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The heart of all modern software-defined radios is the mixer. The mixer circuit establishes the receiver’s basic performance level, making it the most critical part of the radio. Linear Technology’s LTC5551 downconverting mixer brings a whole new level of performance to new receiver designs (see the figure). It targets a wide range of applications including LTE basestations, military and public safety radios, repeaters, backhaul, avionics, radar, phased-array antenna systems, and white space receivers.
The LTC5551 boasts high linearity defined by its input third-order intercept (IIP3) value of +36 dBm and 9.7-dB noise figure (NF). Most passive mixers have a conversion loss of 7 to 9 dB while the LTC5551 delivers 2.4 dB of conversion gain, substantially improving receiver dynamic range. The 1-dB compression point is +18 dBm.
The mixer’s 300-MHz to 3.5-GHz operating frequency range makes it useful in a broad array of wireless devices. Its integrated local oscillator (LO) driver only requires 0 dBm (1 mW) of LO power to meet the full IIP3 spec. The elimination of the need for a high-power LO signal greatly reduces the potential for undesirable radiation. As a result, the filtering and RF shielding requirements for the radio are minimal. A fully integrated IF amplifier is also present.
Both the RF and LO inputs have integrated balun transformers, eliminating the need for external components and circuits while simplifying the design and reducing cost and space requirements.
The LTC5551 receives its dc power from a standard 3.3-V supply and draws 204 mA of current while meeting full specifications. A low-power mode drops the current draw to 142 mA but trades off power for an IIP3 of +29.3 dB. Its shutdown mode decreases the current to 100 µA. Its 500-ns turn-on and turn-off time make it suitable for burst mode receivers.
The LTC5551 comes in a 4- by 4-mm, 16-pin quad flat no-lead (QFN) package. It operates from –40°C to 105°C. Pricing is $7.25 in 1000-unit quantities.
Linear Technology