# Sampling frequency range of an discrete signal

Is it possible to detect each and every frequency above the nyquist frequency? Or this can be also discrete Thank you.

In a simple word no. You can detect the presence of frequencies above the nyquist frequency but you can't categorically "know" what frequency they are. See this: -

This informs you that a sinewave at 0.6 x sampling frequency (or 1.2 x nyquist) can be appear as coming from frequencies of 0.4 Fs, 1.4 Fs or 1.6 Fs.

It's impossible to know which one is correct. Note also this can be extended to frequencies centred around 2.5, 3.5, 4.5 etc of Fs.

Also,exactly at Fs (or integer multiples of Fs) any signal present at those frequencies will be "detected" as zero or a DC level (phase angle dependent).

However it's not all bad news because even though you cannot "know" the frequency you can demodulate any movement on that "out-of-band-signal" and recover a modulating waveform intact. This is useful for software defined radios. It's called "under-sampling".

• I couldnt question it properly. I said that is there any discrete behavior persists in detecting sampling frequency. As an example, if we can detect any signal above with minimum 6 Hz, then over that there are many possibilities. We can detect 6.1 Hz, 6.01 Hz, 6.001 Hz. My question actually whether there is any limit of the rational values. I got my answer, it depends on how much precision can a digital device do. Thank you anyway. Sep 27, 2015 at 8:00
• I don't understand what you are saying. Sep 27, 2015 at 8:10
• I have no idea what you are trying to do. What is a "rational frequency". Maybe we just leave this as (for now) an unanswered question. Sep 27, 2015 at 9:05