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Demodulation of wideband communications systems becomes more challenging with the arrival of every new wireless standard and every next generation of cellular communications system. Fortunately, the model LTC5586 in-phase/quadrature (I/Q) demodulator from Linear Technology helps mitigate the situation by providing the demodulation bandwidth necessary to satisfy emerging wideband requirements.
With a frequency range of 500 MHz to 6 GHz (and 300 MHz to 6 GHz by adding an external matching capacitor), this zero-intermediate-frequency (zero-IF) demodulator features an IF bandwidth of dc to 1 GHz with amplitude flatness of ±1 dB. Its wide bandwidth and high-performance analog signal processing ease the requirements for analog-to-digital converters (ADCs) at the demodulator’s I and Q output ports, supporting practices such as digital predistortion (DPD) to improve transmitter linearity. The demodulator essentially has what it takes to design receivers for next-generation wireless communications systems, including Fifth-Generation (5G) cellular infrastructure systems.
The LTC5586 (Fig. 1) is a densely packed integrated circuit (IC) supplied in a surface-mount-technology (SMT) housing. Since it uses quadrature demodulation, wideband modulation can be achieved within its 1-GHz IF bandwidth without requiring extensive digital signal processing (DSP) of demodulated I and Q outputs to maintain signal linearity. As a result, the demodulator can be paired with lower-cost DSPs and field-programmable gate arrays while still maintaining excellent performance with wideband carriers through 6 GHz.
Subsystem on a Chip
The LTC5586 incorporates I and Q mixers; I and Q IF amplifiers with eight gain settings and maximum gain of 7.7 dB at 1.9 GHz; an RF attenuator with as much as 31-dB attenuation, adjustable in 1-dB steps; and a two-channel single-pole, double-throw switch with 40-dB isolation and 100-ns switching speed for selecting input signals (Fig. 2). In addition, the demodulator provides a serial peripheral interface (SPI) to control the local-oscillator (LO) input to any frequency band from 300 MHz to 6 GHz without external matching components. The demodulator’s I and Q outputs can drive filters and external ADCs when terminated with external 100-⦠differential impedance.
The wideband demodulator achieves typical –70-dBc image rejection at 3.5 GHz. The extremely linear device features output third-order-intercept (OIP3) performance of typically +35 dBm and output second-order-intercept (OIP2) performance of typically +70 dBm, both measured at 3.5 GHz.
The LTC5586, housed in a 5- × 5-mm plastic 32-lead QFN package, is designed to maintain consistent, reliable performance over operating temperatures from –40 to +105°C. To ensure optimum results, the demodulator includes on-chip calibration of the amplitude and phase responses of its I and Q channels. Calibration and other functions are controlled via the SPI connection.
This novel use of on-chip fine-tuning can make adjustments for deviations in amplitude and phase between the I and Q channels, helping to improve the demodulator’s image-rejection performance. Performing these adjustments in the analog realm greatly reduces requirements for such corrections in the digital realm, along with reduced-speed requirements for DSPs and FPGAs at the demodulator’s outputs. The LTC5586 has the potential to be a key building block in simpler, less-expensive, wideband 5G receiver solutions.
