Wideband Frequency Synthesizer with Integrated VCO - LMX2581


Wideband Frequency Synthesizer with Integrated VCO

Recommended alternative parts

  • LMX2541  - The device has SIMILAR FUNCTIONALITY but is not functionally equivalent to the compared device.   RF Synthesizer With Extremely Low Spurs
  • LMX2531  - The device has SIMILAR FUNCTIONALITY but is not functionally equivalent to the compared device.   Very Low Power RF Synthesizer


The LMX2581 is a low noise wideband frequency synthesizer that integrates a delta-sigma fractional N PLL, multiple core VCO, programmable output divider, and two differential output buffers. The VCO frequency range is from 1880 to 3760 MHz and can be sent directly to the output buffers or divided down by even values from 2 to 38. Each buffer is capable of output power from –3 to +12 dBm at 2700 MHz. Integrated low noise LDOs are used for superior noise immunity and consistent performance.

This synthesizer is a highly programmable device and it enables the user to optimize its performance. In fractional mode, the denominator and the modulator order are programmable and can be configured with dithering as well. The user also has the ability to directly specify a VCO core or entirely bypass the internal VCO. Finally, many convenient features are included such as power down, Fastlock, auto mute, and lock detection. All registers can be programmed through a simple 3 wire interface and a read back feature is also available.

The LMX2581 operates on a single 3.3 V supply and comes in a 32 pin 5.0 mm × 5.0 mm WQFN package.


  • Output Frequency from 50 to 3760 MHz
  • Input Clock Frequency up to 900 MHz
  • Phase Detector Frequency up to 200 MHz
  • Supports Fractional and Integer Modes
  • –229 dBc/Hz Normalized PLL Phase Noise
  • –120.8 dBc/Hz Normalized PLL 1/f Noise
  • –137 dBc/Hz VCO Phase Noise @ 1 MHz
    offset for a 2.5 GHz Carrier
  • 100 fs RMS Jitter in Integer Mode
  • Programmable Fractional Modulator Order
  • Programmable Fractional Denominator
  • Programmable Output Power up to +12 dBm
  • Programmable 32 Level Charge Pump Current
  • Programmable Option to Use an External VCO
  • Digital Lock Detect
  • 3-Wire Serial Interface and Readback
  • Single Supply Voltage from 3.15 V to 3.45 V
  • Supports Logic Levels down to 1.6 V

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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
Operating Temperature Range (C)
Approx. Price (US$)
Aux Prescalers
Divider Ratio
LMX2581 LMX2531 LMX2541 TRF3765
50     553     32     300    
3760     3132     4000     4800    
-229     -212     -226     -221    
-125     -104     -125     -106    
135     32     120     190 w/ all 4 buffers on    
3.15 to 3.45     2.8 to 3.2     3.15 to 3.45     3.3    
Yes     Yes     Yes     Yes    
Catalog     Catalog     Catalog     Catalog    
32WQFN     36WQFN     36WQFN     32VQFN    
-40 to 85     -40 to 85     -40 to 85     -40 to 85    
7.00 | 1ku     5.50 | 1ku     6.50 | 1ku     5.65 | 1ku    
1 to 38     1
1 to 127     1
WEBENCH® Clock Architect - LMX2581
Input Frequency
Output Frequency

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