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Software Defined Radio
FM Radio – Arctangent Method
Summary:
Use the NI USRP as a spectrum analyzer to find local FM radio stations and then modify the block diagram to decode the FM radio station and listen to the music.
Requirements:
· NI USRP-2920 (most radio stations fall between 88 MHz and 108 MHz
· Ideally an FM antenna, but any small wire may work
· Radio station reception (not a good demo for the basement, use the video)
Demo Procedure:
Find an FM Radio Station (Spectrum Analyzer)
1. Open the VI: FM_Radio_Spectrum.vit
2. Verify the correct IP Address of your USRP is set under device names (typically 192.168.10.2)
3. Run the VI (Note: if it errors try and decreasing the number of samples to 10,000)
4. Disable Auto Scale X by right clicking on the graph and zoom into a radio station to see its frequency (remember that 0 on the graph represents the carrier frequency on the left)
5. Point out that the bandwidth of the radio station is +/- 100kHz (a total of 200kHz bandwidth)
Troubleshooting:
1. Boost the gain: Try boosting the gain try: 10, 25, and 30
2. Still no stations: Stop the demo here and show the video.
Listen to Live FM Radio
6. Change the carrier frequency to that of the radio station you found
i.e. If the center frequency is 94.7M and the station appears at -1M, set the new radio staion frequency to 93.7M
7. Decrease the IQ rate to 200kHz
8. You may want to turn Auto Scale X back on
9. Change windows to the block diagram (CTRL + E)
10. Grab the VI’s from the Disabled Diagram structure and drop them on the block diagram
[Note: If the VI's are unable to link in the Disable Diagram structure, simply open each of the VIs in the subVIs folder and they should re-link automatically]
11. Wire in the brown wire in the middle to the first VI and memorize the following wiring:
Figure 1: VIs to be wired shown two ways, with icon view and terminal view
12. Talk through the process:
a. Determine the phase of the complex IQ signal
b. Unwrap the phase to remove discontinuities at +/- 180 degrees
c. Take the 1st derivative of the phase, this is frequency and the output is equivalent to the information in the FM signal
d. Finally resample is used to reduce the data rate to be compatible with the soundcard
13. Wire the orange tunnel on the left of the while loop to the
14. Delete the brown waveform wire to the right of Sound Output Wirte.vi and wire the top output terminal of Resample Waveform to the shift register.
15. Finally, delete the VI going into the PS/PSD VI and wire in between the Derivative and Resample Waveform VIs.
Figure 2: Complete VI wiring
16. Switch back to the front panel (Ctrl-E)
17. Run the VI
18. You’ve now demodulated broadcast FM radio using the mathematical approach
Troubleshooting:
1. Distortion: can be caused by applying too little gain or too much! Try increasing the gain or decreasing the gain methodically. Use the first part of this demo, the spectrum analyzer to troubleshoot.
2. Breaks in the audio: The sound VI’s are less than ideal. Try decreasing the number of samples to a very small number like 10,000. Some sound cards just don’t work well.
Files:
· FM_Radio_Spectrum.vit
o Demo VI (saved as a VI Template)
To learn more about the NI USRP visit ni.com/usrp
Hey, I am unable to use the above file.
When I tried to open the (.vit) file. I get this error. I am new to USRP and Labview.
PLEASE HELP ME
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Hiee I am new to labview and i find the error asshown in screen shot please help me resolving the error
i observed these errors and modified the as block diagram as follows.kindly plzz help us solving this issue..thanks in advance 
..
