
MK2069-01
VCXO-BASED LINE CARD CLOCK SYNCHRONIZER
VCXO AND SYNTHESIZER
IDT VCXO-BASED LINE CARD CLOCK SYNCHRONIZER
7
MK2069-01
REV K 051310
External VCXO PLL Components
In general, the loop damping factor should be 0.7 or greater
to ensure output stability. A higher damping factor will create
less peaking in the passband and will further ensure output
stability with the presence of system and power supply
noise. A damping factor of 4 will ensure a passband peak
less then 0.2dB which may be required for network clock
wander transfer compliance. A higher damping factor may
also increase output clock jitter when there is excess digital
noise in the system application, due to the reduced ability of
the PLL to respond to and therefore compensate for phase
noise ingress.
Notes on setting the value of CP
As another general rule, the following relationship should be
maintained between components CS and CP in the loop
filter:
CP establishes a second pole in the VCXO PLL loop filter.
For higher damping factors (> 1), calculate the value of CP
based on a CS value that would be used for a damping factor
of 1. This will minimize baseband peaking and loop
instability that can lead to output jitter.
CP also dampens VCXO input voltage modulation by the
charge pump correction pulses. A CP value that is too low
will result in increased output phase noise at the phase
detector frequency due to this. In extreme cases where
input jitter is high, charge pump current is high, and CP is too
small, the VCXO input voltage can hit the supply or ground
rail resulting in non-linear loop response.
The best way to set the value of CP is to use the filter
response software available from IDT (please refer to the
following section). CP should be increased in value until it
just starts affecting the passband peak.
Loop Filter Response Software
Online tools to calculate loop filter response can be found at
www.idt.com/?app=calculators&source=support_menu.
R
SET
C
P
21
22
23
24
1
2
3
4
5
6
7
8
9
10
11
12
13
14
X1
15
16
X2
17
18
LFR
19
LF
20
ISET
25
26
27
28
36
35
34
33
56
55
54
53
52
51
50
49
48
47
46
45
44
43
42
41
40
39
38
37
32
31
30
29
M
K
2069
XTAL
C
L
C
L
R
S
C
S
Optional
Crystal Tuning
Capacitors
DON'T STUFF
Refer to "Crystal Tuning Load
Capacitors" Section
C
P
C
S
20
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