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Quadrature modulator enables GSM base stations

Posted: 01 Feb 2007     Print Version  Bookmark and Share

Keywords:GSM  CDMA  RFID  Linear Tech  quadrature modulator 

A high-performance quadrature modulator from Linear Technology is optimised for 850MHz to 965MHz GSM, CDMA2000, ISM and RFID modulator applications. The LT5571 accepts In phase (I) and Quadrature phase (Q) base band signals and modulates directly to RF. Its zero- or low-IF-transmitter architecture enables base station designers to achieve high performance while reducing power and parts count, shrinking size and achieving system cost savings. The product features exceptional linearity with an Output 3rd Order Intercept Point (OIP3) of 21.7dBm and an Output 2nd Order Intercept Point (OIP2) of 63.8dBm at 900MHz while drawing only 97mA quiescent current at a 5V supply. The LT5571 has an output noise floor of -159dBm/Hz and low LO leakage of -42dBm. Its image rejection of -53dBc at 900MHz is the best in its class. This device meets or exceeds the dynamic range requirements of GSM cellular base stations as well as other high-performance wireless infrastructure transmitters.

Highly integrated chip
The LT5571 is a highly integrated chip comprising two matched, high linearity mixers, a 50Ω LO input buffer, a precision 0/90° LO phase shifter, and an on-chip RF output transformer operating within the band of 620-1100MHz. The RF transformer sums the modulated signals from the I and the Q channel mixers, and produces a single-ended, 50Ω-matched output. Likewise the LO input is also single-ended, reducing external components while easing the design task.

The LT5571's two on-chip mixers are internally balanced, producing outstanding LO leakage performance. With an LO input power of 0dBm, leakage to the RF output at 900MHz is an exceptional -42dBm, without calibration. Similarly, the on-chip precision phase shifter is accurate to better than one-half degree, enabling an uncalibrated image rejection performance of -53dBc. Both the image rejection and LO leakage can be improved to better than -70dBc with calibration, if desired.

High impedance
The LT5571 has high impedance I and Q inputs with 0.5Vdc common-mode bias to allow flexible AC or DC coupling as well as simple interfaces to many available D/A converters or base band driving circuits.

The LT5571 operates from a single 5V supply voltage. Typical operating current is 97mA. The device can be shut down by an ENABLE pin. When disabled, the chip draws 100µA maximum quiescent current to conserve power. For half-duplexing or time-division multiplexed operating modes, the chip can be switched on and off rapidly using the ENABLE pin.

The LT5571 is offered in a 16-pin 4-by-4mm surface mount QFN package. Pricing starts at Rs.224.33 ($4.95) in 1,000-piece quantities, and the device is available immediately from stock.




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Related Datasheets
Part Number Description Category
LT5571 620MHz - 1100MHz High Linearity Direct Quadrature Modulator Electromechanical - Keyboards, Relays and Switches

 
 
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