6.4GHz Low Noise Fractional N PLL With Ramp/Chirp Generation - LMX2491


6.4GHz Low Noise Fractional N PLL With Ramp/Chirp Generation



The LMX2491 device is a low-noise, 6.4-GHz wideband delta-sigma fractional N PLL with ramp and chirp generation. It consists of a phase frequency detector, programmable charge pump, and high frequency input for the external VCO. The LMX2491 supports a broad and flexible class of ramping capabilities, including FSK, PSK, and configurable piecewise linear FM modulation profiles of up to 8 segments. It supports fine PLL resolution and fast ramp with up to a 200-MHz phase detector rate. The LMX2491 allows any of its registers to be read back. The LMX2491 can operate with a single 3.3-V supply. Moreover, supporting up to 5.25-V charge pump can eliminate the need of external amplifier, leading to a simpler solution with improved phase noise performance.


  • –227-dBc/Hz Normalized PLL Noise
  • 500-MHz to 6.4-GHz Wideband PLL
  • 3.15-V to 5.25-V Charge Pump PLL Supply
  • Versatile Ramp / Chirp Generation
  • 200-MHz Maximum Phase Detector Frequency
  • FSK / PSK Modulation Pin
  • Digital Lock Detect
  • Single 3.3-V Supply Capability

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Parametrics Compare all products in RF PLLs and Synthesizers

Output Frequency (Min) (MHz)
Output Frequency (Max) (MHz)
Normalized PLL Phase Noise (dBc/Hz)
1/f Noise (10 kHz offset at 1 GHz carrier) (dBc/Hz)
Current Consumption (mA)
Integrated VCO
LMX2491 LMX2485E LMX2492
  50    500   
  3000    14000   
-227    -210    -227   
-120    -104    -120   
60    3.3    60   
3.15 to 3.45    2.5 to 3.6    3.15 to 3.45   
No    No    No   
24WQFN    24WQFN    24WQFN   

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