CS2000-CP
4. ARCHITECTURE OVERVIEW
4.1
Delta-Sigma Fractional-N Frequency Synthesizer
The core of the CS2000 is a Delta-Sigma Fractional-N Frequency Synthesizer which has very high-resolu-
tion for Input/Output clock ratios, low phase noise, very wide range of output frequencies and the ability to
quickly tune to a new frequency. In very simplistic terms, the Fractional-N Frequency Synthesizer multiplies
the Timing Reference Clock by the value of N to generate the PLL output clock. The desired output to input
clock ratio is the value of N that is applied to the delta-sigma modulator (see Figure 4).
The analog PLL based frequency synthesizer uses a low-jitter timing reference clock as a time and phase
reference for the internal voltage controlled oscillator (VCO). The phase comparator compares the fraction-
al-N divided clock with the original timing reference and generates a control signal. The control signal is fil-
tered by the internal loop filter to generate the VCO’s control voltage which sets its output frequency. The
delta-sigma modulator modulates the loop integer divide ratio to get the desired fractional ratio between the
reference clock and the VCO output (thus the duty cycle of the modulator sets the fractional value). This
allows the design to be optimized for very fast lock times for a wide range of output frequencies without the
need for external filter components. As with any Fractional-N Frequency Synthesizer the timing reference
clock should be stable and jitter-free.
Timing Reference
Clock
Phase
Comparator
Internal
Loop Filter
Voltage Controlled
Oscillator
PLL Output
Fractional-N
Divider
Delta-Sigma
Modulator
N
Figure 4. Delta-Sigma Fractional-N Frequency Synthesizer
4.2
Hybrid Analog-Digital Phase Locked Loop
The addition of the Digital PLL and Fractional-N Logic (shown in Figure 5) to the Fractional-N Frequency
Synthesizer creates the Hybrid Analog-Digital Phase Locked Loop with many advantages over classical an-
alog PLL techniques. These advantages include the ability to operate over extremely wide frequency ranges
without the need to change external loop filter components while maintaining impressive jitter reduction per-
formance. In the Hybrid architecture, the Digital PLL calculates the ratio of the PLL output clock to the fre-
quency reference and compares that to the desired ratio. The digital logic generates a value of N which is
then applied to the Fractional-N frequency synthesizer to generate the desired PLL output frequency. Notice
that the frequency and phase of the timing reference signal do not affect the output of the PLL since the
digital control loop will correct for the PLL output. A major advantage of the Digital PLL is the ease with which
the loop filter bandwidth can be altered. The PLL bandwidth is set to a wide-bandwidth mode to quickly
achieve lock and then reduced for optimal jitter rejection.
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DS761A2
CIRRUS [ CIRRUS LOGIC ]
