# FFT for FM Tx and Rx

Hi,I implemented the FM transmitter and receiver from the tutorials
(http://files.ettus.com/tutorials/labs/Lab_1-5.pdf), the carrier
frequency is 560 MHz, audio rate = 48 kHz, and frequency deviation = 5
kHz. My question is why don’t the FFT plots look similar? The FFT for
the transmitter is centered on 562 MHz, but for the receiver, the
highest amplitude component is centered on 561.97 MHz, also there is one
more component on 562.09 MHz. Screenshots of the FFT plots are
attached.Thanks a lot,
El

Hi El,

this is a bit hard to tell – are you using a direct cable or antennas
between TX and RX?
If you’re using antennas, that might simply be something else
transmitting there.

Also, if you transmit at 560 MHz, but only observe
$\SI{562}{\mega\hertz}\pm\frac{\SI{0.2}{\mega\hertz}}{2}$, you can’t see
your transmit signal, so I can’t compare the strength of that and what
you see in your RX FFT plot.

Best regards,
Marcus