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数字频率合成Demystified
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数字频率合成Demystified:
This text deals with emerging modern digital techniques used to
generate and modulate sine wav e s. These waveforms are used in
almost all radio applications, communications, radar, digital comm
u n i c a t i o n s, electronic imaging, and more. Such tech n i q u e s
either build the waveform from the “ground up” digitally (i.e., generate
all the signal parameters such as phase, frequency, and
amplitude digitally) and deal with the very fundamental nature of
the waveform and its features (direct digital synthesis) or are part
of the digital heart of modern p h a s e - l o cked loop (P L L) synthesize
r s. This might seem, and is indeed, a common and known subject.
Sine waves are truly natural waveforms and trigonometric functions
that are well known and have been researched for a long
t i m e. Furthermore, frequency synthesis is quite a mature technology
with extensive literature and comprehensive coverage in
the professional meetings. Why another text on the subject? What
is new besides application-specific integrated circuit (A S I C) technologies
and silicon densities, geometry, and integration?
While the above statements are true, there is a continuous evolution
in the tech n o l o g y. The generation of accurate wav e f o r m s
p l ays a crucial role in almost all electronic equipment, from radar
to home entertainment equipment, so the importance of the subject
is cl e a r. Clearly, the most important reason for the utilization
of the now extremely popular PLL synthesizers in consumer elec-
tronics and other very popular applications at extremely low cost
(and the popularization of frequency synthesis from consumer
products all the way to complex requirements) is the advance of
digital technology; integrated, high densities; and low-cost silicon
single-PLL chips and ASICs. However, parallel to the advance of
traditional PLL synthesis, there emerged other synthesis techn
iques, mainly digital in nature, direct digital synthesis (D D S)
and fractional-N PLL synthesis. Thus, the classical PLL synthesizer
is now being supplemented with a sizable element of digital
technology and digital signal processing (DSP). Indeed the application
of DSP techniques to frequency synthesis is still at an early
stage.
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