MK2069-03 Datasheet, PDF (7/19 Pages) Integrated Circuit Systems – VCXO-Based Clock Translator with High Multiplication

MK2069-03
VCXO-Based Clock Translator with High Multiplication
C_P establishes a second pole in the VCXO PLL loop
filter. For higher damping factors (> 1), calculate the
value of C_P based on a C_S 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.
C_P also dampens VCXO input voltage modulation by
the charge pump correction pulses. A C_P 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 C_P 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 C_P is to use the filter
response software available from ICS (please refer to
the following section). C_P 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.icst.com.
Graph of Charge Pump Current vs. Value of R_SET (external resistor)
1E-3
100E-6
10E-6
100E+3
1E+6
R_SET, ohms
Recommended Range
of Operation
10E+6
Charge Pump Current, Example Settings
from Above Graph
R_SET
5 MΩ
3 MΩ
2 MΩ
1 MΩ
480 kΩ
400 kΩ
Charge Pump Current
(I_CP)
25 µA
42 µA
65 µA
125 µA
255 µA
300 µA
Notes on Setting Charge Pump Current
The recommended range for the charge pump current
is 25 µA to 300 µA. Below 25 µA, loop filter charge
leakage, due to PCB or capacitor leakage, can become
a problem. This loop filter leakage can cause locking
problems, output clock cycle slips, or low frequency
phase noise.
As can be seen in the loop bandwidth and damping
factor equations or by using the filter response software
available from ICS, increasing charge pump current
(I_CP) increases both bandwidth and damping factor.
MDS 2069-03 I
7
Revision 050203
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