Power Handling

Maximum power handling is a common concern of our customers. Nothing is worse than plugging in an expensive device only for it to be immediately destroyed. Understanding power handling is important to us as well so that we can address customer concerns about reliability and to develop devices that are able to tolerate higher power levels. The purpose of this document is to describe the failure mode and maximum power handling of several Marki Microwave product lines. It lists the power required to destroy the device and describes what part of the device fails at that power.

How to Measure Mixer IP3 and Identify Potential Error Sources

Mixer linearity is continuously and permanently a critical problem faced in RF system design. The nonlinear action of all physically realizable RF mixers propagates throughout signal chains generating undesired, unfilterable output harmonics, multitone intermodulation, and nonrecoverable nonlinear signal distortion. For example, nonlinear mixing action can cause undesired output harmonics (i.e., spurs) such as the $2f_{RF} \times 2f_{LO}$ or the $2f_{RF} \times f_{LO}$ spur instead of the desired $f_{RF} \times f_{LO}$ converted signal. In multitone applications, such as data transmission and radar tracking, mixing heavily exacerbates the problem of spectral purity by not only introducing a second set of unwanted, unfilterable output harmonics but by also introducing multitone intermodulation distortion (IMD). IP3, or the 3rd order intercept point (TOI), is the figure of merit by which industry judges the linearity of all active, power-consuming RF components and their ability to maintain core linear assumptions about circuits. The mixer is no exception.

A power calibrated vector network analyzer and an external driving synthesizer is the standard tool to do IP3 measurements. While VNAs have their own errors, such as limited RF input power and poor 2nd input harmonic suppressions, it is the most convenient and time efficient way to measure IP3 over broadband frequency sweeps. Spectrum analyzers, although highly susceptible to user error, provide a secondary way of doing comparable IP3 measurements. Both measurement on a spectrum analyzer and a VNA should agree. This document serves as a tool for debugging, developing, and utilizing the spectrum analyzer, it’s supporting synthesizers, and other RF paraphernalia for mixer IP3 measurements.

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:

MM1-2567 Operation to 80 GHz

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:

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

New MMIC Mixers from Marki Microwave Cover 3-24 GHz

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.

What happens when you underdrive a mixer?

The oldest question in mixer tech support is probably “what happens when I drive the mixer with X dBm LO?”, where X is some number lower than what we recommend. In general, and particularly in the past, we have avoided this question. Our recommendation was and is to never underdrive a mixer. The reason for this is that a mixer with insufficient LO drive does not act as a switching device, but as a square law device. If the LO does not turn on the diodes then the physics of mixer operation change completely, and all of our carefully laid design work is thrown out the window.

Indeed, when you underdrive a mixer the conversion loss is not the only thing that changes. All of the specs change, and in unpredictable ways. The LO side in particular responds weirdly, because a lot of deficiencies and inefficiencies on the LO side are exposed when it is underdriven and concealed under normal operation. In this post we will show all the bad things that can happen when you don’t supply an adequate LO drive to the mixer, and then leave it to you as the user to decide what LO drive to design with.

Microlithic Die Attach

We are often asked what the ‘preferred’ method is for attaching Microlithic die to the substrate is. We use, and prefer, silver epoxy die attachment, because epoxy die attach

• is a low temperature process (epoxy can dry at room temperature, vs. very high temperatures for solder or eutectic)
• is easy to do, with a low learning curve and minimal waste of parts
• does not require a solderable die surface (not all chip based Microlithics have a nickel solder barrier layer)
• has a low probability for voids underneath the chip
• is a reversible process, where the part can be removed without damage if performed carefully.

For these reasons, we use silver epoxy (specifically Epotek H20E), place a minimal amount beneath the die, and place the chip into position so that a thin epoxy fillet is created around the chip. The part is then cured at temperature according to the epoxy instructions.

All About Mixers as Phase Modulators

In our last post we showed the physical basis for how mixers are used as phase detectors, concluding by showing that IQ mixers make ideal phase detectors due to their ability to unambiguously identify the relative phase between two signals at any power level. In this post we examine the opposite: how to use mixers as phase modulators. It seems like you should be able to use them in exactly the opposite way, which is to apply a DC voltage to get a linear phase shift. Unfortunately, it’s not that simple.

Note: as with mixers as phase detectors, we as the manufacturers are not the best experts, but our users are. In this case I would recommend Kratos General Microwave, whose application notes I used in preparation of this blog post.

Why phase modulators?

Before examining how to get a phase modulator, let’s look at why you might need one. The main applications are communications and electronic warfare.

Communications: Phase modulation (mathematically identical to frequency modulation) has been used since very early in radio communications, due to FM communications having constant amplitude, better spectral/power efficiency, and convenience. The most common way of understanding phase modulation is with binary phase shift keying (BPSK), or quadrature phase shift keying (QPSK) if both orthogonal components are used. All modern communication systems use these techniques, so they have been written about very extensively, and we will assume that you are familiar with them.

Electronic Warfare: Here it gets interesting. If you have a phase modulator in a jammer, you can trick an enemy radar system into thinking that your plane/boat/tank is not where it actually is. You do this by listening to their radar pulses and responding with frequency shifted radar pulses, making it appear that you are moving at a different speed. This is the classic decoy technique. Modern jammer systems employ much more advanced, exotic, and classified schemes than this that I hope I never have the classification level to learn about. The principles, however, are the same.

All About Mixers as Phase Detectors

Some of the most common questions we receive here are about using mixers as phase detectors. We previously discussed this topic in the post, “DC Offset and Mixers as Microwave Phase Detectors”. In this post we will go into much further depth about the physical mechanisms by which mixers act as phase detectors, and what is important for engineers trying to accomplish this in the lab. First a warning though: we’re just showing experimental results here. The real experts in phase detectors, phase noise, and all things related to phase are the people that do this every day at Holzworth Instrumentation.

Double Balanced Mixers as Phase Detectors

Much has been written about how double balanced mixers work as phase detectors (for example, see this article from Watkins Johnson about the subject). As with most circuit topics the descriptions in the literature are based in math rather than physical principles, so we’ll now consider the physical mechanisms in play when a double balanced mixer is used as a phase detector. Let’s look at what happens when we apply two in phase (frequency matched) voltage signals to an ideal double balanced ring mixer1: