Highest power in 0 Hz

[nadhira]

Hello. I am trying to calculate the rx signal power but it seems like the highest power is at 0Hz frequency. The parameters that I use are (Tx)
tone freq (ch 0) = 10kHz, tone freq (ch 1): 15kHz,
gain (ch 0) = 20 dB, gain (ch 1): 20 dB

 Carrier freq = 900 MHz

no. of samples = 2644

Theoretically, the rx peak power are somewhere between 10kHz until 15 kHz as the tone frequency. but, in my result the highest is 0 Hz and the amplitude of  10kHz until 15kHz is low like other frequencies. what is probably the problem? thank you