The LMX2485 device is a low power, high performance delta-sigma fractional-N PLL with an
auxiliary integer-N PLL. The device is fabricated using Texas Instruments 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 LMX2485 is highly resistant to changes in
temperature and variations in wafer processing. The LMX2485 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 LMX2485 is transferred through a three-line, high-speed
(20-MHz) MICROWIRE interface. The LMX2485 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
LMX2485 is available in a 24-lead 4.0 × 4.0 × 0.8 mm WQFN package.
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