A new quadrature modulator from Linear Technology is optimized for 850 MHz 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. It has a zero-IF transmitter architecture. The product features exceptional linearity with an OIP3 (output third-order intercept point) of 22.9 dBm and an OIP2 (output second-order intercept point) of 63 dBm at 850 MHz.

Article Tools

• 

The LT5568's noise floor is -160.3 dBm/Hz, and its image rejection is –46 dBc with an LO leakage of –43 dBm. This device meets or exceeds the dynamic range requirements of GSM cellular basestations as well as other high-performance wireless infrastructure transmitters.

The LT5568 is a highly integrated chip comprising two matched, high linearity mixers, a precision 0° and 90° phase shifter, a 50-Ω LO input buffer, 50-Ω I and Q inputs, and an on-chip RF transformer with an output matched to 50-Ω within the band of 700 MHz to 1050 MHz. The RF transformer sums the modulated signals from the I and 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. 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 one-half degree mismatch, enabling an uncalibrated image rejection performance of –46 dBc.

The LT5568 operates from a single 5-V supply voltage. Typical operating current is 117 mA. When disabled, the chip draws 50 μA. 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-mm x 4-mm surface-mount QFN package. Pricing starts at $5.35 in 1000-piece quantities, and the device is available immediately.