Mixers are useful components in a large variety of radio frequency (RF) electronic applications. In particular, monolithic microwave integrated circuits (MMICs) are used in low cost, high volume consumer electronics. Many of these consumer electronics are portable, small, and require very small batteries. Given very limited battery life, MMIC circuits in such products must be designed for power efficiency.
The cost of MMIC circuits is proportional to their circuit area. Given a small size requirement, MMICs are designed to use a minimum number of components. Because MMICs are so minute in size, design freedom is actually enhanced because ordinary interconnection parasitics are eliminated along with special tuning components that are often required to cancel the effects of the parasitics.
Conventional radio frequency (RF) mixer circuits use the non-linear characteristics of diodes, driven by a local oscillator (LO) so that the diodes are switched between their non-linear "on" and "off" states. RF signals applied to the non-linear diodes are mixed with the LO drive to produce mixing products (sum and difference frequencies) from which is selected an intermediate frequency (IF). Substantial LO drive power (e.g., 10 milliwatts) is required to switch these diodes to obtain an acceptable IF conversion efficiency with low inter-modulation distortion. The requirement for substantial LO drive necessitates more DC power, reducing battery life when these mixers are used in portable equipment. Conventional mixer circuits also require tuned matching networks to efficiently couple LO and RF energy into the mixing diode elements, making the circuits physically larger and bandwidth limited. Additional circuitry or balancing is required to increase port to port isolation.
The conventional mixer, when directly translated to a MMIC configuration is not power efficient, uses too much circuit area and is relatively expensive, especially for low cost, high volume consumer electronics applications. The conventional mixer circuit is complex because additional components are required to improve voltage standing-wave ratio (VSWR), port-to-port isolation and intermodulation distortion.
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