Tag Archives: mixers

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 handing 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.

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. 

The Why and When of IQ Mixers for Beginners

SSB-IR transmission

Sometimes you need a mixer; sometimes you need an IQ mixer. How do you know which one to buy? Before answering this question, I recommend reading the Mixer Basics Primer to get a good understanding of the fundamentals of mixers, the blog post ‘IQ, Image Reject, and Single Sideband-Mixers’ for and introduction to these mixers, and ‘How to think about IQ mixers’ if you want a deeper physical understanding of the mechanisms of IQ, image reject (IR), and single sideband (SSB) mixers.

What happens when you underdrive a mixer?

Conversion Loss

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.

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:

In phase mixers

 

The T3: A High Dynamic Range Mixer for 4G, LTE, and 5G testing

Spectral regrowth is a big deal for you. In order for the wireless revolution to continue apace, enabling you to watch funny cat videos faster in more crowded environments, spectral regrowth must be conquered wherever it occurs. Spectral regrowth is what occurs when a broadband or spread spectrum signal intermodulates with itself, creating deterministic products that look like noise, limiting the signal to noise ratio of the signal. According to Shannon’s theorem this limits the information capacity of the signal, and thus your video takes longer to load (for some reason this always happens at the worst time).

Spectral regrowth comes from a handful of sources. It can come from mixers, but in installed communication systems it tends to come from the power amplifiers at the transmitter and the connections to the antenna itself (called ‘passive intermodulation’ or PIM). It is made much worse by using higher power and by denser concentrations of signals. Both of these factors are increasingly common as data capacity is increased. This is why highly linear amplifiers and PIM are both big buzzwords in the mobile communications world right now.

What is not always talked about is that ‘spectral regrowth’ in the mobile communication world is the same as ‘two-tone intermodulation distortion’ or ‘IP3’ in the microwave world. Two tone modulation distortion is what causes spectral regrowth, just summed over all of the frequencies involved in the signal. This can be seen by moving from two tone testing to three tone testing in a standard double balanced mixer, the Marki M1-0212SA:

M1-0212SA Comparison

How to think about IQ mixers

There are many ways to think about IQ modulation, and all of them rely on math. This is because ‘quadrature’ modulation is a mathematical construct, a way of thinking about how time domain signals can be manipulated more than a physical reality. In this blog post I will describe how I think of IQ modulation, which is as the cancellation of a signal through two 90° phase shifts that create a 180° phase shift, which is the negative of the original signal. The negative and positive versions of the signals cancel, resulting in suppression of the other signal. This is identical to the math that governs image cancellation in image reject and single sideband mixers, the only difference is that one of the 90° phase shifts occurs at the transmitter in an IQ scheme, while they are both at the receiver in the image reject/single sideband scheme.

IQ-SSB Graphic