LMX2346 National Semiconductor Corporation, LMX2346 Datasheet - Page 19

• Manufacturer Part Number: LMX2346
• Description: Pllatinum? Frequency Synthesizer For Rf Personal Communications
• Manufacturer: National Semiconductor Corporation

OSC
Notes:
1. Test Conditions: R_CNTR = 1000, LD_OUT = 2, PD = 0.
2. Sensitivity limit is reached when the frequency error of the divided RF input is greater than or equal to 1 Hz.
1.0 Functional Description
The basic phase-lock-loop (PLL) configuration consists of a
high-stability crystal reference oscillator, a frequency synthe-
sizer such as the National Semiconductor LMX2346/7, a
voltage controlled oscillator (VCO), and a passive loop filter.
The frequency synthesizer includes a phase detector, cur-
rent mode charge pump, a programmable reference divider,
and a programmable feedback divider. The VCO frequency
is established by dividing the crystal reference signal down
via the reference divider to obtain a frequency that sets the
comparison frequency. This reference signal, f
sented to the input of a phase/frequency detector and com-
pared with another signal, f
IN
Sensitivity Test Setup
p
, the feedback signal, which was
r
, is then pre-
19
obtained by dividing the VCO frequency down by way of the
feedback divider. The phase/frequency detector measures
the phase error between the f
control signals that are directly proportional to the phase
error. The charge pump then pumps charge into or out of the
loop filter based on the magnitude and direction of the phase
error. The loop filter converts the charge into a stable control
voltage for the VCO. The phase/frequency detector’s func-
tion 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 times that of the compari-
son frequency, where N is the feedback divider ratio.
r
and f
p
signals and outputs
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