Trimming the single-chip PLL + VCO solution introduced last spring, National Semiconductor Corp. has developed a new family of monolithic RF voltage-controlled oscillators (VCOs). The first member of this new line of integrated VCOs is aimed at handling triple bands in the cellular arena. The LMX2604 is tailored to meet the specifications for the GSM and GPRS cellular standards. In fact, the small form factor and low phase noise of the device are designed for GSM900/DCS1800/PCS1900 triple-band applications.
In essence, the LMX2604 chip incorporates two VCOs, one for GSM in the 880 MHz to 915 MHz range and a second one for DCS in the 1710 MHz to 1785 MHz band and PCS in the 1850 to 1910 MHz band. In addition, it offers two separate buffer amplifiers to drive an external high-power amplifier, one for the GSM900 band and the other for DCS1800/PCS1900 bands. Furthermore, the monolithic VCO provides a differential buffer amplifier to drive the mixer in offset PLL (see the figure). The resonant circuits of the VCOs are fully integrated on the chip to ease the application of the IC. The oscillator core and the tank circuit are designed to be immune to external noise such as supply and load variations. According to the manufacturer, the high quality factor of the embedded tank circuit achieves very low phase noise characteristics at the VCO output. It has low phase noise of -167 dBc/Hz at 20 MHz offset in the GSM band, and -162 dBc/Hz at 20 MHz offset in the DCS and PCS bands. The only required external components are a couple of supply bypass capacitors and matching components. A control pin for controlling the oscillation frequency is shared by the two VCOs. The new VCO exploits 0.25 µm RF CMOS process for integration.High output power for the integrated VCO is +6 dBm in all bands, which is ideal for use with transmit closed-loop modulation. The LMX2604 also has 2.7 V to 3.3 V operation and low current consumption of 18 mA in GSM mode and 15 mA in DCS/PCS mode. Other salient features of the device include smaller package and fewer external components. To provide a smaller package than alternative triple-band VCO modules, the LMX2604 is housed in a 20-pin leadless leadframe package (LLP), which is 4 × 4 mm in size. This allows designers to save significant board space in mobile handsets. The supplier also has plans to offer additional members in this line throughout the year. Available now, it is priced at $1.35 in 1,000 units.
Publicado por: Jahir Alonzo Linares Mora C.I: 19769430 CRF
Bibliografia: http://rfdesign.com/vlf_to_uhf/rf_circuits/radio_monolithic_vco_tackles/
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