LMX2325TM National Semiconductor, LMX2325TM Datasheet - Page 15

• Manufacturer Part Number: LMX2325TM
• Description: IC FREQ SYNTHESIZER 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PLLatinum™r
• Type: PLL Frequency Synthesizerr

Specifications of LMX2325TM

• Pll: Yes
• Input: CMOS, TTL
• Output: CMOS
• Number Of Circuits: 1
• Ratio - Input:output: 1:1
• Differential - Input:output: Yes/No
• Frequency - Max: 2.5GHz
• Divider/multiplier: Yes/No
• Voltage - Supply: 2.7 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP
• Frequency-max: 2.5GHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LMX2325TM

Application Information
LOOP FILTER DESIGN
A block diagram of the basic phase locked loop is shown.
An example of a passive loop filter configuration, including
the transfer function of the loop filter, is shown in Figure 2 .
Define the time constants which determine the pole and zero
frequencies of the filter transfer function by letting
and
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 and K
loop filter transfer function [Z(s)], the open loop Bode plot
can be calculated. The loop bandwidth is shown on the Bode
plot ( 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 −180˚.
FIGURE 2. 2nd Order Passive Filter
T2 = R2 • C2
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FIGURE 1. Basic Charge Pump Phase Locked Loop
VCO
] and the
DS012339-32
(1)
(2)
15
Open Loop Gain =
Closed Loop Gain =
Thus we can calculate the 3rd order PLL Open Loop Gain in
terms of frequency
From equation 3 we can see that the phase term will be
dependent on the single pole and zero such that
By setting
we find the frequency point corresponding to the phase
inflection point in terms of the filter time constants T1 and T2.
This relationship is given in equation 6.
= K Z(s) K
FIGURE 3. Open Loop Transfer Function
( ) = tan
VCO
−1
/Ns
(
i
/
o
e
/
• T2) − tan
= H(s) G(s)
i
= G(s)/[1 + H(s) G(s)]
−1
(
• T1) + 180˚
DS012339-31
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DS012339-29
(3)
(4)
(5)
(6)