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Quadrature modulator offers base station system cost savings

Posted: 16 Nov 2005 ?? ?Print Version ?Bookmark and Share

Keywords:quadrature modulator? linear technology? cdma2000? lt5568?

The new high performance quadrature modulator from Linear Technology Corp. accepts I (In phase) and Q (Quadrature phase) baseband signals and modulates directly to the RF transmission frequency.

Designed for 850MHz to 965MHz GSM, cdma2000, ISM and RFID modulator applications, the LT5568s Zero-IF transmitter architecture enables base station designers to attain high performance while reducing parts count, shrinking size and achieving systems cost savings. The product features exceptional linearity with an OIP3 (Output 3rd Order Intercept Point) of 22.9dBm and an OIP2 (Output 2nd Order Intercept Point) of 63dBm at 850MHz. The LT5568's noise floor is -160.3dBm/Hz, and its image rejection is -46dBc with an LO leakage of -43dBm.

The LT5568 is a highly integrated chip comprising two matched, high linearity mixers, a precision 00 and 900 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 ohms within the band of 700MHz to 1,050MHz. 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, thus easing the design task.

Linear Tech explained that the two on-chip mixers are well balanced, producing exceptional LO leakage performance. At LO input power of 0dBm, leakage to the RF output at 850MHz is -43dBm, uncalibrated. Similarly, the on-chip precision phase shifter is accurate to better than one-half degree mismatch, enabling an uncalibrated image rejection performance of -46dBc.

The LT5568 operates from a single 5V supply voltage. Typical operating current is 117mA. This product can be shut down by an ENABLE pin. When disabled, the chip draws 505A 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 LT5568 is offered in a 16-pin 4-by-4mm surface mount QFN package. Pricing starts at $5.35 in 1,000-piece quantities.

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