Re: [Discuss-gnuradio] Weird (or maybe not) received signal amplitude behaviour

[Josh Blum]

On 02/24/2011 10:13 PM, Arya Santini wrote:
> Hi List,
> 
> I'm transmitting a pure sinusoid of 50kHz using the tx_waveforms UHD
> example and receiving the same using rx_samples_to_file example. I'm
> using N210/WBXs for Tx and Rx at 250MHz, at 500 ksps, --ampl=1.0,
> buffer size of 200 at Tx, and 0 dB for Rx,Tx gain.
> 

Can you repeat the results with the default amplitude? An amplitude of
1.0 is maximum, and could lead to clipping in the FPGA DUC.

-josh

> I plot the received file (the I stream, after de-interleaving the
> complex shorts) using gnuplot and here's what I see. The peak to peak
> amplitude is 40. The wave's dancing nicely about 0 and its a
> spectacular sine wave at almost precisely 50 kHz. But here's the
> catch: I can see this happening only after about sample # 8000
> thereabouts in the graph. Before this, the wave starts out (at sample
> # 10 ) at 1400-1440 peak to peak and gradually makes its way
> down(maintaining its sinusoidal shape during the descent) to settle at
> -20 to +20 peak to peak. It takes 8000 samples for this to happen, or
> 8000*2us = 16ms.
> 
> Why is this happening?
> 
> Arya
> 
> P.S. At Tx/Rx gains of 15 dB, same thing happens but the signal starts
> at around 1000-2000 p2p and settles at -500 to 500 at around sample
> #6000.
> 
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