DFT after the Nyquist limit

1. When the Nyquist limit is exceeded what occurs?
2. What happens to the signal if it is sampled below Nyquist limit?
3. When a frequency above the Nyquist limit is sampled what happens?
4. What is the formula for the Nyquist limit?

When the Nyquist limit is exceeded what occurs?

Aliasing will occur when the velocity exceeds the Nyquist limit. The Nyquist limit is equal to one half of the pulse repetition frequency. Use the baseline shift to "stretch" the Nyquist limit.

What happens to the signal if it is sampled below Nyquist limit?

As the sampling frequency decreases, the signal separation also decreases. When the sampling frequency drops below the Nyquist rate, the frequencies will crossover and cause aliasing.

When a frequency above the Nyquist limit is sampled what happens?

Sounds above the Nyquist limit (in this case 4000 Hz) will seem to fold back into the allowed range. For example, a tone 100 Hz above the limit will appear as a phantom tone 100 Hz below it. So a frequency of 4100 will seem to appear at 3900, one at 4200 will appear at 3800, and so on.

What is the formula for the Nyquist limit?

The Nyquist limit always equals Pulse Repetition Frequency (PRF)/2. The US machine can display the Nyquist limit either as the maximum measurable blood flow velocity, or in kHz, the latter representing the maximum measurable Doppler shift.