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Analog Angle Article

LTE/WiMax synthesizer IC offers extremely low noise, improves EVM, sensitivity, spectral purity

Santa Clara, CA—Claiming significantly better performance compared to its nearest competitors, a family of six frequency synthesizers from National Semiconductor Corp. covers 33 MHz to 4 GHz via different VCO ranges with ultralow phase noise, PLL noise, and in-band spurs. The LMX2541 ICs are designed for local oscillator applications in LTE and WiMax baseband designs as well as microwave backhaul links. Their performance allows system designers to either extend the range of the link and increase coverage or increase SNR and thus data capacity for the same range.



National Semiconductor LMX2541 frequency synthesizer for 33 to 4000 MHz

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The LMX2541 devices specify root-mean-square (rms) noise below 2 milliradians (mrad) at 2.1 GHz and 3.5 mrad at 3.5 GHz, along with phase lock loop (PLL) noise floor of -225 dBc/Hz (normalized to 1 Hz) and in-band spurs below -60 dBc. National claims that the rms noise of this dvice is one-fifth that of the closest competitor, while the PLL noise floor is 10 dB lower and the in-band spur performance specification is 12-dB lower.



Internal block diagram of LMX2541

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Within the SiGe IC is a delta-sigma fractional-N PLL, voltage controlled oscillator (VCO), divider, and output driver. Note that users can use their own external VCO if desired, to take advantage of the LMX2541's PLL itself. The frequency divider, for ratios oif 1 through 63, produces 50% duty cycle for all division ratios.

A single 3.15 to 3.45 V supply is required, as integral low-noise, low dropout regulators (LDO) generate the needed internal supply rails, while typical dissipation is 500 mW. Programming of the synthesizer is done using a three-wire Microbus interface, which allows setting of output power, divider ratio, and other factors.—Bill Schweber

Packaging, price, and availability : The LMX2541 is housed in a 6×6 mm, 36-lead LLP package, and is sampling now, with full production scheduled for Q4 2009. It is priced at $9.50 in 1000-piece lots.
For more information : contact National Semiconductor Corp., http://www.national.com. Samples and an evaluation board are available at http://www.national.com/pf/LM/LMX2541.html.

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