GaAs or GaN: Best Choice for Highly Linear Mixers?

Everywhere you look in the microwave industry press or at the international microwave symposium you see one topic mentioned over and over again: Gallium Nitride (GaN). There is presently a gold rush in the industry to produce new varieties of products in GaN. The wide bandgap and high electron mobility of GaN mean that it is capable of a much higher power density than Gallium Arsenide (GaAs).  The introduction and availability of a 0.15 micron commercial GaN processes means that fabless integrated circuit companies (such as Marki) can produce GaN microwave products at frequencies comparable to GaAs. This has led to a profusion of products including power amplifiers, low noise amplifiers, driver amplifiers, and many other amplifiers using GaN that set new records for power at high frequency.

One thing that is fueling the drive to GaN is the increasingly stringent linearity requirements placed on modern RF/Microwave systems. Since the noise floor of a system can only be reduced so much, the only way left to increase the dynamic range is to increase the power compression level and decrease the intermodulation products to create a wider spur free dynamic range. At the heart of most RF receivers is a mixer that limits the linearity and therefore the dynamic range of the receiver. So the next step looks obvious; GaN is increasing the dynamic range of many components in the system, so it should increase the dynamic range of the mixer as well!

There are five main reasons that this logic doesn’t work:

  1. GaN MMIC mixers are more expensive while offering no advantage over GaAs MMIC mixers: Unlike amplifiers, which are based on transistors, high performance microwave mixers are based on diodes. Unlike transistors, which have to be split into balanced configurations to increase the power capability, diodes can simply be stacked! The voltage splits across the two diodes, and for microwave performance reasons they act as a single diode. The high turn on voltage of a GaN mixer is easily replicated using two stacked GaAs diodes. This gives you the benefit of the cost, repeatability, and scalability of GaAs, while providing the high turn on voltage of GaN diodes.GaN mixer vs GaAs Mixer
  2. GaN Mixers require massive LO Power Levels: Due to the lossy designs used for GaN mixers, the conversion loss is high and the LO drive requirements are higher. A 3dB increase in LO drive levels equates to twice as much LO driver power. From experience I can say that finding enough LO drive to characterize high frequency, high level mixers is a real challenge. While it is possible to use a high power GaN amplifier to provide this LO power, is it worth the cost? In addition to more money on the LO amplifier, these amplifiers also require high (20+Volt) drain voltages and sequenced negative voltages. In contrast an LO buffer amplifier such as the ADM-0026-5929SM operates with only a single positive supply voltage. Also since spur free dynamic range is degraded by conversion loss, the OIP3 is the most important spec. For OIP3, the increased conversion loss in GaN is a major liability.
  3. Higher LO drives lead to higher LO-RF leakage power: While higher LO powers may lead to lower single tone intermodulation tones, they definitely lead to higher LO-RF/LO-IF leakage and harmonic LO-RF/LO-IF leakages. While there are some applications where this can be easily filtered out, in many applications such as IQ mixing and low IF upconversions the high LO feedthrough will cause many problems. Also the high gain in the LO amplifier can lead to noise bleeding through the mixer and degrading the noise figure of the converter. In a balanced mixer the number 1 design parameter is LO-RF isolation, and this is inherently degraded with higher LO drives. In contrast, you can use an excellent MM1 mixer from Marki with world class isolations, all at a price below competing MMIC mixers. These offer LO-RF better than 40 dB typically, such as in the MM1-0626SSM:MM1-0626S Isolation
  4. Single tone spurious tones are better suppressed through improved design: In a similar vein, single tone spurious tones are mildly reduced in a double balanced mixer by increasing the drive level. Physically you would expect each higher order mixer spur to be improved by 6 dB (in power) for a doubling of the diode turn on voltage. Therefore, this is the spurious improvement you would expect from either a GaN diode mixer or a GaAs double junction mixer:Diode Level Spur Improvement Chart_revAIndeed this is what you see if you look at the results of an experiment we actually performed by varying the turn on voltage in a standard GaAs process: MM1-0726 intermods vs diode levelNow lets compare this to what can be seen by improving the design of the baluns that do the spurious suppression. Below is a table comparing the ML1-0626I (an obsolete, early generation Microlithic mixer) with an MM1-0626H (a newer generation MMIC mixer). The circuit topology is the same, and the diode level is the same, but the MM1 uses newer design techniques honed over many generations of mixer design, and available only from Marki. As you can see, the spurious improvement for the lower order spurs that typically matter the most is somewhere between equivalent and dramatically better. All this is gained with no increase in power consumption or LO feedthrough and with a reduction in cost:MM1-0626H vs ML1-0626IThese only include the released products that Marki has. There are several different types of circuit topologies under development at Marki that have the promise to offer dramatically reduced single tone intermodulation distortion, none of which use exotic materials systems such as GaN.
  5. (Most Important) Two Tone intermodulation distortion is better suppressed through different circuit topologies: The biggest promise of GaN for signal processing is lower two tone intermodulation distortion products (IP3). In amplifiers, the only way to improve IP3 is to increase the ‘headroom’ of the amplifier. That is, to increase the 1 dB compression of the amplifier such that the signal cannot compress the amplifier and create intermodulation tones (more on this in our forthcoming amplifier basics primer part 2). Fortunately in mixers we do not have this problem. In mixers the IP3 can be improved by using more advanced circuit topologies such as the T3.  We have written quite a bit  about the T3 mixer, so I won’t repeat all of the magic that lies inside of it. The highlights are: highest IP3, flexible LO drive, low conversion loss, improvement with square wave drive, ultra-broadband overlapping LO/RF/IF bands, and super high single tone spurious suppression. The only drawback from the T3 traditionally has been that it is a complex handbuilt product, meaning that scalability is limited and pricing does not reduce significantly in volume. This limitation has been overcome, however, with the new MMIC T3 products, the MT3-0113HCQG, MT3-0113LCQG, and MT3H-0113HCH. These new MMIC mixers offer the ultrahigh-IP3 (OIP3 levels of , low conversion losses, flexible LO drive capability, high spurious suppression, and the cost and scalability benefits MMIC mixers are known for. MT3-0113HCQG IIP3 MT3-0113HCQG OIP3

Currently released high linearity MMIC mixers from Marki include:

These are all products that are available right now for volume design ins based on proven, high reliability GaAs MMIC technology.

If we want to look into the future there is much, much more available. Marki does not publicly share its R&D pipeline obviously, but I can say that there are many other techniques to improve mixer linearity that we are aware of. Some of them we have proven and have not yet released, and some we are still working on. If you have a challenging mixer linearity problem and you are looking for a solution, contact [email protected] for the best mixer in the world.

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