Negative pulse after xbee-pro received data

[pnt]

Xbee-pro receiver module:
A low pulse(500microS) shows on the input of the 3.3V regulator but it’s not on the output.
When set PL=0 on the receiver the low pulse about 500mV
Set PL=4 on the receiver the low pulse about 1V
The low pulse shows right after received the data. And after another 500 microSec the data will send to pin 2 (Dout)
Help.

[mvut]

You are most likely seeing the ACK response packet being sent back.

[pnt]

Thanks
Can I turn the ACK off?