub/openbsc_spektralanalyse.png Why are there 2 carriers emitted by the BTS? I thought that openBSC uses only one TRX. I have verfied that the carrier must be emitted by the BTS. If i stop the BTS the carrier vanishes and the T-Mobile carrier comes through (much weaker, of cause) I have no explaination for this effect, can anyone give me a hint? regards Philipp