W3000 Datasheet, PDF (19/28 Pages) Agere Systems – W3000 PLL Dual-Band Frequency Synthesizer

Advance Data Sheet
December 1999
Application Information
A typical PLL system can be modeled as follows:
W3000 PLL Dual-Band Frequency Synthesizer
fREF
1/R
+
KPD
Z(s)
KVCO
s
fVCO
1/N
Figure 11. Typical PLL Model
KPD = Phase detector in mA/2π rad
Z(s) = Loop filter
KVCO = VCO gain in MHz/V
N = Total divide ratio
R = Reference divide ratio
Where the open loop gain is:
G(s)_OPEN =
KPD
*
Z(s) * Kvco
Ns
Where
s = jω
The circuit shown in Figure 12 uses a passive third-order loop filter for the element Z(s), defined by the network:
CHARGE PUMP
OUTPUT
C1
R2
R1
C2
VCO
C3
Figure 12. Third-Order Loop Filter
The purpose of the loop filter is to provide response with bandwidth sufficient not only to allow a quick lock time
but also to meet phase-noise and reference sideband requirements. Addition of a third pole formed by R2 and C3
will improve reference sideband performance with little overall impact on the loop performance. A reasonable
practical limit is that the f comparison is greater than 5 times loop bandwidth.
Lucent Technologies Inc.
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