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Bandpass Signaling Principles and Circuits Chap. 4
Microprocessor
(with digitized samples
of waveform stored
in random-access memory
or read-only memory)
PCM
signal
Digital-to-analog
converter
Synthesized
waveform
Figure 4–26
Direct digital synthesis (DDS).
reduced to a low level [Conkling, 1998]. More complicated PLL synthesizer configurations
can be built that incorporate mixers and additional oscillators.
4–15 DIRECT DIGITALSYNTHESIS
Direct digital synthesis (DDS) is a method for generating a desired waveform (such as a sine
wave) by using the computer technique described in Fig. 4–26. To configure the DDS system
to generate a waveform, samples of the desired waveform are converted into PCM words and
stored in the memory (random-access memory [RAM] or read-only memory [ROM]) of the
microprocessor system. The DDS system can then generate the desired waveform by “playing
back” the stored words into the digital-to-analog converter.
This DDS technique has many attributes. For example, if the waveform is periodic, such as
a sine wave, only one cycle of samples needs to be stored in memory. The continuous sine wave
can be generated by repeatedly cycling through the memory. The frequency of the generated sine
wave is determined by the rate at which the memory is read out. If desired, the microprocessor
can be programmed to generate a certain frequency during a certain time interval and then switch
to a different frequency (or another waveshape) during a different time interval. Also, simultaneous
sine and cosine (two-phase) outputs can be generated by adding another DAC. The signal-toquantizing
noise can be designed to be as large as desired by selecting the appropriate number of
bits that are stored for each PCM word, as described by Eq. (3–18).
The DDS technique is replacing analog circuits in many applications. For example, in
higher-priced communications receivers, the DDS technique is used as a frequency synthesizer
to generate local oscillator signals that tune the radio. (See Sec. 4–16.) In electronic pipe
organs and music synthesizers, DDS can be used to generate authentic as well as weird sounds.
Instrument manufacturers are using DDS to generate the output waveforms for function generators
and arbitrary waveform generators (AWG). Telephone companies are using DDS to
generate dial tones and busy signals. (See Chapter 8.)
4–16 TRANSMITTERS AND RECEIVERS
Generalized Transmitters
Transmitters generate the modulated signal at the carrier frequency f c from the modulating
signal m(t). In Secs. 4–1 and 4–2, it was demonstrated that any type of modulated signal could
be represented by
v(t) = Re{g(t)e jvct }
(4–113)