LMX2353 National Semiconductor Corporation, LMX2353 Datasheet - Page 7

• Manufacturer Part Number: LMX2353
• Description: Pllatinum? Fractional N Single 2.5 Ghz Low Power Frequency Synthesizer
• Manufacturer: National Semiconductor Corporation

Typical Performance Characteristics
(Continued)
LMX2353 Sensitivity vs Frequency
Oscillator Input Sensitivity vs Frequency
DS101124-15
DS101124-16
RF Input Impedence
Vcc = 2.7 V to 5.5 V, f
= 50 MHz to
IN
3 GHz (f
Capacitor = 100 pF)
INB
DS101124-14
1.0 Functional Description
The basic phase-lock-loop (PLL) configuration consists of a high-stability crystal reference oscillator, a frequency synthesizer
such as the National Semiconductor LMX2353, a voltage controlled oscillator (VCO), and a passive loop filter. The frequency
synthesizer includes a phase detector, current mode charge pump, as well as programmable reference [R] and feedback [N]
frequency dividers. The VCO frequency is established by dividing the crystal reference signal down via the R counter to obtain
a frequency that sets the comparison frequency. This reference signal, fr, is then presented to the input of a phase/frequency
detector and compared with another signal, fp, the feedback signal, which was obtained by dividing the VCO frequency down by
way of the N counter and fractional circuitry. The phase/frequency detector’s current source outputs pump charge into the loop
filter, which then converts the charge into the VCO’s control voltage. The phase/frequency comparator’s function is to adjust the
voltage presented to the VCO until the feedback signal’s frequency (and phase) match that of the reference signal. When this
“phase-locked” condition exists, the RF VCO’s frequency will be N+F times that of the comparison frequency, where N is the
integer divide ratio and F is the fractional component. The fractional synthesis allows the phase detector frequency to be
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