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Sec. 5–9 Binary Modulated Bandpass Signaling 361
is called a binary FSK signal. Of course, a multilevel input signal would produce a multilevel
FSK signal. We will assume that the input is a binary signal in this section and examine the
properties of binary FSK signals.
In general, the spectra of FSK signals are difficult to evaluate since the complex
envelope, g(t), is a nonlinear function of m(t). However, the techniques that were developed in
Sec. 5–6 are applicable, as we show in the next example.
Example 5–9 SPECTRUM FOR THE BELL-TYPE 103 FSK MODEM
Today, personal computers are often connected to internet servers via a modem connected to a
DSL twisted-pair telephone line (see Section 8–3) or via a coaxial cable-system modem that uses
QAM signaling (described in the next section). Historically, the personal computer was
connected to a remote computer via a modem connected to an analog dial-up telephone line. The
Bell-type 103 modem is an example of this type of dial-up connection and it uses FSK signaling.
FSK signaling continues to be used for telephone caller ID signaling (Bell 202 standard; see
Tables 8–1) and in many wireless applications, because it is cost effective.
The evaluation of spectra for FSK signals will be demonstrated by using the 300-bs
Bell System 103 modem standard as an example. Referring to Fig. 5–24, we find that each
modem contains both an FSK transmitter and an FSK receiver, so that the computer terminal
can both “talk” and “listen.” Two FSK frequency bands are used (one around 1 kHz and another
around 2 kHz), making it possible to talk and listen simultaneously. This approach is called
full-duplex capability. (In half-duplex, one cannot listen while talking, and vice versa; in
simplex, one can only talk or only listen.) The standard mark and space frequencies for the two
bands are shown in Table 5–5. From this table, it is seen that the peak-to-peak deviation is
2∆F = 200 Hz.
The spectrum for a Bell-type 103 modem will now be evaluated for the case of the widestbandwidth
FSK signal, obtained when the input data signal consists of a deterministic (periodic)
square wave corresponding to an alternating data pattern (i.e., 10101010). † This is illustrated in
Fig. 5–25a, where T b is the time interval for one bit and T 0 = 2T b is the period of the data modulation.
The spectrum is obtained by using the Fourier series technique developed in Sec. 5–6 and
Keyboard
computer
terminal
Digital
data
FSK
modem
(originate)
Dial-up phone line
Telephone
office
FSK
modem
(answer)
Computer
center
Figure 5–24
Computer communication using FSK signaling.
† For the case of random data, the PSD for g(t) is given by the h = 0.7 ≈ 0.67 curve of Fig. 5–27.