HDM8515 Datasheet, PDF (75/75 Pages) Hynix Semiconductor – DVB/DSS Compliant Receiver

A4. Nyquist Criteria Considerations
The HDM8515 is clocked at 60MHz, yet processes signals with symbol rates as high as 45M
symbols-per-second. At first thought, this might appear to be violating the Nyquist criteria which
states that the sampling rate must be at least twice the highest frequency component. The total
bandwidth of the 45Msps signal, with 35% excess bandwidth, is about 60MHz.
The samples provided to the HDM8515 are complex samples, which is equivalent to 120M
samples-per-second, which does satisfy the Nyquist criteria. Another way of looking at this is to
examine the baseband signal. The signal bandwidth covers 60MHz, but the baseband spectrum
covers from -30MHz to +30MHz. There are no baseband frequency components greater than
30MHz, and the 60MHz clock is adequate as long as complex samples are taken.
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