Reference : http://gnuradio.4.n7.nabble.com/Artefacts-in-usrp-fft-py-tc11359.html
Sometimes we see several unknown frequency components visible in our FFT plot.
These are called "receiver spurs" or "rf spurs" or "a/d spurs"
You can check whether it is because of internal hardware imperfections or some external source. Following is a very simple test to verify it :
Note down the frequency of the spur, now tune the receiver again say by 1KHz and see the frequency of the spur again
If it doesn't move or if it moves in the opposite direction or it moves with an amount other than 1KHz , then it is an internal artifact not external.
** Histogram of a noisy input will be of Gaussian in shape
You can also check the proper functioning of the ADC by observing the histogram.
Spikes in the histogram may indicate the bits which are stuck
A/D's also have quantization effects that are periodic and show up as spurs
Adding dithering noise helps to break up the periodicity and reduce the spurs.
** Dithering is a process that adds broadband noise to a digital signal. You may wonder why adding noise would make a signal sound better? It is really a trade off. The introduction of noise lessens the audibility of the digital distortion that comes from the quantization errors discussed above. In essence, low-level hiss-like noise is traded for a reduction of digital distortion.Dithering adds amplitude to all the signals in a digital sample. It forces the lower level amplitude values up to the next threshold level.
Reference : http://www.pcrecording.com/dither.htm
Sometimes we see several unknown frequency components visible in our FFT plot.
These are called "receiver spurs" or "rf spurs" or "a/d spurs"
You can check whether it is because of internal hardware imperfections or some external source. Following is a very simple test to verify it :
Note down the frequency of the spur, now tune the receiver again say by 1KHz and see the frequency of the spur again
If it doesn't move or if it moves in the opposite direction or it moves with an amount other than 1KHz , then it is an internal artifact not external.
** Histogram of a noisy input will be of Gaussian in shape
You can also check the proper functioning of the ADC by observing the histogram.
Spikes in the histogram may indicate the bits which are stuck
A/D's also have quantization effects that are periodic and show up as spurs
Adding dithering noise helps to break up the periodicity and reduce the spurs.
** Dithering is a process that adds broadband noise to a digital signal. You may wonder why adding noise would make a signal sound better? It is really a trade off. The introduction of noise lessens the audibility of the digital distortion that comes from the quantization errors discussed above. In essence, low-level hiss-like noise is traded for a reduction of digital distortion.Dithering adds amplitude to all the signals in a digital sample. It forces the lower level amplitude values up to the next threshold level.
Reference : http://www.pcrecording.com/dither.htm
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