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參數資料
| 型號： | LMX2301TM |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | XO, clock |
| 英文描述： | PLLatinumTM 160 MHz Frequency Synthesizer for RF Personal Communications |
| 中文描述： | PLL FREQUENCY SYNTHESIZER, 160 MHz, PDSO20 |
| 封裝： | 0.173 INCH, PLASTIC, TSSOP-20 |
| 文件頁數： | 11/14頁 |
| 文件大小： | 225K |
| 代理商： | LMX2301TM |
Application Information
LOOP FILTER DESIGN
A block diagram of the basic phase locked loop is shown.
TL/W/12458–24
FIGURE 1. Basic Charge Pump Phase Locked Loop
An example of a passive loop filter configuration, including
the transfer function of the loop filter, is shown in Figure 2.
TL/W/12458–25
Z(s)
e
s (C2
#
R2)
a
1
s
2
(C1
#
C2
#
R2)
a
sC1
a
sC2
FIGURE 2. 2nd Order Passive Filter
Define the time constants which determine the pole and
zero frequencies of the filter transfer function by letting
T2
e
R2
#
C2
and
C1
#
C2
C1
a
C2
The PLL linear model control circuit is shown along with the
open loop transfer function in Figure 3. Using the phase
detector and VCO gain constants
[
K
w
and K
VCO
]
and the
loop filter transfer function
[
Z(s)
]
, the open loop Bode plot
can be calculated. The loop bandwidth is shown on the
Bode plot (
0
p) as the point of unity gain. The phase margin
is shown to be the difference between the phase at the unity
gain point and
b
180
§
.
(1a)
T1
e
R2
#
(1b)
TL/W/12458–27
Open Loop Gain
e
i
i
/
i
e
e
H(s) G(s)
e
K
w
Z(s) K
VCO
/Ns
Closed Loop Gain
e
i
o
/
i
i
e
G(s)/
[
1
a
H(s) G(s)
]
TL/W/12458–26
FIGURE 3. Open Loop Transfer Function
Thus we can calculate the 3rd order PLL Open Loop Gain in
terms of frequency
G(s)
#
H(s)
l
s
e
j
#
0
eb
K
w
#
K
VCO
(1
a
j
0
#
T2)
0
2
C1
#
N(1
a
j
0
#
T1)
From equation 2 we can see that the phase term will be
dependent on the single pole and zero such that
w
(
0
)
e
tan
b
1
(
0
#
T2)
b
tan
b
1
(
0
#
T1)
a
180
§
By setting
#
T1
T2
(2)
(3)
d
w
d
0
e
T2
1
a
(
0
#
T2)
2
b
T1
1
a
(
0
#
T1)
2
e
0
(4)
we find the frequency point corresponding to the phase in-
flection point in terms of the filter time constants T1 and T2.
This relationship is given in equation 5.
0
p
e
1/
T2
#
T1
For the loop to be stable the unity gain point must occur
before the phase reaches
b
180 degrees. We therefore
want the phase margin to be at a maximum when the magni-
tude of the open loop gain equals 1. Equation 2 then gives
C1
e
K
w
#
K
VCO
#
T1
(5)
0
p2
#
N
#
T2
ó
(1
a
j
0
p
#
T2)
(1
a
j
0
p
#
T1)
ó
(6)
http://www.national.com
11
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