Quadrature modulator

A high performance quadrature modulator from Linear Technology is optimised for 850 to 965 MHz GSM, CDMA2000, ISM and RFID modulator applications.

The LT5568 accepts I (in phase) and Q (quadrature phase) baseband signals and modulates directly to the RF transmission frequency. Its zero-IF transmitter architecture enables base station designers to attain high performance while reducing parts count, shrinking size and achieving cost savings.

The product features fine linearity with an OIP3 (output 3rd order intercept point) of 22.9 dBm and an OIP2 (output 2nd order intercept point) of 63 dBm at 850 MHz. The LT5568's noise floor is -160.3 dBm/Hz and its image rejection is -46 dBc with an LO leakage of -43 dBm.

The device meets or exceeds the dynamic range requirements of GSM cellular base stations as well as other high performance wireless infrastructure transmitters.

The device is a highly integrated chip comprising two matched, high linearity mixers, a precision 0 and 90° phase shifter, a 50 ohm LO input buffer, 50 ohm I and Q inputs, and an on-chip RF transformer with an output matched to 50 ohm within the band of 700 to 1050 MHz.

The RF transformer sums the modulated signals from the I and the Q channel mixers' outputs, and converts the differential signal to that of a single-ended output. The LO input is also single-ended.

The two on-chip mixers produce good LO leakage. At LO input power of 0 dBm, leakage to the RF output at 850 MHz is -43 dBm, uncalibrated. Similarly, the on-chip precision phase shifter is accurate to better than half a degree mismatch, enabling an uncalibrated image rejection performance of -46 dBc.

The device operates from a single 5 V voltage. Typical operating current is 117 mA. 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-duplexing or time-division multiplexed operating modes, the chip can be switched on and off rapidly using the enable pin.