ACS8946 Semtech Corporation, ACS8946 Datasheet - Page 12

• Manufacturer Part Number: ACS8946
• Description: Jitter Attenuating, Multiplying Phase Locked Loop, with Protection Switch, for Oc-12/stm-4 and GbE
• Manufacturer: Semtech Corporation

such that if any phase error signal is present, the LOCKB
output drives out a +10 µA current, otherwise it is off.
Consequently this output (LOCKB) is a pulse width
modulated (PWM) pulse stream whose mark/space ratio
indicates the current input phase error. Filtering this
signal with a simple external RC parallel filter as shown in
Figure 9 will give a signal whose output level indicates PLL
phase and frequency lock.
Figure 9 Lock Filter Components
The filtering components are external so that the time to
indicate lock or not locked can be optimized for the
application. The output indicates both phase and
frequency lock. During off-frequency conditions the
LOCKB output will be predominately high in its PWM
generation with the filtered version giving a constant high
state.
PLL Bandwidth Setting
The bandwidth is set by two identical sets of passive RC
components that connect to the differential charge pump
outputs and internal VCO control inputs. Pins VCN and
VCP are the combined differential charge pump outputs
and VCO control voltage inputs. Figure 10 shows the
arrangement.
Figure 10 Loop Filter Components
All capacitors should be low leakage and low ESR
(Equivalent Series Resistance). Tantalum, or ceramic
where possible, are suitable. Tables 6 to 9 are based on a
Revision 3/November 2006 © Semtech Corp.
ADVANCED COMMUNICATIONS
C1
F8943_010Loopfilter_02
LOCKB
R1
C2
C3
LOCK_B
R2
C4
R1
470K
C1
220nF
F8944_011Lockfilter_02
VCP
VCN
GND
GND
FINAL
Page 12
damping factor of 1.2 (phase margin 80.2°). Higher
damping factors may be used if lower transfer peaking is
required. Contact Semtech Sales Support for further
details.
RC Components Required to Achieve Bandwidth at
Given Input Frequencies (Tables 6 to 9).
Table 6 77.76 MHz or 19.44 MHz Input Frequency
Table 7 155.52 MHz or 38.88 MHz Input Frequency
Note...* not available at 155.52 MHz input
Table 8 125 MHz Input Frequency
Note...All bandwidths are subject to ± 20% variation due to
component tolerancing.
1500
2000
4000
8000
1500*
2000
4000
8000
1500
2000
4000
8000
Closed Loop
Closed Loop
Closed Loop
Bandwidth
Bandwidth
Bandwidth
56
75
150
270
110
150
300
560
68
91
180
360
R1 & R2/
R1 & R2/
R1 & R2/
Ω
Ω
Ω
ACS8946 JAM PLL
33
15
4.7
0.68
15
6.8
2.2
0.47
22
15
3.3
0.68
C2 & C4/
C2 & C4/
C2 & C4/
µF
µF
µF
DATASHEET
www.semtech.com
200
100
33
7.5
91
47
15
3.9
150
91
20
6.2
C1 & C3
C1 & C3
C1 & C3
nF
nF
nF