Calculating maximum signal frequency knowing its sampling rate? I was curious as to how I could calculate the maximum frequency of a signal knowing only its sampling rate (for example 125 nanoseconds). Does the sampling theorem have anything to do with it (sampling rate is at least two times the maximum frequency)? So is the max frequency half of the sampling rate?
The maximum possible frequency in the sampled signal is half the sampling rate, assuming that you are not bandpass sampling and/or there is an anti-aliasing filter. The question is whether the signal has content up to that rate or whether it stops at a lower frequency.

One way to determine this would be to take your sampled signal $x[n]$ and transform it to the frequency domain via FFT to get $X[k]$.

Then find the largest value of $k$ for which
$$
\left |X[k] \right| &gt; t
$$
for $0 \le k \le \frac{K}{2}$ where $K$ is the length of $X$ and $t$ is some "threshold" below which you decide the signal level is effectively zero.