The first step in evaluating the baseband performance of a head-end modulator is to obtain an "ideal" demodulator. Otherwise, shortcomings of the test equipment and those of the device under test will be difficult to separate, A similar requirement plagues the evaluation of the head-end demodulator. In this case the "perfect" modulator is needed. But what is "perfect"? How are the "perfect" instruments tested? This paper answers these questions.