GS4915 Data Sheet
3.3.1 Phase Detector
3.3.2 Charge Pump
The GS4915's phase detector can identify phase misalignment between the
selected input clock and the reference clock provided by the external VCO, and
correspondingly signal the charge pump to alter the VCO control voltage.
The charge pump block of the PLL is powered externally by +2.5V DC applied to
CP_VDD. This is provided by the GS4915 itself at the VCO_VDD pin. An external
RC filter at the CP_VDD pin is recommended to reduce supply noise for best jitter
performance. Please refer to the Typical Application Circuit on page 22.
An external resistance connected to the CP_RES pin is used to set the charge
pump reference current of the device. Typically, the CP_RES pin will be connected
through 10kΩ to VCO_GND.
3.3.3 Loop Filter
The GS4915 PLL loop filter is an external first order filter formed by a series RC
connection as shown in Table 3-1: Loop Filter Component Values. The loop filter
resistor value sets the bandwidth of the PLL and the capacitor value controls its
stability and lock time. A loop filter resistor value between 1 Ω and 20 Ω and a loop
filter capacitor value between 1µF and 33µF are recommended.
The GS4915 uses a non-linear, bang-bang, PLL, therefore its bandwidth scales
linearly with the input jitter amplitude - greater input jitter results in a smaller loop
bandwidth causing more of the input jitter to be rejected. For a given input jitter
amplitude, a smaller loop filter resistor produces a narrower loop bandwidth. With
an input jitter amplitude of 300ps, for example, the PLL bandwidth can be adjusted
from 2KHz to 40KHz by varying the loop filter resistor, as shown in the table below.
For use with GS4911, a narrow loop bandwidth is recommended.
Increasing the loop filter capacitor value increases the stability of the PLL, but
results in a longer lock time. For loop filter resistors smaller than 7Ω, a capacitor
value of 33µF is recommended, while larger resistor values can accommodate
smaller capacitors. Sample combinations of the loop filter resistor and capacitor
values are shown in the table below, along with the resulting loop bandwidth.
Additional loop bandwidths can be achieved by using different loop filter resistor
values.
Table 3-1: Loop Filter Component Values
Loop Filter
R
Typical Loop
Bandwidth*
Recommended
Loop Filter C
Comments
1Ω
2kHz
8kHz
40kHz
33μF
10μF
1μF
Narrow bandwidth - provides maximum jitter reduction. Long lock-time.
7Ω
20Ω
Note:
Wide bandwidth. Fast lock-time.
1. *Measured with 300ps pk-pk input jitter on CLK.
39145 - 3 November 2007
14 of 26
GENNUM [ GENNUM CORPORATION ]
