Modulator directly converts from baseband to 900MHz

A new quadrature modulator from Linear Technology is designed for direct conversion from baseband to the 850MHz-965MHz frequency band. The solution targets GSM, EDGE, CDMA2000 base stations and 900MHz RFID readers.

The LT5558's In phase (I) and Quadrature phase (Q) baseband inputs offer high impedance and a common mode voltage of 2.1V, allowing high input signal drive levels and convenient baseband coupling to active filters. The product also has a direct-to-RF transmitter architecture.

The product features linearity with an Output 3rd Order Intercept Point of 22.4dBm and an Output 2nd Order Intercept Point of 65dBm at 900MHz. It has a low noise floor of -158dBm/Hz, combined with an image rejection of -49dBc and LO leakage of -43.7dBm. According to Linear, this device meets or exceeds the dynamic range requirements of GSM cellular base stations as well as other high performance wireless infrastructure transmitters.

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 600MHz to 1,100MHz. 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.

At an LO input drive level of 0, leakage to the RF output at 900MHz is only -43.7dBm, uncalibrated. Similarly, the on-chip precision phase shifter is accurate to more than one-half degree mismatch, producing an uncalibrated image rejection performance of -49dBc.

The device operates from a single 5V supply. Typical operating current is 108mA. The device can be shut down by an Enable pin. When disabled, the chip draws 50&μA 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-by-4mm surface mount QFN package. Pricing starts at $5.35 each in 1,000-piece quantities and is available immediately from stock.