LMX2371TMX National Semiconductor, LMX2371TMX Datasheet - Page 21

• Manufacturer Part Number: LMX2371TMX
• Description: IC FREQ SYNTH DUAL 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PLLatinum™r
• Type: PLL Frequency Synthesizerr

Specifications of LMX2371TMX

• Pll: Yes with Bypass
• Input: CMOS, TTL
• Output: CMOS
• Number Of Circuits: 1
• Ratio - Input:output: 3:1
• Differential - Input:output: Yes/No
• Frequency - Max: 2GHz, 1.2GHz
• 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: 2GHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LMX2371TMX

Application Information
A block diagram of the basic phase locked loop is shown in Figure 1 .
LOOP GAIN EQUATIONS
A linear control system model of the phase feedback for a
PLL in the locked state is shown in Figure 2 . The open loop
gain is the product of the phase comparator gain (K ), the
VCO gain (K
the gain of the feedback counter modulus (N). The passive
loop filter configuration used is displayed in Figure 3 , while
the complex impedance of the filter is given in Equation (2) .
The time constants which determine the pole and zero fre-
quencies of the filter transfer function can be defined as
and
The 3rd order PLL Open Loop Gain can be calculated in
terms of frequency,
and the design constants K , K
VCO
FIGURE 3. Passive Loop Filter
FIGURE 2. PLL Linear Model
/s), and the loop filter gain Z(s) divided by
, the filter time constants T1 and T2,
T2 = R2 • C2
VCO
DS101026-40
, and N.
FIGURE 1. Basic Charge Pump Phase Locked Loop
DS101026-39
(1)
(2)
(3)
(4)
21
From Equation (3) we can see that the phase term will be de-
pendent on the single pole and zero such that the phase
margin is determined in Equation (6) .
A plot of the magnitude and phase of G(s)H(s) for a stable
loop, is shown in Figure 4 with a solid trace. The parameter
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
double the frequency which gave us our original loop band-
width, wp, the loop response time would be approximately
halved. Because the filter attenuation at the comparison fre-
quency also diminishes, the spurs would have increased by
approximately 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
complications or compromises related to our original design
criteria. We would ideally like to momentarily shift the curve
of Figure 4 over to a different cutoff frequency, illustrated by
the dotted line, without affecting the relative open loop gain
and phase relationships. To maintain the same gain/phase
relationship at twice the original cutoff frequency, other terms
in the gain and phase Equations (5), (6) will have to compen-
sate by the corresponding “1/w” or “1/w
of Equations (3), (4), (6) indicates the damping resistor vari-
able R2 could be chosen to compensate the “w” terms for the
phase margin. This implies that another resistor of equal
value to R2 will need to be switched in parallel with R2 during
the initial lock period. We must also ensure that the magni-
tude of the open loop gain, H(s)G(s) is equal to zero at wp’ =
2wp. K
changed by a factor of 4, to counteract the w
in the denominator of Equations (3), (4) . The K term was
chosen to complete the transformation because it can
readily be switch between 1X and 4X values. This is accom-
plished by increasing the charge pump output current from 1
mA in the standard mode to 4 mA in Fastlock.
p
shows the amount of phase margin that exists at the point
VCO
( ) = tan
, K , N, or the net product of these terms can be
−1
(
• T2) − tan
−1
(
2
DS101026-38
” factor. Examination
• T1) + 180˚
2
term present
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(5)
(6)