n FPGA, the DDC filtering can be done within the ADC to see just one-eighth or one-sixteenth of the total bandwidth. When used in conjunction with a synthesized numerically controlled oscillator (NCO), the precise placement of the converter's DDC filter in the band can be tuned with accurate resolution. This permits a lower output rate and eliminates the need to move and pro- cess large amounts of unwanted data in an FPGA. When two DDCs are available, each with a unique NCO, they can alternately be stepped across the spectrum to sweep for expected sig- nals, without loss of visibility. This is often typical in some radar applications. 5.5.1 Nyquist Zones and Sampling of Wideband Signals In the theory of sampling, a signal must be sampled at more than twice the maximum fre- quency fmax in the spectrum of the signal, in order to be able to regenerate the signal from its sampled values. This sampling frequency is known as the Nyquist rate fs 2fmax. If the signal is sampled below the Nyquist rate, aliasing occurs. As a general rule, for a sample rate of fs, one would not be able to differentiate between any sine waves with frequencies New Radio Access RF and Transceiver Design Considerations Nfs/2 = Af, where N is an integer. In this case, the signal power for these frequencies will concentrate at fs/2- Af. As long as, for each Af, there is only a signal at one of the aliasing frequencies, the signal has not been irreversibly corrupted. That means the signal can still be reconstructed, if one knows which of the frequencies that alias occurs fs/2 + Af were occupied by the desired signal. In this case, the aliasing decisively can be useful, which leads to the concept of the Nyquist zones. However, if the desired signal has significant power at both fs/2 - Af and fs/2 + Af frequencies, then aliasing occurs and you no longer have the information to distinguish these two components since these formerly distinct signal frequencies now appear as a single frequency, thus it is not possi- ble to determine the power of each co