# Nyquist limit

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Related to Nyquist frequency: Nyquist Theorem, Nyquist limit

## Nyquist limit

the maximum frequency shift that can be accurately interpreted in a pulsed Doppler ultrasound unit.
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Although Figure 2 shows a well behaved (magnitude) spectrum, we cannot know from the reconstructed waveform alone whether this data has been corrupted by energy aliased from the frequency spectrum above the Nyquist frequency.
If we define "in-band" as the desired signal band extending up to Nyquist frequency (half the sample rate), and "out-of-band" as all other energy, then corruption of the in-band signal can result from:
The higher the slope, the closer to the Nyquist frequency that the bandwidth can approach.
Because of the very gradual transition between passband and stopband for a Bessel filter, the attenuation at the 1-MHz Nyquist frequency is about 54 dB.
Linear's LTC2248 maintains 74 dB SNR and 80 dB SFDR for inputs well beyond the Nyquist frequency, as shown in Figure 2.
Another common goal of filtering is to guard against aliasing--the misrepresentation of a signal due to acquiring the data too slowly--by limiting the frequency components that are sampled to those at or below the Nyquist frequency.
It is quite straightforward to realize that the instrument output will be equivalent to a higher speed DAC so the equivalent Nyquist frequency will be increased accordingly.
The signal components that will be aliased are those that are passed through the amplifier and which are above the Nyquist frequency.
To illustrate, assume the digitizer's maximum sample rate is 100 MS/s, which makes its Nyquist frequency 50 MHz, and the signal has two components: the fundamental at 10 MHz and a spur frequency component at 20 MHz.
In contrast, Butterworth or elliptical filters allow much higher frequencies to be monitored, almost up to the Nyquist frequency or one-half of the sample rate.

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