High Linearity 620MHz-1100MHz Quadrature Modulator

20th August 2007

ES Admin

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A new high performance quadrature modulator from Linear Technology is optimized for 850MHz to 965MHz GSM, CDMA2000, ISM, and RFID modulator applications. The LT5571 accepts I (In phase) and Q (Quadrature phase) baseband signals and modulates directly to RF. Its Zero or Low IF transmitter architecture enables basestation 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 OIP3 (Output 3rd Order Intercept Point) of 21.7dBm and an OIP2 (Output 2nd Order Intercept Point) 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 basestations as well as other high performance wireless infrastructure transmitters.

The LT5571 is a highly integrated chip comprising two matched, high linearity mixers, a 50-Ohm LO input buffer, a precision 0°/90° LO phase shifter, and an on-chip RF output transformer operating within the band of 620MHz to 1100MHz. The RF transformer sums the modulated signals from the I and the Q channel mixers, and produces a single-ended, 50-Ohm-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.

The LT5571 has high impedance I and Q inputs with 0.5V DC common-mode bias to allow flexible AC or DC coupling as well as simple interfaces to many available D/A converters or baseband 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 100uA 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 4mm x 4mm surface mount QFN package. Pricing starts at $4.95 in 1,000-piece quantities, and the device is available immediately from stock.