The LMX2485Q-Q1 is a low power, high performance delta-sigma fractional-N PLL with an
auxiliary integer-N PLL. The device is fabricated using TI’s advanced process.
With delta-sigma architecture, fractional spurs at lower offset frequencies are pushed
to higher frequencies outside the loop bandwidth. The ability to push close in spur and phase noise
energy to higher frequencies is a direct function of the modulator order. Unlike analog
compensation, the digital feedback technique used in the LMX2485Q-Q1 is highly resistant to changes
in temperature and variations in wafer processing. The LMX2485Q-Q1 delta-sigma modulator is
programmable up to fourth order, which allows the designer to select the optimum modulator order to
fit the phase noise, spur, and lock time requirements of the system.
Serial data for programming the LMX2485Q-Q1 is transferred through a three-line,
high-speed (20-MHz) MICROWIRE interface. The LMX2485Q-Q1 offers fine frequency resolution, low
spurs, fast programming speed, and a single word write to change the frequency. This makes it ideal
for direct digital modulation applications, where the N-counter is directly modulated with
information. The LMX2485Q-Q1 is available in a 24-lead 4.0 × 4.0 × 0.8 mm WQFN package.
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