Category Archives: IQ-IR-SSB Mixers

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

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.

The Why and When of IQ Mixers for Beginners

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.

Top 7 Ways to Create a Quadrature (90˚) Phase Shift

We’ve talked a lot about IQ mixers in the last few posts, about their theoretical underpinnings and applications as a phase detector and phase modulator. In upcoming posts we will discuss applications of image reject and single sideband mixers as well. The key to all of these circuits is the quadrature phase shift, both at the LO side for an IQ mixer, and at the LO and IF side for an image reject or single sideband mixer.

Remember: a phase shift is not the same as a time shift. This is one of the most difficult concepts to grasp in RF, microwaves, and optics. We will begin with the trivial example of a time delay, just to show that this doesn’t work for anything but the most simple circumstances. We go on to show how different techniques can be used to create more flexible and useful quadrature phase shifts to ultimately realize our goal of an ideal, broadband IQ/image reject/single sideband mixer.

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


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

IQ, Image Reject, and Single-Sideband Mixers

IQ and Image Reject or Single Sideband mixers use similar circuitry to solve two different fundamental problems in communications and signal processing. IQ mixers address the problem of maximizing information transmission by allowing the user to modulate both the in-phase and quadrature components of a carrier simultaneously, multiplexing two signals onto the carrier*. Image Reject (IR) mixers allow a user to select a signal in a crowded signal environment while suppressing the adjacent image signal, relaxing receiver filtering requirements. Single Sideband (SSB) mixers allow a user to upconvert a signal onto a carrier while suppressing the same image frequency signal, relaxing transmitter filter requirements.

By using quadrature hybrid couplers to manipulate the phase of a signal during conversion, IQ, Image Reject (IR) and Single-Sideband (SSB) mixers enable advanced signal processing capabilities. An IQ mixer allows a system to send twice the information content in a double sideband transmission without using any more bandwidth by using ‘quadrature’ modulation. An IR mixer allows the selection of only one of either the LO + IF frequency or the LO – IF frequency while rejecting the other ‘image’ frequency. This allows for unambiguous signal detection in a crowded signal environment without a tunable RF filter. The SSB mixer is the same structure as the IR mixer and allows the upconversion of an IF signal to only one of the LO + IF or LO – IF frequencies without creating an image of the signal at the other frequency. These mixers are particularly useful when using low IF frequencies, making preselection filtering difficult.

IQ Mixer Operation and Structure

The IQ mixer is the basic building block for higher order IR and SSB mixers. The structure is shown below:

Block Diagram Structure of an IQ mixer