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GS9035ACPJE3 Datasheet PDF : 14 Pages
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WZ WP1
WBW
FREQUENCY
WP2
Fig. 16 Bode Plot for PLL Transfer Function
The 3dB bandwidth of the transfer function is approximately
w3dB
=
------------------------------w----B----W-------------------------------
1
–
2 w-----B----W--
wP2
+
(---w----B----W------⁄--w-----P---2---)--2-
1
–
2
w-----B---W---
wP2
≈
-w----B----W--
0.78
9.2 Transfer Function Peaking
There are two causes of peaking in the PLL transfer function
given by Equation 1.
The first is the quadratic
s2CLF2L
+
s
----L-----
RLF
+
1
which has
wO
=
---------1-----------
CLF2L
and
Q = RLF
-C----L---F---2-
L
This response is critically damped for Q = 0.5.
Thus, to avoid peaking:
RLF
-C----L---F---2-
L
<
1--
2
or
----------1------------
RLFCLF2
----L-----
RLF
>
4
Therefore,
wP2 > 4 wBW
L=
2N
ΙCPKƒ
However, it is desirable to keep wP2 as low as possible to
reduce the high frequency content on the loop filter.
The second is the zero-pole combination:
---s--- + 1
------------s---C-----L---F---1---R----L---F----+-----1------------- = --w-----Z------------
s
CL
F
1RLF
–
-----L-----
RLF
+
1
----s----- + 1
wP1
This causes lift in the transfer function given by
20
LOG
w-----P---1-
wZ
=
20
LOG
----------1-----------
1 – ---w----Z----
wBW
To keep peaking to less than 0.05dB,
wZ < 0.0057 wBW
9.3 Selection of Loop Filter Components
Based on the above analysis, select the loop filter
components for a given PLL bandwidth, ƒ3dB, as follows:
1. Calculate
where
ΙCP is the charge pump current and is a function of the
RVCO resistor and is obtained from Figure 17.
Kƒ = 90MHz/V for VCO frequencies corresponding to
the ƒL curve.
Kƒ = 140MHz/V for VCO frequencies corresponding to
the ƒH curve.
N is the divider modulus.
(ƒL, ƒH and N can be obtained from Table 2 or Table 3).
2. Choose RLF = 2(3.14) ƒ3dB (0.78)L
3. Choose CLF1 = 174 L / (RLF)2
4. Choose CLF2 = L/4(RLF)2
400
350
300
250
200
150
100
50
0
0 200 400 600 800 1000 1200 1400 1600 1800
RVCO (Ω)
Fig. 17 Charge Pump Current vs. RVCO
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