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.