Linear Technology Corporation introduces new low-power I/Q modulators

     Linear Technology Corporation has introduced a new low-power I/Q modulator, the LTC5589, that enables high-performance, battery-powered wideband transmitters operating in the 700MHz to 6GHz range. The LTC5589 modulator is powered by a single 2.7V to 3.6V supply and draws only 29.5mA of current, which is 50% less power than other solutions. When uncalibrated, the new modulator exhibits best-in-class sideband rejection performance of -50 DBC and carrier leakage typical values of -43 DBM. Using the built-in fine-tuning function via the SPI bus, sideband and carrier rejection can be further improved to better than -60 DBC and -60 DBM, respectively. In addition, the device has a very low output noise layer of -158.8dBm/Hz, which, combined with 19dBm OIP3, enables superior transmitter performance.

     The LTC5589 supports both narrowband and wideband transmitters. Its baseband bandwidth is extended to 92MHz with a gain flatness of ±1dB, providing 184MHz RF bandwidth at 1.8GHz. The device's low power consumption and reliable performance make it suitable for a variety of demanding radio and wireless communication applications. These include broadband modems, femtocellular and picocellular broadband wireless access, wireless amplifiers and portable audio systems, broadband portable field radios, unlicensed band radios, train communications, software-defined radios, portable RF test equipment, low-power microwave backhaul and repeater stations, telemetry radios, and satellite modems.

     Previous zero IF modulators had excessive carrier leakage and poor sideband rejection, resulting in poor error vector amplitude (EVM) and adjacent channel power ratio (ACPR). Calibration is challenging and relies on digital tuning algorithms in baseband FPGas with external Dacs. Tuning resolution is usually limited. The LTC5589 provides on-chip tuning capabilities for phase and amplitude mismatches between the I and Q channels, which affect sideband rejection. In addition, the device also has a DC offset balance adjustment function, which affects carrier leakage. With the built-in serial port controlled calibration capability, the LTC5589 simplifies calibration and can be achieved in a cost-effective manner.

     The gain of the LTC5589 can be set through the on-chip serial port. A coarse gain control provides 1dB step and 0.1dB adjustable fine gain control. The total gain ranges from -19dB to 0dB. Changing the modulator gain affects the device power supply current (from 9mA to 39mA), so the device can be configured to operate at lower power consumption by slightly reducing the gain and performance, depending on the needs of the specific application. Once set, the gain can be automatically temperature compensated by activating the built-in temperature calibration function.

     The LTC5589 comes in a 4mm x 4mm plastic QFN package, providing a small solution to the board area. The specified operating temperature range is -40 °C to 105°C (housing). The device has an enable pin for TDD or burst mode transmitter operation. When deactivated, the LTC5589 saves power by absorbing 0.6μA (typical) standby current. The device can be switched on with complete orthogonal accuracy within 350ns.