US Patent 7403580 Software defined radio system

The invention provides a technique that saves rate conversion accompanying complicated calculations and shares an analog filter for anti-aliasing in a sampling device. The sampling device employs an anti-alias filter having a fixed cutoff frequency, an A/D converter and a D/A converter each having a clock signal input, which are capable of varying a sampling frequency, and a PLL circuit capable of varying the frequency of a clock signal; and it sets the sampling frequency not lower than an integral multiple of a signal bandwidth and double the cutoff frequency for sampling.