[Discuss-gnuradio] Frequency dependant behaviour of wired transmission using USRP

[Harry Jones]

Hello,

I am currently doing a project at university on channel sounding. I have been using pseudo-random noise and complex chirps as the base signals in order to do this task. In order to get used to using the USRP, I have been using a coaxial cable of around 3m connecting the two USRPs (on two different computers). I am currently using the DC-30/40MHz daughterboards for the initial test stage. I am using a very basic transmission and receive structure. I write a file in MATLAB, generally pseudo-random noise, read this in Python and then use it as a vector source into the USRP. The transmit works fine and has been tested using oscilloscopes etc. The received side takes the data into a complex file sink. The receive end also takes in data well. I am running a NetBSD system which has proven itself to work well.

When I look at the frequency content of the received data however I see a strange tappering on the low/high frequencies sides of the signal, almost as if the coaxial cable is applying a bandpass filter to the data (I apply an FFT to the data on MATLAB after receiving it. For example I take a 1000 chunk of the data, if the transmitted data was 1000 long, I then take a complex fft.). The complex chirp and PR noise are both spectrally flat and should not be changed drastically by the wired channel. When I do a time domain cross correlation, I get nice delta-esque peaks, which implies that the impulse response of the wired channel is a delta (what I expect). Thus the frequency domain and time domain descriptions are conflicting.

I have:
- Checked this phenomenom at DC, still occurs
- Occurs at 10MHz and 20MHz (the coaxial should surely work up into the GHz range)
- Changed USRP, still occurs

I know:
- The transmitted data definately has a flat spectral density at transmission
- The received signal is complex and seems to have been demodulated correctly (I take into account the frequency offset)
- This channel shouldn't be behaving like this!

Attached are the transmit/receive python files as well a few figures showing this problem.

Thank you for your help in advance,
Harry

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