High linearity direct conversion quadrature modulator

5 September 2007 Telecoms, Datacoms, Wireless, IoT

A high performance quadrature modulator from Linear Technology is optimised for direct conversion from baseband to the 850-965 MHz frequency band, suited for GSM, EDGE, CDMA2000 basestations 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 allows parts count reduction. The product features superior linearity with an OIP3 of 22,4 dBm and an OIP2 of 65 dBm at 900 MHz. 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.

The LT5558 combines two matched, high linearity mixers, a precision 0° and 90° phase shifter, a 50 Ω LO input with integrated buffer and an on-chip RF transformer with its output matched to 50 Ω 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 Ω output, requiring no external matching components.

The two on-chip mixers are well balanced, producing exceptional LO leak-age performance. Similarly, the on-chip precision phase shifter is accurate to better than one-half degree mismatch. The LT5558 operates from a single 5 V supply voltage.

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