Detection of FSK signals

Chapter Detection of FSK signals

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • This demodulation scheme is called heterodyne detection. A receiver that makesuse of a heterodyne detector is called a direct-conversion receiver.Detection of FSK signalsFrequency-shift keying can be detected by using the same methods as CW detection.The carrier beats against the BFO in the mixer, producing an audio tone that alternatesbetween two different pitches. The block diagram of Fig. 27-5 can therefore apply toFSK reception as well as to CW reception.With FSK, the BFO frequency is always set well above, or well below, both the markand the space carrier frequencies. The offset affects the audio tone frequencies and is setso that certain standard pitches result. There are several sets of standard tone frequen-cies, depending on whether the communications is amateur, commercial, or military. Toget the proper pitches, the BFO must be set precisely at the right frequency. There is lit-tle tolerance for error.Detection of SSB signalsA single-sideband signal is really just an AM signal, minus the carrier and one of thesidebands. If the BFO frequency of a CW receiver is set exactly where the carrier shouldbe, the sideband, either upper or lower, will beat against the BFO and produce audiooutput. In this way, the receiver of Fig. 27-5 can be used to detect SSB.The BFO frequency is critical for good SSB reception. If it is not set exactly at thefrequency of the suppressed carrier, the voice will sound bizarre, like “monkey chatter.” A more advanced method of receiving CW, FSK, and SSB makes use of a productdetector. This is a specialized form of mixer and is discussed a little later in this chapter.Detection of FM signalsFrequency-modulated or phase-modulated signals can be detected in several ways. Thebest FM receivers respond to frequency/phase changes, but not to amplitude changes.Slope detectionAn AM receiver can be used to detect FM. This is done by setting the receiver near, butnot exactly on, the FM signal.An AM receiver has a narrowband filter with a passband of about 6 kHz. This givesa selectivity curve such as that shown in Fig. 27-6. If the FM carrier frequency is nearthe skirt, or slope, of the filter response, modulation will cause the signal to move in andout of the passband. This will make the receiver output vary with the modulating data.Because this scheme takes advantage of the filter slope, it is called slope detection.It has two disadvantages. First, the receiver will respond to amplitude variations (be-cause that’s what it’s designed for). Second, there will be nonlinearity in the receivedsignal, producing distortion, because the slope is not a straight line.506 Data reception