Latest Advancements Lend Active Mixers to New, Broader Applications
By Wes Boyd, Product Marketing Manager, Skyworks Solutions, Inc.

The latest generation of active mixers has leapfrogged previous solutions to improve cost, power and space efficiency while circumventing tough technical challenges, not only in basestation transceivers, but in a growing array of medical, scientific and industrial applications as well.

Mixers play an important role in the wireless transceiver chain. They follow the low noise amplifier (LNA) and/or image reject filter and translate the RF frequency to the IF or baseband signal, which allows for easier filtering and signal processing. While mixers haven't tended to share the limelight very often with DSPs and integrated RF transceiver chips, they are extremely important for optimizing wireless system performance. Mixers are intrinsically non-linear devices that can be thought of as a frequency multiplier -- they work on the basis of trigonometric identity, with two signals multiplied together. The linearity of the mixer is a critical parameter since unwanted interferers, if not significantly attenuated or filtered, might be mixed down to the band of interest, and block the reception of the desired signal.

Any nonlinear device can be adopted for frequency translation, but the most common and best mixers until recently were passive mixers based on high-speed switching diodes or field effect transistors (FETs). These mixers were configured as a bridge and driven by a high-frequency local oscillator (LO). In these solutions, the input signal and the LO supplied the energy for the frequency-translated output signal.

The early passive mixers were widely adopted and easy to use. They were supplied with impedance-matched, single-ended inputs and outputs. They also offered low-power dissipation, low noise, and high linearity. They served a vital role in meeting stringent requirements for transmitting a clean signal that had a low noise floor with low intermodulation and harmonic distortion. Their ability to provide high linearity was particularly important because higher-order modulation is generally required while transmitting multiple carrier signals in basestation and other applications.

Linearity is not sufficient in and of itself, however. Besides linearity, gain and low noise are two other important characteristics of a mixer, and they are major determinants for receiver noise figure and sensitivity. Designers also need a mixer solution to deliver easy LO drive and little or no conversion loss in order to offer the necessary cost efficiency and performance. Passive mixers increasingly fell short in these departments because they are inherently lossy devices. They typically have 6 to 8 dB of conversion loss, calling for additional amplifier gain to compensate for the loss. Because the output signal had only about 10 to 15 percent of the energy level of the input signal, the LO generally needed to be a large amplitude signal to switch the diodes or FETs, and it produced spurious electromagnetic radiation that had to be filtered and shielded from other circuitry. This need for high LO signals required the addition of LO amplifiers, which also added cost and compromised isolation. Finally, passive mixers also suffered from high sensitivity to the LO-signal input amplitude, therefore requiring tight control of LO signal flatness. The bottom line: passive mixers offered good linearity at the expense of several critical disadvantages.

Active mixers provided an answer to these problems. Typically, they were configured as double balanced current steering topologies. The active mixer output power came from its Dc power supply, rather than the LO. This meant that smaller, more easily managed LO levels could be used. Also, active mixers have an associated gain, while passive mixers can achieve, at best, a gain of one. Finally, active mixers can be implemented in either single-ended or differential configurations, the latter being preferred due to their superior ability to cancel undesired noise. These differential configurations also are available in single- and double-balanced configurations, the latter with the ability to eliminate LO-to-IF and RF-to-IF feed-through, which can increase linearity while decreasing susceptibility to supply voltage noise.
Despite their many benefits, however, active mixers weren't initially considered suitable for high-performance infrastructure applications like cellular basestations, or other emerging non-cellular applications. There was too much noise, and substandard linearity performance as compared to passive mixers.

Other hurdles had to be cleared before active mixers could be widely used. While they offered lower, circuitry-simplifying LO levels, much less sensitivity to LO level variations, and superior port-to-port isolation as compared to passive mixers, they couldn't match the 3rd Order Input Intercept Point (IIP3) performance and other key criteria.

To meet these needs, the latest active mixers dramatically improve linearity while simultaneously improving such key metrics as OIP3 performance. Today's best solutions reach 38.5 dBm for lower frequency (400 Mhz to 1 Ghz) solutions and 35.5 dBm for higher frequency (1.7 GHz to 2.2 GHz) solutions (see Figure 1).

Today's active mixers also offer a modest conversion gain of 2.5 dB (1.9 GHz) and the noise figure is only 9.0 dB when LO power is at 0 dBm. At this LO power level, the OIP3 and conversion gain are also near their respective optimal levels, and all three parameters work together to enhance the dynamic range. No sacrifices to NF or IP2 are needed either when using active mixers. Considering the NF of the passive mixer with the NF of an IF amplifier, needed to make up for the loss of the mixer, the NF of the active mixer is approximately the same. The differential and double balanced nature of some active mixers also provide excellent IP2 performance. These various advances are made possible through the use of specialty semiconductor process technology and the incorporation of low-noise, high-linearity transistors.

Linearity was the most important metric to improve. Breakthroughs in active mixer linearity have been achieved through a higher level of RF integration, which secondarily has improved both form factor and cost through the elimination of a negative power supply and additional amplifiers.

The latest generations of active mixers offer ease-of-use and performance much like passive mixers, while simultaneously providing everything that already makes active mixers popular in lower-performance applications. They offer inherently low-LO drive, superior LO suppression, gain, reduced filter requirements, and easier integration. They require 0 dBm (or less) signal to drive their LO port, as compared to a passive mixer, which would need at least +17 dBm LO signal with its associated strong source of undesirable radiation. At 1 GHz to 2 GHz frequencies, small PC board parasitic elements can couple enough LO signal to adversely affect the system's other sensitive circuits. Therefore, passive mixers have tended to require RF shields, which can lead to several time-consuming PC board spins. Furthermore, the linearity performance of passive mixers can be significantly degraded by as little as 2 dB to 3 dB of LO power change.

Leading the field of today's active mixers is Skyworks' SKY42070 solution. An integrated, high-dynamic range, low-noise receiver down converter, it includes a double-balanced active mixer, LO amplifiers and dual-LO inputs selected by an external switch interface. The LO switch function is managed using an externally controlled complementary metal oxide semiconductor (CMOS)-compatible interface. The SKY42068 provides an OIP3 of 38.5 dBm and an NF of 9.5 dB, and the SKY42070 features an IIP3 of 35.5 dBm with an NF of 9.0 dB. Both devices offer 2.5 dB gain, which can potentially reduce the need for additional gain stages. (See Figure 2).

Active mixers have come a long way since their initial debut in lower-performance applications. Today, they deliver all of the benefits of original passive mixers along with a variety of new capabilities. They fundamentally disprove the notion that active mixers cannot achieve the same linearity as passive mixers, while offering advantages that were previously unavailable to RF designers.
For more information, please visit www.skyworksinc.com

SKYWORKS SOLUTIONS, INC.
www.skyworksinc.com
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