The LTC5569 dual mixer was used in an uplink receiver for LTE time-division-duplex (TDD) operation from 2496 to 2690 MHz. The Intermediate Frequency (IF) outputs were optimized for the best possible return loss at higher IFs to improve the IF output frequency response flatness. It is optimized for a supply of +3.3 VDC. At this supply voltage, each mixer draws only 90 mA DC current, for power consumption of only 300 mW/channel. With such low power consumption, an eight-channel MIMO receiver consumes only 2.4W power. Since the dual mixer operates as low as 300MHz, its RF input ports can be matched for the 700MHz LTE band and 800MHz GSM bands.
In this application circuit, the mixer's local oscillator (LO) port matching was optimized for low side LO injection of 2281 to 2475 MHz. The mixer circuit's measured performance for an RF input sweep from 2496 to 2690 MHz yielded conversion gain of 1.5 dB, flat within 0.3 dB, and output third-order intercept point (OIP3) of +26.0 to +27.2 dBm. Each mixer's RF input and the common LO input have integrated balun transformers rendering external transformers unnecessary.
The 120-nH pull-up inductors in parallel with the LTC5569 mixer IF output capacitance (1.3-pF differential) and other parasitic circuit elements form a wide-bandwidth, single-pole bandpass filter at the IF outputs. Each IF output pin conducts 28 mA of DC current from the Vcc supply. The total IF DC current of 56 mA is split between the secondary winding of the TC8-1 IF transformer and the two 120-nH IF output inductors. The junction between the two pull-up inductors and the center tap of the TC8-1 transformer requires a good AC ground, which is provided by the 10-nF bypass capacitors.
