Direct conversion modulator for battery-powered transmitters

August 12, 2014 // By Graham Prophet
LTC5599 is a 92mW direct conversion I/Q modulator with better than 50 dB sideband and carrier suppression; this low-power device for battery-powered transmitters operating in the 30 MHz to 1.3 GHz frequency bands, claims new level of performance in power consumption, sideband suppression, carrier leakage and dynamic range performance.

The modulator, powered from a single 2.7V to 3.6V supply, draws 28 mA current, less than 60% that of alternative solutions - with no performance sacrifice. The LTC5599 delivers native -52.6 dBc sideband suppression and -51.5 dBm carrier leakage without calibration. With on-chip calibration resources, performance can be further improved to -60 dBc and -65 dBm, respectively. It achieves an output noise floor of -156 dBm/Hz with an OIP3 of 20.8 dBm, for superior transmitter performance.

The LTC5599 combines low power consumption and robust performance to suit a wide range of demanding battery-powered radios and wireless communications applications that are exposed to strong radio interference. These include wireless professional microphones, frequency hopping narrowband and broadband portable field radios, public safety radios, train communications, as well as broadband VHF/UHF white space modems, software-defined radios, portable RF test equipment, picocell base stations, low-power microwave backhaul, small wireless repeaters and satellite modems.

The device's gain can be set via the on-chip serial port. A coarse gain control provides 1 dB/step, along with adjustable fine gain control of 0.1 dB. Total gain ranges from -19 dB to 0 dB. Varying the modulator gain enables device supply current from 8 mA to 35 mA, allowing the device to be set to lower power consumption with slightly reduced gain and performance, as needed for specific applications. Once set, the gain can be automatically temperature compensated by activating the on-chip temperature correction feature.

The LTC5599 supports narrowband and wideband radio applications. Its I and Q inputs are each capable of -1 dB modulation bandwidth of up to 37 MHz, supporting a total of 74 MHz RF bandwidth at 900 MHz frequency.

The LTC5599 is available in a 4 x 4 mm QFN package, specified for case operating temperature from -40°C to 105°C, supporting reliable operation in extended temperature environments. The device can be shut down with an enable control pin. When disabled, the device conserves power by