A method of testing an analog, or mixed analog and digital, circuit designed for operation at a clock frequency comprises multiplexing a plurality of low frequency stimulus signals using a high frequency clock to produce a circuit input signal, applying the input signal to the circuit to obtain a circuit output signal; sampling the circuit output signal synchronously with the high frequency clock at a frequency equal to the clock frequency divided by the number of the low frequency signals; storing the samples and measuring properties of the signal samples to determine properties of the output signal of the circuit.