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[Discuss-gnuradio] USRP2 FM TX and FM RX working together

From: Jorge Miguel
Subject: [Discuss-gnuradio] USRP2 FM TX and FM RX working together
Date: Tue, 21 Sep 2010 18:03:54 +0200

Hello community!

I initiated myself in GNU radio two weeks ago and I am learning as fast as I can. To begin with, I decided to play with GRC and my URSP2.
I successfully built a FM modulator that works fine.
I successfully built a FM demodulator that works fine.
But there are still several things I do not understand.

While building the FM modulator:
1)Wave file source
2)Rational resampler
4)Rational resampler
5)Frequency Mod
6)Multiply const
7)USRP2 sink

Why is block 6 necessary? I tried with lots of values over 2 and all of them are ok. I realized that the smaller the number, the higher the noise in my receiver (my mobile phone). Is it related to the amplitude of the modulated signal?

Another thing very strange is that if I create a GRC file with both, transmitter and receiver with exactly the same blocks and the same parameters I cannot hear any demodulated signal. I can see information with a FFT block connected to the receiver chain, and I am able to demodulate the signal with my mobile phone when the example is running but in my computer I do not hear anything else besides noise mixed with some sort of non-understandable signal. Thus, I guess I have to change something in the receiver chain although it works alone in my FM demodulator. I changed every single parameter but I cannot get any improvement.

Any suggestions?

Many thanks in advance,

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