LMX2331LSLBX National Semiconductor, LMX2331LSLBX Datasheet - Page 20

• Manufacturer Part Number: LMX2331LSLBX
• Description: PLL Frequency Synthesizer Dual 45MHz to 2000MHz 24-Pin LAM CSP T/R
• Manufacturer: National Semiconductor
• Type: PLL Frequency Synthesizerr

Specifications of LMX2331LSLBX

• Package: 24LAM CSP
• Number Of Elements Per Chip: 2
• Operating Temperature: -40 to 85 °C
• Operating Supply Voltage: 3 V

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Application Information
A block diagram of the basic phase locked loop is shown in
Figure
LOOP GAIN EQUATIONS
A linear control system model of the phase feedback for a PLL
in the locked state is shown in
is the product of the phase comparator gain (Kφ), the VCO
gain (K
of the feedback counter modulus (N). The passive loop filter
configuration used is displayed in
impedance of the filter is given in
The time constants which determine the pole and zero fre-
quencies of the filter transfer function can be defined as
1.
VCO
/s), and the loop filter gain Z(s) divided by the gain
FIGURE 3. Passive Loop Filter
FIGURE 2. PLL Linear Model
Figure
Figure
Equation
12806 Version 9 Revision 2
FIGURE 1. Basic Charge Pump Phase Locked Loop
1280616
2. The open loop gain
3, while the complex
1.
1280615
(1)
Print Date/Time: 2011/07/11 15:54:42
20
and
The 3rd order PLL Open Loop Gain can be calculated in terms
of frequency, ω, the filter time constants T1 and T2, and the
design constants K
From Equations 2, 3 we can see that the phase term will be
dependent on the single pole and zero such that the phase
margin is determined in
A plot of the magnitude and phase of G(s)H(s) for a stable
loop, is shown in
φ
the gain drops below zero (the cutoff frequency wp of the
loop). In a critically damped system, the amount of phase
margin would be approximately 45 degrees.
If we were now to redefine the cut off frequency, wp', as dou-
ble the frequency which gave us our original loop bandwidth,
wp, the loop response time would be approximately halved.
Because the filter attenuation at the comparison frequency
also diminishes, the spurs would have increased by approxi-
mately 6 dB. In the proposed Fastlock scheme, the higher
spur levels and wider loop filter conditions would exist only
during the initial lock-on phase—just long enough to reap the
benefits of locking faster. The objective would be to open up
the loop bandwidth but not introduce any additional compli-
cations or compromises related to our original design criteria.
We would ideally like to momentarily shift the curve of
4
line, without affecting the relative open loop gain and phase
p
over to a different cutoff frequency, illustrated by the dotted
shows the amount of phase margin that exists at the point
φ(ω) = tan
−1
Figure 4
(ω • T2) − tan
φ
, K
T2 = R2 • C2
VCO
Equation
, and N.
with a solid trace. The parameter
−1
5.
(ω • T1) + 180°
Figure
1280614
(2)
(3)
(4)
(5)