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As the wireless infrastructure morphs into the next generation, it will use new systems based on Long-Term Evolution (LTE) or WiMAX. Both use orthogonal frequency-division multiplexing (OFDM) with modulation schemes to 64QAM (quadrature amplitude modulation) and 256QAM. These new systems absolutely must have a very low receiver noise floor to provide maximum receiver sensitivity. The secret to good, low-noise performance is clean clocks and frequency synthesizers with minimum jitter.

To help provide such demanding specifications, National Semiconductor’s LMX2541 phase-locked loop (PLL) frequency synthesizer integrates a voltage-controlled oscillator (VCO). According to National, it’s the industry’s lowest-noise frequency synthesizer as well as a good fit for LTE and WiMAX basestations. It’s also applicable to backhaul and point-to-point microwave radios. The chip can replace the bulky and expensive synthesizer modules now used in basestations.

The LMX2541 comes in six models with VCO frequency ranges from 31 to 4000 MHz. Customized frequency ranges are also available. Its key feature is its low noise performance. The chip produces less than 2 milli-radians (mrad) root-mean-square (RMS) noise at 2.1 GHz and 2.5 mrad RMS noise at 3.5 GHz. National claims those specs best the nearest competition by 10 dB in both in-band PLL noise and spurious performance.

When paired with the National LMK04000 clock jitter cleaner, the LMX2541 significantly improves system error vector magnitude (EVM), producing improved receiver sensitivity and transmitter spectral purity. Other National products that work well with the LMX2541 include the ADC16V130 analog-to-digital converter (ADC) and the LMH6517 digital variable gain amplifier (VGA).

Also, the LMX2541 offers a normalized noise floor of –225 dBc/Hz. It can be operated with up to a 104-MHz phase-detector rate (comparison frequency) in both integer and fractional modes. Other features include programmable output power, digital frequency shift-keying (FSK) modulation, and a FastLock mode with cycle-slip reduction. A crystal oscillator is also provided on-chip as well as output driver matching networks.

The LMX2541 operates from a supply of 3.15 to 3.45 V. The PLL has its own low-dropout (LDO) regulator. It is made with silicon-germanium (SiGe) biCMOS and comes in a 36-pin, 6- by 6-mm leadless lead-frame package (LLP). Prices begin at $9.50 in 1000-unit quantities. Samples and an evaluation board are ready to go.