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Re: [Discuss-gnuradio] The sinusiodal waveform is drifting from input to

From: Mattias Kjellsson
Subject: Re: [Discuss-gnuradio] The sinusiodal waveform is drifting from input to output.
Date: Wed, 03 Mar 2010 12:35:32 +0100
User-agent: Thunderbird (X11/20090817)

srinivas naga vutukuri wrote:
> Hi,
> I am transmitting the sinusoidal wave form (IQ samples), in the 
> following methods, and found that when i stem plot in the MATLAB both 
> the input wave form data on one USRP2 and the received data on another 
> USRP2, initially the waveform is exactly fitting (i mean overlapping), 
> and after certain samples, it starts ie., in the input wave form and the 
> output data waveform drifting slightly and at the end samples both the 
> waveforms overlaps perfectly. Why this slight drifting is happening when 
> transmitting the I Q samples from one USRP2 to another USRP2.
> 1st method:
>     I used, tx_samples.cc and receive_streaming_samples.cc.
> 2nd method:
>    Using the GRC, created a signal source, and file sink and usrp2 sink 
> on One PC and another PC usrp2 source and file source. So plotting both 
> the input stored samples and output received samples, observed the above 
> mentioned drift of mismatch of samples of the waveform and again at the 
> end and start it perfectly matches or overlaps.
> Kindly help me in understanding this phenomena, why this is happening on 
> raw data (IQ) samples sending. But this never happens, when i checked 
> with the benchmark_rx.py and benchmark_tx.py used (as internally these 
> two are using GMSK modulation, sync work and packets framing method).
Is it possible that this is due to a phase/frequency- drift between the
two units? I think benchmark_rx implements a PLL (Costas- loop) for
frequency/phase- offsets, and might (don't remember) as well implement a
form of early- late- gate loop (Optimized Müller & Müller algorithm) for
time- synchronizing the samples.

I don't know if the phenomena you see is due to this, but it might...


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