In the circuit of diode sensor-based wattmeters shown in Figure 3.37, the measurand vx, terminated on the matching resistor Rm, is applied to the diode sensor Ds working in its squared region in order to produce a corresponding output current ic in the bypass capacitor Cb.
If Cb has been suitably selected, the voltage Vc between its terminals, measured by the voltmeter amplifier Va, is proportional to the average of ic, i.e., to the average of the squares of instantaneous values of the input signal vx, and, hence, to the true average power.
In the true-average power measurement of nonsinusoidal waveforms having the biggest components at low frequency, such as radio-frequency AM (Amplitude Modulated), the value of Cb must also satisfy another condition.
The voltage vd on the diode must be capable of holding the diode switched-on into conduction even for the smallest values of the signal.
Otherwise, in the valleys of the modulation cycle, the high-frequency modulating source is disconnected by the back-biased diode and the measurement is therefore misleading.