Question about Nyquist frequency and noise

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SUMMARY

The discussion centers on the Nyquist frequency and its implications for a time series sampled at 250 Hz, particularly in the presence of noise at 300 Hz and 475 Hz. The Nyquist frequency for this sampling rate is 125 Hz, which is below the noise frequencies, leading to aliasing effects. When noise exceeds the Nyquist frequency, it folds back into the spectrum, causing distortion in the digitized signal. Understanding the mathematical relationship between the frequencies involved is crucial for analyzing the effects of noise on sampled data.

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Homework Statement



A time series is sampled at 250 Hz. What is the sampling interval and Nyquist frequency of the original signal, before digitising was contaminated with noise at about 300 Hz. At what frequency would this noise appear in the spectrum of the digitised sequence ? Repeat the calculation for noise contamination at 475 Hz.

The Attempt at a Solution


[/B]
I can calculate the sampling interval and Nyquist frequency fine but I don't understand the noise part of the question at all. Any help is appreciated, thanks.
 
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bert2002 said:
At what frequency would this noise appear in the spectrum of the digitised sequence ?
Look at the upper curve here:
alias_ex2.gif

The dots are the sampled values.

Say the curve is the 300 Hz noise. If you sketch a ( low frequency ) curve through the dots, what will the frequeny of the sketched curve be ?
 
bert2002 said:

Homework Statement



A time series is sampled at 250 Hz. What is the sampling interval and Nyquist frequency of the original signal, before digitising was contaminated with noise at about 300 Hz. At what frequency would this noise appear in the spectrum of the digitised sequence ? Repeat the calculation for noise contamination at 475 Hz.

The Attempt at a Solution


[/B]
I can calculate the sampling interval and Nyquist frequency fine but I don't understand the noise part of the question at all. Any help is appreciated, thanks.
The noise is well above the Nyquist frequency so you will get aliasing. How does the noise spectrum fold about the Nyquist frequency when its spectrum is above the Nyquist frequency?
A little hint: cos(w1t)cos(w2t) = ?
 

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