Wideband 6 GHz zero-IF I/Q demodulator with deep sideband suppression

July 28, 2016 // By Graham Prophet
A high linearity I/Q demodulator, the LTC5586, offers –1 dB flatness over bandwidth of greater than 1GHz, supporting next-generation 5G wireless infrastructure equipment such as DPD (digital predistortion correction) receivers for base stations, microwave backhaul and software-defined radios (SDR), as well as other broadband receiver applications.

The LTC5586’s RF input port is, says Linear Technology, truly broadband. With a single network, it is 50Ω matched from 500 MHz to 6 GHz, covering all the LTE bands as well as the emerging 4.5G and 5G bands at 3.6 GHz and 5 GHz. For operation from 300 MHz to 500 MHz, an additional capacitor suffices. The device also has built-in SPI tunability, allowing the demodulator to set its LO input to any frequency band from 300 MHz to 6 GHz with no external matching components. The demodulator supports very high OIP2 of 80 dBm and 60 dBc sideband suppression, ensuring superior receiver performance.

The LTC5586 is a true zero-IF design that demodulates wideband RF signals directly to baseband DC or low IF coupling. The demodulator exploits the quadrature relationship between the two I and Q channels, sufficiently containing all the information available when both channels are sampled at one-half the bandwidth of the full RF bandwidth from which they came. As a result, the A/D converters’ (ADC) sampling rate and cost can be reduced.

In the front end, the LTC5586 has an RF switch that selects one of its two inputs to a programmable attenuator via the on-chip serial bus. This front end attenuator has a gain control range of 31 dB in 1 dB steps, allowing the receiver to accommodate a wide range of signal inputs. The demodulator consists of I and Q mixers, with their respective LO driven by an accurate wideband quadrature phase shifter. After the signal is demodulated, two programmable gain amplifiers provide gains to the I and Q baseband signal paths. Maximum gain of 7.7 dB can be set by the serial port in 1 dB steps, allowing users to optimally set the signal levels while driving a pair of external A/D converters directly.


The IC has a unique on-chip tuning facility for simple calibration via the SPI serial port. Through this