The diode in Fig. 20-2 acts to recover the audio from the radio signal. This is calleddetection; the circuit is a detector. If the detector is to be effective, the diode must beof the right type. It should have low capacitance, so that it works as a rectifier at radiofrequencies, passing current in one direction but not in the other. Some modern RFdiodes are actually microscopic versions of the old cat whisker, enclosed in a glass casewith axial leads. You have probably seen these in electronics hobby stores.Details about detector circuits are discussed in chapter 27. Some detectors usediodes; others do not. Modulation methods are examined in chapter 26.Frequency multiplicationWhen current passes through a diode, half of the cycle is cut off, as shown in Fig. 20-1.This occurs no matter what the frequency, from 60-Hz utility current through RF, aslong as the diode capacitance is not too great.The output wave from the diode looks much different than the input wave. Thiscondition is known as nonlinearity. Whenever there is nonlinearity of any kind in a cir-cuit—that is, whenever the output waveform is shaped differently from the input wave-form—there will be harmonic frequencies in the output. These are waves at integermultiples of the input frequency. (If you’ve forgotten what harmonics are, refer back tochapter 9.)Often, nonlinearity is undesirable. Then engineers strive to make the circuit linear,so that the output waveform has exactly the same shape as the input waveform. Butsometimes a circuit is needed that will produce harmonics. Then nonlinearity is intro-duced deliberately. Diodes are ideal for this.A simple frequency-multiplier circuit is shown in Fig. 20-3. The output LC circuit istuned to the desired nth harmonic frequency, nfo, rather than to the input or funda-mental frequency, fo.372 Some uses of diodes20-3 A frequency multipliercircuit.For a diode to work as a frequency multiplier, it must be of a type that would alsowork well as a detector at the same frequencies. This means that the component shouldact like a rectifier, but not like a capacitor.