LMX2320TM National Semiconductor, LMX2320TM Datasheet - Page 16

• Manufacturer Part Number: LMX2320TM
• Description: IC FREQ SYNTH 2.0GHZ 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PLLatinum™r
• Type: PLL Frequency Synthesizerr

Specifications of LMX2320TM

• Pll: Yes
• Input: CMOS, TTL
• Output: CMOS
• Number Of Circuits: 1
• Ratio - Input:output: 1:1
• Differential - Input:output: Yes/No
• Frequency - Max: 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: 2.5GHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LMX2320TM

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Application Information
For the loop to be stable the unity gain point must occur
before the phase reaches −180 degrees. We therefore want
the phase margin to be at a maximum when the magnitude
of the open loop gain equals 1. Equation 3 then gives
Therefore, if we specify the loop bandwidth,
phase margin,
late the two time constants, T1 and T2, as shown in equa-
tions 8 and 9. A common rule of thumb is to begin your
design with a 45˚ phase margin.
From the time constants T1, and T2, and the loop bandwidth,
10 to 12.
K
K (mA)
p
VCO
, the values for C1, R2, and C2 are obtained in equations
(MHz/V)
p
, Equations 1 through 7 allow us to calcu-
Voltage Controlled Oscillator (VCO) Tuning
Voltage constant. The frequency vs voltage
tuning ratio.
Phase detector/charge pump gain con-
stant. The ratio of the current output to the
input phase differential.
(Continued)
p
, and the
(10)
(12)
(11)
(7)
(8)
(9)
16
N
RF
f
In choosing the loop filter components a trade off must be
made between lock time, noise, stability, and reference
spurs. The greater the loop bandwidth the faster the lock
time will be, but a large loop bandwidth could result in higher
reference spurs. Wider loop bandwidths generally improve
close in phase noise but may increase integrated phase
noise depending on the reference input, VCO and division
ratios used. The reference spurs can be reduced by reduc-
ing the loop bandwidth or by adding more low pass filter
stages but the lock time will increase and stability will de-
crease as a result.
THIRD ORDER FILTER
A low pass filter section may be needed for some applica-
tions that require additional rejection of the reference side-
bands, or spurs. This configuration is given in Figure 4 . In
order to compensate for the added low pass section, the
component values are recalculated using the new open loop
unity gain frequency. The degradation of phase margin
caused by the added low pass is then mitigated by slightly
increasing C1 and C2 while slightly decreasing R2.
The added attenuation from the low pass filter is:
Defining the additional time constant as
Then in terms of the attenuation of the reference spurs
added by the low pass pole we have
We then use the calculated value for loop bandwidth
equation 12, to determine the loop filter component values in
equations 16–18.
frequency jump lock time will increase.
ref
opt
(kHz)
(MHz)
ATTEN = 20 log[(2 f
Main divider ratio. Equal to RF
Radio Frequency output of the VCO at
which the loop filter is optimized.
Frequency of the phase detector inputs.
Usually equivalent to the RF channel spac-
ing.
c
is slightly less than
T3 = R3 • C3
ref
• R3 • C3)
p
2
, therefore the
+ 1]
opt
/f
ref
(13)
(14)
(15)
(16)
(17)
(18)
c
in