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Is my BIST correct?

Question asked by tanybin on Mar 7, 2018
Latest reply on Mar 12, 2018 by sripad

My tx LO is 2.4GHz. Tx_sample_clk is 7.68MHz. When the tx_data_port is not inputed digital data, I measure a signal of 2.4GHz from spectrum analyzer. Then I enable BIST,set the register 0x3F4=0x01. According to the  reference of BIST,this will get a tone of 240KHz. I observed a spectrum at 2.400240GHz. But I have a question, I consider that It should only two frequency ,one is 2.4GHz-240KHz, other is 2.4GHz+240KHz  when the 2.4GHz and 240KHz is mixed. Why the signal of 2.4GHz still exist?Do my knowledge mistake, or my BIST test error?

the attachment1 is screenshot of spectrum analyzer when BIST disable.

the attachment1 is screenshot of spectrum analyzer when BIST enable.