# FFT for FM Tx and Rx

**URL:** <https://www.ruby-forum.com/t/fft-for-fm-tx-and-rx/242392>\
**Category:** GNU Radio\
**Created:** [June 21, 2015, 3:48pm UTC](https://www.ruby-forum.com/t/fft-for-fm-tx-and-rx/242392 "2015-06-21T15:48:55Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\
**Post date:** [June 21, 2015, 3:48pm UTC](https://www.ruby-forum.com/t/fft-for-fm-tx-and-rx/242392/1 "2015-06-21T15:48:55Z")

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Hi,I implemented the FM transmitter and receiver from the tutorials  
([http://files.ettus.com/tutorials/labs/Lab\_1-5.pdf](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

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**Author:** ![El\_El](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/ac8455/32.png) [@El\_El](https://www.ruby-forum.com/u/El_El)\
**Post date:** [June 22, 2015, 10:57am UTC](https://www.ruby-forum.com/t/fft-for-fm-tx-and-rx/242392/2 "2015-06-22T10:57:25Z")

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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