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XR215A Datasheet, PDF (18/32 Pages) Exar Corporation – Monolithic Phase-Locked Loop
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XR-215A
Low-Pass Filter
The low-pass filter section is formed by connecting an
external capacitor or RC network across terminals 2 and
3. The low-pass filter components can be connected
either between pins 2 and 3 or, from each pin to ground.
Typical filter configurations and corresponding filter
transfer functions are shown in Figure 15 where R1 (6kW)
is the internal impedance at pins 2 and 3. It should be
noted that the rejection of the low pass filter decreases
above 2MHz when the capacitor is tied from pin 2 to 3.
Lag Filter
2
3
C1
t1 = 2R1C1
F(s) =
1
St1
Lag Lead Filter
2
R2
3
C1
t1 = 2R1C1
t2 = R2C1
F(s)
=
1
1 + St2
+ S(t1+t2)
2
3
C1
C1
t1 = R1C1
F(s) =
1
St1
2
3
C1
C1
R2
R2
t1 = R1C1
t2= R2C1
F(s) =
1 + St2
1 + S(t1 + t2)
Note:
R1 = 6kW internal resistor.
Figure 15.
The natural frequency wn can be calculated from the VCO conversion gain K0, the phase comparator conversion gain
Kd, and the low pass filter time constants t1 and t 2 as follows:
ǒ Ǔ j
+
wn
2
t2
)
K0
1
·
Kd
Then the damping factor j can be calculated using:
Ǹ wn +
K0 · Kd
t1 ) t2
Rev. 1.01
18
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