LMX2485 - Delta-Sigma Low Power Dual PLL for RF Personal Communications

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.

• Quadruple Modulus Prescaler for Lower Divides
  • RF PLL: 8/9/12/13 or 16/17/20/21
  • IF PLL: 8/9 or 16/17
• Advanced Delta-Sigma Fractional Compensation
  • 12-Bit or 22-Bit Selectable Fractional Modulus
  • Up to 4th Order Programmable Delta-Sigma
    Modulator
• Features for Improved Lock Time
  • Fastlock / Cycle Slip Reduction With Integrated
    Time-Out Counter Which Requires Only a
    Single-Word Write
• Wide Operating Range
  • LMX2485 RF PLL: 500 MHz to 3.0 GHz
  • LMX2485E RF PLL: 50 MHz to 3.0 GHz
• Useful Features
  • Digital Lock Detect Output
  • Hardware and Software Power-Down Control
  • On-Chip Input Frequency Doubler.
  • RF Phase Detector Frequency up to 50 MHz
  • 2.5 to 3.6 V Operation With I_CC = 5.0 mA