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Let's say we have 2 signals of frequencies 4 kHz and 8 kHz.

If we sample both signals with sampling frequency 10 kHz and pass the signal through a low pass filter with cut off frequency of 10 kHz, what frequencies will be present in the output signal?

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    $\begingroup$ Homework question ? What have you tried so far ? $\endgroup$ – Paul R Dec 10 '13 at 8:18
  • $\begingroup$ Strange question: 1/ what is a "signal of frequency 4kHz"? It could be a sine wave or something else and the result will be different. 2/ Signal 2's fundamental frequency is above nyquist, is that normal? $\endgroup$ – pichenettes Dec 10 '13 at 10:50
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    $\begingroup$ The cutoff frequency of your filter is also above the Nyquist rate. I think this question is a bit malformed. $\endgroup$ – Jason R Dec 10 '13 at 12:56
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The 8kHZ signal causes aliasing as we should sample at 2fmax = 2*8kHz = 16kHz (normally we should sample faster than 2fmax). It is also strange that you have 2 signal @4kHz and 8kHz but you want to use a LP at 10kHz.

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This, I think is a homework question. When the signal of 4 KHz is sampled We will have 4KHz, 10 +/- 4KHz, 20 +/- 4KHz signals being present.

When a signal of 8 Khz is samplrd, We will hve 8, 10+/-8KHz, 20 +/-8KHz etc.

There for we Will have 2KHz,KHz, 6KHz,and 8KHz being present when you use a low pass filter

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