PLL Synthesizer Offers High Output Frequency And Lowest Phase Noise

The Texas Instruments LMX2581 phase-locked loop (PLL) frequency synthesizer integrates a voltage-controlled oscillator (VCO) that covers from 50 to 3760 MHz and offers perhaps the best phase-noise specification around. Normalized PLL phase noise is –229 dBc/Hz, and typical phase noise is –137 dBc/Hz at 1 MHz with a 2.5-GHz carrier. The normalized PLL 1/f noise is –120 dBc/Hz. The ultra-low phase noise is the result of design features such as its very high 200-MHz phase detector frequency.

The LMX2581 also offers the option of using integer or fractional PLL modes and optional dithering for spur reduction with the multi-order delta-sigma PLL. The integrated multicore VCO comes with its own tank circuit. The VCO may be bypassed in favor of an external VCO. A low-dropout regulator (LDO) is integrated to power the VCO.

Also included is a clock input divider and a programmable PLL output divider with a range of 1 (bypass) to 38. The noise floor is –155 dBc/Hz, and the in-band spur specification is –46 dBc. The chip has two 50-Ω differential outputs with programmable power levels from –4 to +10 dBm as well.

Typical applications include wireless infrastructure basestations, broadband wireless devices, test and measurement, and clock generation. The LMX2581 also suits a variety of radar and other defense and aerospace applications in addition to medical imaging. With the very low phase noise, the chip makes a superior precision local oscillator in an RF front end for excellent receiver sensitivity.