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[Discuss-gnuradio] Jagged Data


From: Andrew Buck
Subject: [Discuss-gnuradio] Jagged Data
Date: Sun, 29 Mar 2009 11:17:54 -0400
User-agent: Thunderbird 2.0.0.21 (Windows/20090302)

Hi,

I'm trying to use a USRP to decode a CPFSK signal at 980MHz and 1Mb/s. The decoder successfully detects the sync pattern, but only about once a minute when the pattern is transmitted once a second. And when the decoder does detect the sync pattern, the data following the pattern is incorrect. I think the issue may be that the sample rate is too low. When I record the raw signal data to my hard drive, graph it with Matlab, and zoom way in, it's very jagged and I can't see the uniform changes in frequency I would expect with CPFSK. Clearly there is some data there since my code detects the sync pattern once in awhile, but I feel like I may be right on the edge of the minimum sampling rate. When I tell the USRP to sample at 4MHz, I do get periodic overflow errors and when I sample at 2MHz I don't, but obviously that's only 2 samples per bit.

What I don't understand is how if the USRP is capable of recording an HDTV signal, which has a bitrate well about 1MB/s, why I can't seem to get a sample rate fast enough for my slower signal. Can anyone tell me what I'm doing wrong? Maybe it's not sample rate, but something else? If so, why is the data so jagged?

Thanks,
Andrew




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