The LT5571 quadrature modulator from Linear Technology is optimized for 850 MHz to 965 MHz GSM, CDMA2000, ISM, and RFID modulator applications. The device accepts in-phase (I) and quadrature-phase (Q) baseband signals and modulates directly to RF. It features a zero or low IF transmitter architecture, enabling designers to achieve high performance while reducing power and parts count. The LT5571 has high impedance I and Q inputs with 0.5 Vdc common-mode bias to allow flexible ac or dc coupling as well as simple interfaces to DAC converters or baseband driving circuits.

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The IC features an output third-order intercept point (OIP3) of 21.7 dBm and an output second-order intercept point (OIP2) of 63.8 dBm at 900 MHz while drawing only 97 mA quiescent current at a 5 V supply. The LT5571 has an output noise floor of –159 dBm/Hz and low LO leakage of –42 dBm. It provides an image rejection of –53 dBc at 900 MHz. This device meets or exceeds the dynamic range requirements of GSM cellular base stations as well as other high-performance wireless infrastructure transmitters.

The LT5571 is a highly integrated chip comprising two matched, high-linearity mixers, a 50 W LO input buffer, a precision 0°/90° LO phase shifter, and an on-chip RF output transformer operating within the band of 620 MHz to 1100 MHz. The RF transformer sums the modulated signals from the I and the Q channel mixers and produces a single-ended, 50 W 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 0 dBm, leakage to the RF output at 900 MHz is –42 dBm, without calibration. Similarly, the on-chip precision phase shifter is accurate to better than one-half degree, enabling an uncalibrated image rejection performance of –53 dBc. Both the image rejection and LO leakage can be improved to better than –70 dBc with calibration, if desired.

The LT5571 operates from a single 5 V supply voltage. Typical operating current is 97 mA. The device can be shut down by an "enable" pin. When disabled, the chip draws 100 mA 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. Available now, the LT5571 is offered in a 16-pin 4 mm x 4 mm surface-mount QFN package. Pricing starts at $4.95 in 1000-piece quantities.