Voltage sampling is traditionally used for analog-to-digital (A/D) conversion. In a voltage sampler, a sampling switch is placed between a signal source and a capacitor. Between two sampling moments, the capacitor voltage tracks the signal voltage accurately. At the sampling moment, the switch is turned off to hold the capacitor voltage. The two processes become increasingly difficult when the signal frequency increases. For a given accuracy, thermal noise and switching noise set a minimum allowable capacitance while the tracking speed set a maximum allowable capacitance or switch resistance. It becomes impossible when the maximum is smaller than the minimum. Moreover, the clock jitter and finite turning-off speed (nonzero sampling aperture) make the sampling timing inaccurate. In fact, the bandwidth of a voltage sampling circuit must be much larger than the signal bandwidth. This makes direct sampling of high frequency radio signal extremely difficult. Sub-sampling can reduce the sampling rate but not the bandwidth of the sampling circuit and not the demands on small clock jitter and small sampling aperture.