There is spike when sending zeros/signals (DC bias)?

[t-leikan]

Hi, we want to send signals in on/off duty cycle (40ms/40ms for example), in off cycle, we want there is no signal at all. But there is big spike in the center of the frequency can be seen in spectrum analyzer.
After we take a closer look, it seems the spike is always there, in the center of the frequency, until we shut bladerf down completely by disable the tx module, but it takes too much time to enable and disable the module.
Any suggestions will be much appreciated.