Unexpected Received Chirp Attenuation

I have a question regarding the quality of the received signal. I am attempting to implemented a stepped frequency continuous wave chirp. For now I am just attempting to receive a chirp signal, which I can do. My issue is that the received signal seems okay, but the if I make my output buffer larger then the signal appears to attenuate. 

I have a direct connection from transmitter to receiver with some anntenuators. I have tried using just a sine wave and have seen similar results. I am not sure if this is an issue with board set up. I attached a snip below of the input signal and output. The output buffer is set to 120,000 to capture multiple chirps. The input buffer size is 32768. I would expect the received signal to have relatively uniform amplitude throughout. This trend is seen with a direct connection and with antennas. I am using matlab along with the AD-FMCOMMS5 eval board and ZC702.

Thanks,

Patrick