36705482563117027503324650252sect6| Application Note
SECTION 6 HIGH SPEED DACs AND DDS SYSTEMS Walt Kester
INTRODUCTION
A frequency synthesizer generates multiple frequencies from one or more frequency references. These devices have been used for decades, especially in communications systems. Many are based upon switching and mixing frequency outputs from a bank of crystal oscillators. Others have been based upon well understood techniques utilizing phase-locked loops (PLLs). This mature technology is illustrated in Figure 6.1. A fixed-frequency reference drives one input of the phase comparator. The other phase comparator input is driven from a divide-by-N counter which is in turn driven by a voltage-controlled-oscillator (VCO). Negative feedback forces the output of the internal loop filter to a value which makes the VCO output frequency N-times the reference frequency. The time constant of the loop is controlled by the loop filter. There are many tradeoffs in designing a PLL, such a phase noise, tuning speed, frequency resolution, etc., and there are many good references on the subject (see References 1, 2, and 3).
FREQUENCY SYNTHESIS USING OSCILLATORS AND PHASE-LOCKED LOOPS
OSCILLATOR BANK
MIXER XO 1 fc FIXED FREQUENCY REFERENCE fout
PHASE COMPARATOR
PHASE-LOCKED LOOP
VCO LOOP FILTER f out
XO 2
XO 3 SW XO n
MIXER fout = N f c
N
a
6.1
With the widespread use of digital techniques in instrumentation and communications systems, a digitally-controlled method of generating multiple frequencies from a reference frequency source has evolved called Direct Digital Synthesis (DDS). The basic architecture is shown in Figure 6.2. In this simplified model, a stable clock drives a programmable-read-only-memory (PROM) which stores one or more integral number of cycles of a sinewave (or other arbitrary 1
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