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:

An Introduction to Microwave Amplifiers Part 1: Microwave Amplifier Applications

types-of-microwave-amplifiers

This is a (virtually) math-free introduction to microwave amplifiers from an applications standpoint. There are many references available for the aspiring amplifier designer; this series of posts will attempt to quickly elucidate the relevant factors for the RF system design engineer working to evaluate the appropriate amplifier for her system design.

Types of Microwave Amplifiers

types-of-microwave-amplifiers

There are many ways to classify microwave amplifiers, but we will group them into four categories based on what role they would play in a generic superheterodyne receiver (shown above). The above system could represent a cellular, backhaul, satellite, or other radio communications link; it could also represent a radar or other imaging system. The transmitter alone could represent a jammer or exciter, and the receiver alone could represent an electronic warfare scanner or a test instrument. The requirements that drive the amplifier selection will be the same for most applications.

How to Determine the Maximum Power Handling of an RF/Microwave Directional Coupler

directional-coupler-power-flow-fix

The most common question we receive about our stripline directional couplers, low loss airline directional couplers, and high directivity directional bridges is ‘How much power can it handle?’. The reason is that directional couplers are frequently used for load-pull testing of amplifiers or monitoring a signal after a power amplifier. In either case, the directional coupler will be placed as close to the output of the power amplifier as possible, which means it must perform within spec at high operating powers.

Despite the importance of directional coupler performance under high power inputs, most directional coupler vendors offer a single number without any background or context. As we will show in this post, the static value commonly provided for the power handling of a directional coupler is an oversimplification of the matter.

typical-directional-coupler-use-case

Driving Doublers with Amplifiers

Frequency multipliers are used to generate higher harmonics from an input sinusoid.  In particular, the job of the doubler is to output only the 2nd harmonic.  Invariably, the fundamental tone and higher harmonics will leak to the output as well; how much lower the power of these tones are compared to the 2nd harmonic is known as their “suppression”.

Doublers generally require a large input signal (greater than 12 dBm) to work properly.  To reduce stress on your signal generator, you might consider feeding a smaller signal into an amplifier, and using the amplifier to drive the doubler.  Unfortunately, you will observe that the suppressions of the fundamental and 3rd harmonic are much worse with the amplifier attached.  What happened?  Why should the amplifier make a difference?
doubler_graphs1

MM1-2567 Operation to 80 GHz

MM1-2567all_traces

We occasionally receive requests for a mixer that will operate above our highest frequency mixer, the MM1-2567LS. The truth is that, contrary to the datasheet, the MM1-2567 actually operates above 67 GHz.  It was designed to operate up to a frequency of 80 GHz on the RF and LO sides. The issue is that our test equipment can only measure conversion losses up to 67 GHz directly. To solve this problem, I devised an experiment to prove whether the mixer frequency actually extended to it’s simulation limit or not. Here is the experimental setup:

MM1-2567 High Frequency Test Setup

Repeatability of T3 Mixers and Other Handmade Microwave Components in Six Charts

T3 mixers are the highest dynamic range mixer available. They are also handbuilt parts, subject to unit to unit and lot to lot variability. In this blog post we attempt to quantify that variability. Our sample is 10 T3-08LQP mixers from 5 different date codes. All the date codes are separated by at least a month, totaling nearly two years.  Therefore, the variation you see in the plots below accurately represent the variation a designer could expect across two years in the life of their product. Of course there are always outliers, but the following represents typical performance variation.

Conversion Loss

T3 Conversion Loss Variability

New MMIC Mixers from Marki Microwave Cover 3-24 GHz

Mask 2_blacked_out

Marki is bringing advanced mixer designs to a broader market with four new models of GaAs Schottky diode double balanced mixers covering S and K band applications.  These designs combine the legendary mixer design expertise of Marki Microwave with the repeatability and economies of scale intrinsic in the MMIC production method. 

Table of IF Hybrids

For many years Marki Microwave has sold Image Reject and Single Sideband Mixers to both laboratory/research customers and industrial/military customers. Due to the advancement of digital to analog and analog to digital converters, significantly better image rejection and sideband suppression is now possible by connecting the IQ mixer directly to the ADC/DAC. Therefore, the industrial and military customers have migrated towards using IQ mixers without hybrids. Following this trend, and to facilitate maximum flexibility, Marki offers its new line of MLIQ mixer primarily without an IF hybrid, requiring the user to select their own hybrid to create an IR/SSB mixer. While we sell many of these, the bad news is that we do not offer quadrature hybrids below 700 MHz as a strategic decision.

The good news is that we don’t offer them because they are offered by so many different companies. Quadrature hybrids are required to make quadrature balanced amplifiers, one of the most common amplifier topologies. There is a cottage industry in supplying these quadrature splitters at low frequencies, and so there are many, many options available to the user looking to buy an IF hybrid to match with an MLIQ mixer. The sortable and filterable table below is an incomplete list of models below 2 GHz that can be used as IF hybrids. For frequencies above 2 GHz we recommend our own very well balanced quadrature hybrids.

5 Ways to Compensate for Passive IQ Mixer Imbalance

Analog IQ compensation Techniques

The IQ mixer is the backbone of modern communications architectures, as well as advanced vector signal analyzers for electronic warfare and test and measurement receivers. The backbone of the IQ mixer is vectorial cancellation based on phase an amplitude balance. Any imperfection in the phase and amplitude balance of the baluns that constitute the double balanced mixer cores of the IQ mixer will lead to increased LO feedthrough, RF/IF feedthrough, and spurious products. Any imperfection in the phase balance of the LO or the amplitude or phase balance of the I/Q channels will lead to imperfect cancellation of the sidebands in a single sideband (SSB) or image reject (IR) mixer, or imperfect rejection of the unwanted channel in an IQ mixer.

In this blog post we will examine various ways to compensate for the fact that these structures are built with real components with imperfect phase and amplitude balance.

6 Ways to Make an N-Way Power Splitter

N Way Branched Resistive Power Divider

One of the unique products that we have at Marki Microwave is our broadband, high isolation 3-way and 4-way power dividers. In this blog post we will answer some common questions we receive, including:

  • How to make a 5 way power divider
  • How to make a 6 way power divider
  • How to make a 7 way power divider
  • How to make an 8 way power divider
  • How to make a 10 way power divider
  • How to make a 12 way power divider
  • How to make a 16 way power divider
  • How to make a 32 way power divider
  • How to make an way power divider