A new high-performance quadrature modulator (QM) from Linear Technology is optimized for direct conversion from baseband to the 850-965 MHz frequency band, providing a cost-effective solution for GSM, EDGE, CDMA2000 base stations and 900 MHz RFID readers. The LT5558's I (in-phase) and Q (quadrature-phase) baseband inputs offer high impedance and a common mode voltage of 2.1 V, allowing high input signal drive levels and convenient baseband coupling to active filters. Its direct-to-RF transmitter architecture enables base station designers to attain high performance while reducing parts count, shrinking solution size, and achieving systems cost savings. The product features exceptional linearity with an output third-order intercept point (OIP3) of 22.4dBm and an output second order intercept point (OIP2) of 65 dBm at 900 MHz. Moreover, the LT5558 has a low noise floor of -158 dBm/Hz, combined with an image rejection of –49 dBc and LO leakage of -43.7 dBm. This device meets or exceeds the dynamic range requirements of GSM cellular base stations as well as other high-performance wireless infrastructure transmitters.

Article Tools

• 

The LT5558 combines two matched, high-linearity mixers, a precision 0° and 90° phase shifter, a 50-Ohm LO input with integrated buffer and an on-chip RF transformer with its output matched to 50 Ohm within the band of 600 MHz to 1100 MHz. The RF transformer sums the modulated signals from the I and the Q channel mixers' outputs, and converts the resulting differential signal to that of a single-ended, 50 Ohm output, requiring no external matching components. The two on-chip mixers are well balanced, producing exceptional LO leakage performance. At an LO input drive level of 0 dBm, leakage to the RF output at 900 MHz is only -43.7 dBm, uncalibrated. Similarly, the on-chip precision phase shifter is accurate to better than one-half degree mismatch, producing an uncalibrated image rejection performance of –49 dBc.

It operates from a single 5 V supply voltage. Typical operating current is 108 mA. The device can be shut down by an ENABLE pin. When disabled, the chip draws 50 uA maximum quiescent current to conserve power. For half-duplex or time-division multiplexed operating modes, the chip can be switched on and off rapidly using the ENABLE pin. The LT5558 is offered in a 16-pin 4 mm x 4 mm surface-mount QFN package. Pricing starts at $5.35 each in 1000-piece quantities and is available immediately from stock.