Active frequency mixers provide the frequency-translation function required in modern communications systems, with the added benefit of conversion gain in contrast to the conversion loss of passive mixers. Model LT5579 from Linear Technology (www.linear.com) is such an active mixer, designed for upconversion applications from 1.5 to 3.8 GHz. It offers enough bandwidth to cover the 1.9-GHz cellular communications bands as well as the 3.5-GHz WiMAX frequency band. The active mixer's wide dynamic range makes it suitable for current and next-generation wireless base-station transmitters.
The LT5579 (Fig. 1) achieves its wide dynamic range by merit of a noise floor of -158 dBm/Hz at an output level of -5 dBm and output third-order intercept point (IP3) of +27.3 dBm at 2.14 GHz. Compared to the typical 7 to 8 dB conversion loss for a passive mixer, the LT5579 features 2.6 dB conversion gain at 2.14 GHz (Fig. 2). It also boasts low 9.9-dB noise figure at 2.14 GHz (see table).
The upconverting mixer is optimized for use from 1.5 to 3.8 GHz with singleended local oscillator (LO) and RF ports for simplified system layouts. It has integrated RF transformers at both its RF output and LO input to provide differential to single-ended conversion at both ports. The mixer requires only -1-dBm LO power and is designed to operate from a single +3.3-VDC supply. The mixer incorporates an LO buffer amplifier driving a double-balanced mixer to achieve frequency conversion with high linearity. The intermediatefrequency (IF) input ports are tied to the emitters of the double-balancedmixer's transistors (Fig. 2). Optimum DC current for the mixer core is set by external resistors. The mixer draws 226 mA quiescent current at +3.3 VDC and exhibits low LO leakage of -35 dBm.
The LT5579 mixer is designed for maximum LO power of +10 dBm and maximum differential IF power of +13 dBm. The mixer is supplied in a compact 5 x 5 mm QFN24 surface-mount package. The LT5579 is designed for operating temperatures from -40 to + 85C. P&A: $5.50 and up (1000 qty.); stock. Linear Technology, 1630 McCarthy Blvd., Milpitas, CA 95035-7417; (408) 432-1900, FAX: (408) 434-0507, Internet: www.linear.com.