cc2520 transmit and receive sfd timing

Hi,

I have two boards with a cc2520 on each one. I'm using the default configuration for GPIO4, which outputs sfd to the pin(GPIOCTRL = 0x2a). When I send a packet from one board to the other, I see that the receive sfd is delayed from the transmit sfd by roughly 3.3 to 3.5 us. I was expecting the two signals to be more closely aligned. What causes the receive sfd to be delayed? Will the length of this delay be consistent? I'm trying to time align some sample data that is sent between the two boards, so I would like to compensate for this delay if possible.

thanks,

galen