Methods and apparatus for sampling and determining the frequency of periodic digital signals. An exemplary digital sampling apparatus includes a polyphase sampling apparatus configured to sample a periodic digital signal according to a polyphase clock system having multiple phases. The multiple phases provided by the polyphase clock system are successively distributed in time so that consecutive phases have a periodic phase difference. By using a polyphase clock system, a sampling rate that is equivalent to a sampling clock having a period equal to the phase difference in time between phases of the polyphase clocks is realized. Accordingly, the effective sampling rate of a given periodic digital signal can be increased, or the sampling of higher frequency periodic digitals signals can be achieved, while the underlying logic circuitry used to capture the samples is clocked at a much lower rate.