enough capacitance so that it follows the carrier frequency. A transistor, biased for class-Boperation, can also be used.The rapid pulsations are smoothed out by passing the output of the diode or tran-sistor through a capacitor of just the right value. The capacitance should be largeenough so that it holds the charge for one carrier current cycle, but not so large that itsmooths out the cycles of the modulating signal.This scheme is known as envelope detection. It is used extensively for reception ofAM audio and video.Detection of CW signalsIf you tune in an unmodulated carrier with an envelope detector, you won’t hear any-thing. A keyed carrier might produce barely audible thumps or clicks, but it will be im-possible to read the code.For detection of CW, it’s necessary to inject a signal into the receiver a few hundredhertz from the carrier. This injected signal will beat against the carrier, producing a tonewhose frequency is the difference between the carrier and injected-signal frequencies.The injected signal is produced by a beat-frequency oscillator (BFO). The beating oc-curs in a signal combiner or mixer.A block diagram of a simple CW receiver is shown in Fig. 27-5. The BFO is tunable.Suppose there is a CW signal at 3.550 MHz. As the BFO approaches 3.550 MHz from be-low, a high-pitched tone will appear at the output. When the BFO reaches 3.549 MHz,the tone will be 3.550–3.549 MHz, or 1 kHz. This is a comfortable listening pitch for mostpeople. The BFO setting isn’t too critical; in fact, it can be changed to get a differenttone pitch if you get tired of listening to one pitch. As the BFO frequency passes 3.550MHz, the pitch will descend to a rumble, then to a swish-swish sound. As the BFO fre-quency continues to rise, the tone pitch will increase again, eventually rising beyond therange of human hearing.Detection of CW signals50527-5Block diagram of asimple direct-conversionreceiver for CW, FSK,and SSB.