LIBRARY - Microwaves & RF · PDF filetwo main styles of mixers are discrete (hybrid) and integrated (monolithic) ... for active types and FETs for passive types. Discrete mixers can

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CONTENTS

2 | Welcome 3 | RF Mixers Pine For Linearity And Dynamic Range 9 | Understanding Mixers and Their Parameters 16 | What is the Deal with IP2 in Mixers? 20 | Converting Frequencies With Microwave Mixers 25 | More Resources From Microwaves & RF

R IL

fJ fRF

fLO

Jammer DesiredSignal

LO Input

DownconvertedJammer

fIF

IF Filter

JammingSpur

DownconvertedDesired Signal

fLO fJ fRF

Leakage Terms

Downconversion

Frequency

Radiofrequency

(f RF)

Local oscillator(fLO)

Intermediatefrequency

(f IF )

Radiofrequency

(f RF)



(f IF )

DC

R IL

Pow

er

Upconversionf IF = |f LO – f RF| fRF1 = f LO – f IF LO + f IF fRF2 = f

RFLO

IF

FrequencyDC

IRL

Pow

erRF1

LOIF

RF2

MICROWAVES & RF LIBRARY

FOCUS ON: MIXERS

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MICROWAVES & RF LIBRARY FOCUS ON: MIXERS


EDITORIAL

Welcome to Microwaves & RF’s e-book on mixers. Thanks to their han-dling of frequency conversion, mixers are critical to high-frequency applications ranging from radar to cellular base stations. This selec-tion of articles was carefully chosen to provide an in-depth look at mixers of various types, their parameters, applications driving their

development, and more. Here is what you will find in this edition:In “Understanding Mixers and their Parameters,” Technology Editor Chris

DeMartino provides a fundamental overview of these three-port components while delving into crucial parameters like conversion loss, isolation, intermodulation distortion, and the 1-dB compression point. Also covered are mixer topologies ranging from single-diode to balanced, single-balanced, double-balanced, and triple-balanced.

Marki Microwave’s Doug Jorgesen provides an in-depth look into IP2 in the article, “What Is the Deal with IP2 in Mixers?” By looking at intermodulation products in amplifiers, Jorgesen is able to explain similar issues in mixers. In addition to delving into isolations, fundamentals, and second-order distortions, Jorgesen makes a great case for the need to be very specific when talking about IP2. Even among experts, questions commonly arise, leading to instances in which someone confuses, for example, second-order distortion products for two-tone IP2 problems.

Rounding out this compendium are two more informative reports. “RF Mixers Pine for Linearity and Dynamic Range” provides an overview of the applications driving mixer development and how their needs are impacting mixer development, while “Converting Frequencies with Microwave Mixers” details the mixer aspects, types, and performance issues behind downconversion and upconversion.

We hope you enjoy this e-book and find it helpful in your own work in microwaves and RF products and applications. We welcome any feedback you may have. n

The Microwaves & RF Editorial Team





While Reginald Fessenden worked for the U.S. Weather Bureau in the early 1900s, he was determined to advance radio princi-ples to the point where a network of coast-al radio stations could transmit weather information over long distances. Studiously

working for his $3,000 a year, Fessenden invented a princi-ple of combining two radio signals to form a reduced com-posite of the signals in the audible spectrum—a process known as “heterodyning.” Building on Fessenden’s work, Edwin Armstrong later developed the first super-hetero-dyne receiver.

Modern mixers have moved far beyond this heritage to multiply or divide RF signals well into the millimeter-wave range. Yet the advanced behavior of these mixers requires increasing complexity in an industry that demands nev-

By Jean-Jacques DeLisle Contributing Technical Editor

RF MIXERSPINE FOR LINEARITY AND DYNAMIC RANGE

The technology for RF mixers must keep pace to avoid being the limiting factor in new devices.





RF Mixers Pine for Linearity

FOCUS ON: MIXERS

er-ending performance enhancements.Mixers are used throughout the RF

industry wherever significant frequency translation is needed. Using the dynam-ics of a nonlinear node and clever cir-cuitry, which forces some linear behavior from the mixing device, two input signals can be multiplied or divided (Fig. 1). Di-odes, Schottky diodes, bipolar-junction transistors (BJTs), or field-effect tran-sistors (FETs) can be used as nonlinear elements.

Creating an upconverted or downcon-

verted output signal requires the use of a local-oscillator (LO) input signal, inter-mediate-frequency (IF) input/output sig-nal, and an RF input/output signal. The most common mixers use the switching action of the LO to drive a nonlinear junction in and out of conduction, there-by clipping the RF signal. As no device is ideal, several unwanted frequency products are generated by the mixing action. One figure of merit for mixers re-veals how well these products are sup-pressed. Other figures of merit include

spurious-free dynamic range (SFDR), noise figure (NF), input third-order intercept point (IIP3), the 1-dB compression point, con-version gain, and isolation (Fig. 2).

Among the various types of mix-ers, there are two main catego-ries: passive and active mixers. Passive, or current-commutating, mixers generally have higher 1-dB compression points, a lower NF, and a higher IIP3. Active mixers tend to have much lower power consumption and even a potential increase in conversion gain—al-though they trade off lower power for lower linearity. Both passive and active mixers can be divided into several classes: single-device, single-balanced, double-balanced,

Downconversion

Frequency

Radiofrequency

(fRF )



(fIF)

Radiofrequency

(fRF )



(fIF)

DC

R IL

Pow

er

UpconversionfIF = |f LO - f RF| fRF1 = fLO - fIF fRF2 = f LO + f IF

RFLO

IF

FrequencyDC

IRL

Pow

er

RF1

LOIF

RF2

1. Upconversion and downconversion are two critical operations for mixers. For high-frequency signal transmission, upconversion is often used. For test and measurement applications, in contrast, the downconversion of signals from higher frequencies allows for precision measurements. (Courtesy of Marki Microwave)

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and triple-balanced (double-double-bal-anced mixer).

Single-device mixers use a nonlinear component for the mixing action. Often, they require injection filters, as they do not attenuate the LO signal in the out-put. The benefit of single-device mix-ers is that they are capable of reaching millimeter-wave frequencies. Using two nonlinear devices connected via a 180° or 90° hybrid can improve the isolation between the LO and output signal. The resulting device is called a single-bal-anced mixer. Balanced mixers tend to have lower frequency capability and re-quire baluns. According to Christopher

Marki, director of operations for Marki Microwave, “Mixer frequency cover-age is [usually] governed by the balun. Magnetic baluns work very well below 3 GHz, and many companies make such mixers. Non-magnetic, or capacitive coupled, baluns are fantastic in 2-GHz and higher applications. Those baluns are [usually] limited to several octaves. Mixers have a sort of ‘gain/bandwidth’ product that is mostly related to baluns.”

Using a network of four nonlinear de-vices with multiple hybrids, baluns, or transformers produces a higher-per-forming mixer class, which are referred to as double-balanced mixers. The double-balanced mixer benefits from the rejection of spurious/intermodu-

RF Mixer Pine For Linearity

Input Signals

F1

F2

Mixer

Output Signals

F1

F2

F1+F2

F1–F2

2. A mixer generates multiple products from the input of two time-varying signals. Even-order and odd-order products are among a few of the “mixed” outputs.

3. Mixer response is described using several key parameters. Linearity and dynamic range are two of the most critical design considerations for practical devices.

Mixer outputpower (dBm)

1-dB compressionpoint

FundamentalIF outputpower

SFDR

Third-orderintercept point

Two-tone(spurious) response

Noise floor

RF input power (dBm)




lation products, better isolation among all ports, and modu-lated noise rejection in the LO signal. Compared to single-device mixers, balanced mixers boast limited requirements for additional filters and generally broader bandwidth. Higher LO power is necessary for balanced mixer operation to avoid high conversion loss. Often, LO amplifiers are included in mixer designs for this reason.

Because the inclusion of an LO ampli-fier can add noise and proper matching between components and at the IF out-put is essential to avoid conversion loss and undesired harmonic increases, the higher required LO power is a neces-sary feature of the operation of balanced mixer classes. If the LO power drops below the recommended level, all of the mixer parameters will degrade. Such degradation is a non-predictable func-tion of the diode characteristics, circuit tolerances, and temperature.

Industry demands for better mixer per-formance often require more advanced mixer classes. Marki says, “The crowd-ed frequency spectrum puts a huge premium on ultra-linear analog hardware for everything from commercial use, high-end test equipment, and sophisti-

cated electronic warfare. The name of the game is dynamic range. If you can make an efficient frequency converter that doesn’t add nonlinear distortion, you will win a lot of business.”

To meet these demands, the design figures of merit for linearity and SFDR must increase. The devices also need to operate with overlapping or near-over-lapping input and output signals. Using an unbalanced mixer with various filters for increased isolation does not always provide the necessary frequency oper-ation. In these cases, more advanced circuit typologies may be required.

For example, the triple-balanced mix-er is the culmination of two double-bal-anced mixers combined using a push-pull drive configuration. These mixers can support overlapping RF, LO, and intermediate frequencies while flaunting the highest spurious noise suppression of the classes. The higher-bandwidth


4. Mixers that require magnetic baluns are still large compared to

most RF/microwave circuit components. Even for surface

mount, the smallest offering from Marki Microwave is only 0.32 inches on a side. (Courtesy of Marki

Microwave)





capabilities and low-conversion-loss aspects of the triple-balanced mixer are among its additional design advan-tages. Using near-ideal commutating switching techniques and internal feed-back circuitry to enhance performance, triple-balanced mixers can be pushed to very high linear behavior.

Challenges of Discrete and Integrated Mixer Technology

Mixers are manufactured to many size, frequency, and power constraints. The two main styles of mixers are discrete (hybrid) and integrated (monolithic) mixers. Discrete mixers typically use silicon-based Schottky diodes, whereas integrated mixers use NPN transistors for active types and FETs for passive types. Discrete mixers can achieve a higher linearity response and lower NF response than active mixers. Yet they also are known for increased cost, a larger footprint, and often higher power requirements (Fig. 4).

Discrete mixers require matched di-ode structures and carefully chosen components. The demands for de-creased size and lower-power operation translate into more challenging require-ments for these mixers. Marki says, “Mixers that require magnetic baluns are

still very large; our smallest is 0.32 in. on a side. These mixers use magnetic baluns that often dwarf the rest of the RF circuitry, and they are very tall. There is no clearly demonstrated and viable way to eliminate these ‘old school’ wire-wound ferrite cores from the assembly.” Minimizing component sizes to reduce the overall footprint generally requires a decrease in a device’s RF power, which impacts the response’s linearity.

Integrated mixers can be designed to high tolerances. Enabled by integrat-ed matching techniques, such mixers may incur lower costs by taking ad-vantage of silicon processes. As noted by Tom Schiltz, design section leader of high-frequency products for Linear Technology, “Advanced silicon-process technologies offer higher speed and a wider operating frequency. But they come at the expense of lower break-down voltages, which can limit mixer IIP3 and input-signal handling capa-bility.” He also warns that “integrated mixers are limited by the parasitics of packaging technologies available today and the electrostatic-discharge (ESD) circuits that are required to protect the device.”

When it comes to limitations in active mixers, another consideration is the size of the baluns used in the designs.




Baluns require significant space on chip. Off-chip baluns may be used, but they increase the overall mixer footprint while adding parasitics to the design. These factors lead to double-balanced mixers being the popular choice for in-tegrated mixers. After all, the footprint and performance drawbacks of on-chip baluns make triple-balance implementa-tions challenging for low frequencies.

Clearly, the increased performance re-quirements for RF/microwave devices lie heavily on the side of mixer manufactur-ers and designers. Schiltz notes, “In RF systems, the mixer is usually the limiting factor for system linearity. Furthermore, mixers produce undesired mixing prod-ucts that must be filtered and increase the system noise floor, which limits re-ceiver sensitivity.”

Linearity is one of the major enabling factors for higher data rates for com-munications technology, increased accuracy for test and measurement applications, and reliability for electron-ic-warfare and military applications. As such, the pressure is on to push the boundaries of mixer technology.

For sub-6-GHz RF mixers, tunability features are beginning to emerge that limit the effects of process, temperature, power, and frequency variations. These techniques use toggle pins enabled by

DC control in critical locations through-out the mixer circuit. Software-control and feedback methods may be used to enhance the linearity or sideband sup-pression of the next generation of RF mixers. Marki says, “The reconfigura-tion of 6-GHz and higher mixers is not supported in a meaningful way yet. But I think some microwave and millime-ter-wave mixers may support this in the future.”

Above 10 GHz, however, there are additional challenges in producing com-mutating LO waveforms. Yet mixers are steadily breaking new ground in terms of performance. To gauge cutting-edge performance today, Marki shared a fig-ure of merit: If a 10+-GHz mixer has a 30-dBm IIP3 with conversion loss of 8 dB, it is on the upper edge of modern mixer performance. n

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A mixer is a three-port com-ponent, which performs the task of frequency con-version. Mixers translate the frequency of an input signal to a different fre-

quency. This functionality is vital for a wide range of applications, including military radar, satellite-communications (satcom), cellular base stations, and more. Mixers are used to perform both frequency upconversion and downcon-version.

In the tutorial, “Mixer Basics Primer,” Ferenc Marki and Christopher Marki ex-

plain, “Two of a mixer’s three ports serve as inputs, while the other port serves as an output port. An ideal mixer produces an output that consists of the sum and difference frequencies of its two input signals. In other words:

fout = fin1 ± fin2

The three ports of a mixer are known as the intermediate-frequency (IF), ra-dio-frequency (RF), and local-oscillator (LO) ports. The LO port is usually an input port.”

The RF and IF ports can be used inter-changeably, depending on whether the

By Chris DeMartino Technology Editor

MIXERS AND THEIR PARAMETERS

UNDERSTANDING

Mixers remain an important part of most RF/microwave systems, prompting the need to understand this vital component and its parameters.



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Mixers and Their Parameters

FOCUS ON: MIXERS

mixer is being used to perform upcon-version or downconversion. The LO sig-nal is typically the strongest signal inject-ed into a mixer. The required LO drive level is dependent on several factors, including the mixer’s configuration and device technology.

When a mixer is used to perform downconversion, an input signal enters the RF port and an LO signal enters the LO port. These two input signals pro-duce an output signal at the IF port. The frequency of this output signal is equal to the difference of the RF input signal’s frequency and the LO signal’s frequency.

When a mixer is used to perform up-conversion, an input signal enters the IF port and an LO signal enters the LO port.

These two input signals produce an out-put signal at the RF port. The frequency of this output signal is equal to the sum of the IF input signal’s frequency and the LO signal’s frequency. Both downconver-sion and upconversion are shown graph-ically in Fig. 1. Upconversion is normally part of a transmitter, while downconver-sion is typically used in a receiver.

Mixer Performance Parameters

A mixer’s performance is determined by several metrics. These performance metrics are specified in most mixer data-sheets. The specifications, which are described below, help a system design-er select an appropriate mixer to meet

system requirements.Conversion Loss: In pas-

sive mixers, conversion loss is defined as the difference in signal level between the am-plitude of the input signal and the amplitude of the desired output signal. In a mixer used for downconversion, the con-version loss is the difference between the RF input signal’s amplitude and the IF output signal’s amplitude. In a mixer used for upconversion, the conversion loss is the differ-

Downconversion

Frequency

Radiofrequency

(f RF)



(f IF )

Radiofrequency

(f RF)



(f IF )

DC

R IL

Pow

er

Upconversionf IF = |f LO – f RF| fRF1 = f LO – f IF LO + f IF fRF2 = f

RFLO

IF

FrequencyDC

IRL

Pow

er

RF1

LOIF

RF2

1. These simple diagrams provide an illustration of frequency conversion. (Courtesy of Marki Microwave)



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ence between the IF input signal’s am-plitude and the RF output signal’s ampli-tude.

Conversion loss is expressed as a positive number in decibels. “Typical values of conversion loss can range from approximately 4.5 to 9 dB, depending on the mixer,” noted the Markis in their paper. Conversion loss values of 6 to 8 dB are common in standard double-bal-anced mixers, while triple-balanced mix-ers generally have a higher conversion loss than double-balanced mixers. It is also possible to achieve conversion gain in active mixers.

Isolation: As stated by the Markis, “Iso-lation is a measurement of the amount of power that leaks from one port to anoth-er. Isolation is defined as the difference in signal level between the amplitude of an input signal and the amplitude of the leaked power from that input signal to another port.” When isolation is high, the amount of power leaked from one port to a different port is small.

They continue, “Three types of isolation are commonly quoted in microwave mix-ers: LO-RF isolation, LO-IF isolation, and RF-IF isolation.”

As an example, if a 5-GHz signal with an amplitude of +15 dBm is injected into the LO port, a portion of this signal will leak into the RF port. If this LO input

signal causes a 5-GHz signal with an amplitude of -20 dBm to be measured at the RF port, the LO-RF isolation is 35 dB. LO-IF isolation and RF-IF isolation are calculated in the same manner.

“LO-RF isolation is critical in frequen-cy downconverting systems because LO power can leak into the RF circuitry,” note the Markis. “If the LO-RF isolation is poor, the LO power can contaminate the RF line. Poor LO-RF isolation can also cause problems in frequency upcon-verting systems when the LO frequency is very close to the RF output frequen-cy. Because the LO frequency and the RF output frequency are so close, no amount of filtering can remove the LO leakage. As a result, that leakage inter-feres with the RF output, potentially de-grading the RF output circuitry.”

LO-IF isolation specifies the amount of leakage from the LO input signal to the IF port. The Markis add, “When the LO-IF isolation is poor, problems can occur if the LO frequency is close to the IF frequency. In this case, the LO signal can contaminate the IF circuitry.” With sufficient LO leakage, the IF amplifier will potentially be saturated. The loss flat-ness also may be degraded if the LO-IF isolation is insufficient.

RF-IF isolation is the final mixer isola-tion metric. Because the amplitudes of




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both the RF and IF signals are usually significantly lower than the amplitude of the LO signal, most sys-tems designers will not find RF-IF isolation to be a major issue. However, high RF-IF isolation is usu-ally a sign that the mixer will exhibit low conversion loss with good conversion loss flatness.

1-dB Compression Point: A mixer’s conversion loss remains constant when the mixer is in linear op-eration. As the amplitude of the input signal increases, the amplitude of the output signal rises by the same amount. However, once the input signal’s am-plitude reaches a certain level, the am-plitude of the output signal ceases to exactly follow the input signal. The mix-er deviates from linear behavior and its conversion loss begins to increase. A graphical representation of this is shown in Fig. 2.

When a mixer’s conversion loss in-creases by 1 dB, the 1-dB compression point has been reached. The 1-dB com-pression point of a mixer is defined as the amplitude of the input signal required to increase the conversion loss by 1 dB. A mixer’s 1-dB compression point deter-

mines the upper limit of its dynamic range.

A mixer’s 1-dB compres-sion point is usually relat-ed to the LO drive level. Mixers with higher LO drive level requirements have a higher 1-dB com-pression point. Yet higher LO power also must have to be delivered to such mixers. In general, the 1-dB compression point is

anywhere from 4 to 7 dB below the mix-er’s minimum recommended LO drive level.

Intermodulation Distortion: Two-tone third-order intermodulation distortion (IMD) occurs when two signals simulta-neously enter the mixer’s IF or RF input port. In practice, this could happen in a multi-carrier signal environment. These two signals interact with each other and with the LO signal, which creates distor-tion. In a receiver, two-tone third-order IMD is a serious problem because it can generate third-order distortion products that fall within the IF bandwidth.

If fRF1 and fRF2 represent two separate RF input signals and fLO represents the LO signal, the third-order distortion prod-ucts generated at the mixer’s IF port are:

Interferer1 = 2fRF1 - fRF2 -fLO


Out

put p

ower

(dBm

)

Input power (dBm)

Linea

r ran

ge

1 dB

Input 1-dBcompression

point

2. Shown is a graphical representation of 1-dB compression point. (Courtesy of Marki Microwave)



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Interferer2 = 2fRF2 - fRF1 -fLO

These third-order distor-tion products are extremely close to the desired IF output frequency. No amount of fil-tering can remove these un-wanted distortion products. Thus, the signal-to-noise ratio of the received signal is degraded, highlighting the need to sup-press these distortion products.

The third-order input intercept point (TOI or IP3) is a widely accepted figure of merit used to describe a mixer’s ca-pability to suppress third-order distor-tion products. TOI is used in predicting the nonlinear behavior of a mixer as the amplitude of its input signal increases, which causes the third-order products to increase by a 3:1 ratio. For any 1-dB increase in the input signal’s amplitude, the third-order products increase by 3 dB (Fig. 3).

The TOI is the value of the input pow-er when the line representing the fun-damental output intersects with the line representing the third-order distortion products. The TOI is really just an extrap-olated point, however, because the mix-er compresses before the lines actually intersect. Due to the negative impact that third-order distortion products can have

in a system, it is desir-able for a mixer to have a high TOI.

In theory, any non-linear device can be used to create a mixer circuit. However, only a few devices satisfy the requirements needed to design mixers with

acceptable performance. Devices that are commonly used to design modern mixers include Schottky diodes, galli-um-arsenide (GaAs) field-effect tran-sistors (FETs), and CMOS transistors. Various topologies can be used to de-sign mixers. Mixers can be designed as either passive or active compo-nents.

Passive mixers primarily use Schott-ky diodes, although the FET resistive mixer has recently become another popular passive mixer. Active mixers use either FETs or bipolar devices. Schottky diodes, in comparison with FETs and bipolar devices, have the advantage of possessing an inherently wide bandwidth. This is a major reason why diodes are still widely used to de-sign mixers.

Mixers can be designed with just a single diode, which is the simplest mixer topology. Balanced mixers,


Out

put p

ower

(dBm

)

Input power (dBm)

TOI

Actual fundamental response

Idealfundamental

response

Third-orderdistortion

(3:1 slope)

3. A graphical representation of how TOI is derived.



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which consist of two, four, or even eight di-odes in a balanced structure, build upon the single-diode mixer. The majority of mixers avail-able today incorporate some form of mixer balancing.

A single diode can be used to create a mixer (Fig 4). Here, the RF and LO signals combine at the anode of the diode. The LO signal needs to be large enough to switch the diode on and off, which causes the actual mixing process. The frequency components generated by single-diode mixers are:fIF = nfLO ±mfRF (m and n are all integers)

where:fLO = the LO input signal frequencyfRF = the RF input signal frequencyfIF = the IF output signal frequencyAlthough only one output frequency is

desired (when n = 1 and m = 1), addi-tional unwanted harmonics are gener-ated by the diode’s current-voltage (I-V) characteristics and the transconduc-tance modulation caused by the RF sig-nal. Because the single-diode mixer has no inherent isolation between the RF and LO ports, external filters also are needed to achieve isolation between ports. This need for external filtering makes it diffi-

cult to achieve wideband mixers with just a single diode.

Balanced mixers over-come some of the limita-tions of single-diode mix-

ers. They do require baluns or hybrids, which largely determine the bandwidth and overall performance of the mix-er. Inherent isolation between ports is achieved by balanced mixers as well as increased cancellation of intermod-ulation products. Common-mode noise cancellation is another advantage gained by balanced mixers. However, balanced mixers do require a higher LO drive level.

Single-balanced mixers consist of two diodes along with a hybrid (Fig. 5). Al-though 90-deg. and 180-deg. hybrids can both be used to design single-bal-anced mixers, the majority of single-bal-anced mixers incorporate a 180-deg. hybrid. The 180-deg. hybrid’s input ports are mutually isolated, enabling the LO




FOCUS ON: MIXERS

RFinput

LO input

IFoutput

RF

LO

IF

–180º 0º0º

0º

5. A single-balanced mixer consists of two diodes and a hybrid.

4. A mixer can be designed with a single diode.



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FOCUS ON: MIXERS

port to be isolated from the RF port. This provides frequency-band independence and equal power division to the load. In comparison with single-diode mixers, single-balanced mixers also have 50% fewer intermodulation products.

Two single-balanced mixers can be combined to form a double-balanced mixer. Traditional double-balanced mixers are typically based on four Schottky diodes in a quad ring config-uration. Baluns are placed at both the RF and LO ports, while the IF signal is tapped off from the RF balun. The IF signal can also be tapped off from the LO balun, but this would worsen the LO-IF isolation.

For this reason, it is usually pre-ferred to tap off the IF signal from the RF balun instead of the LO balun. An example of a double-balanced mixer is shown in Fig. 6. This mixer has high LO-RF isolation and LO-IF isolation along with moderate RF-IF isolation. Double-balanced mixers also have the benefit of reducing intermodulation products by as much as 75% in com-parison with single-diode mixers.

An even more complex mixer circuit is the triple-balanced mixer. Triple-bal-anced mixers have separate baluns for the LO, RF, and IF ports, which en-ables them to achieve high LO-RF iso-

lation, LO-IF isolation, and RF-IF isola-tion. Triple-balanced mixers also offer higher suppression of intermodulation products than double-balanced mix-ers. The downside of triple-balanced mixers is that they need a higher LO drive level. They also are greater in both size and complexity.

In summary, mixers are available in a variety of forms, with varying levels of performance. Several parameters are used to characterize a mixer’s perfor-mance. Insight into mixers, as well as their parameters, can help system de-signers select the proper mixer for their system. The key parameters of a mixer, such as conversion loss, isolation, and intermodulation distortion, are critical factors for any system design.nReferencesMarki Microwave, “Mixer Basics Primer.” Available: http://www.markimicrowave.com/Assets/appnotes/mixer_basics_primer.pdf



RFLO

IF

6. A double-balanced mixer uses four diodes in a quad ring configuration.



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IP2 MIXERS?



By Doug JorgesenApplication Support / Engineering Manager,Marki Microwave

WHAT IS THE DEAL WITH

Every day we work on high linearity mixers: high IP3, high P1dB, and high spurious suppression. Every once in a while we get a request for a high IP2 mixer. This is much more rare than complaints about IP3 or spurs. Let’s see why.

To start understand-ing IP2 in mixers, lets look at intermodula-tion products in am-

plifiers. Start by imagining a single tone into a single non-ideal amplifiers, with a nonlinearity. (see Fig. 1 on next page)

A single input tone is am-plified to a larger tone, as desired. Due to the nonlin-earity in the amplifier, high-er order tones are created. These are only created at integer multiples of the in-put tone, since the system is time invariant. These decrease in power as the frequency increases, and they are generally referred to as second, third, Nth or-der distortion products.

A new problem arises if we put two tones into the same amplifier: (see Fig. 2 on next page)

This diagram only shows

This article was originally published by Marki Microwave on October 7, 2014. The original document can be found here: http://www.markimicrowave.com/blog/2014/10/what-is-the-deal-with-ip2-in-mixers/.

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IP2 Mixers

FOCUS ON: MIXERS

the second harmonics, the second or-der intermodulation, and the third order intermodulation in the relevant bands. As you can see, the second harmonics (at 2f1 and 2f2) are easily filtered out as they are at a very high frequency, unless you are in a very broad-band multioctave system. The second order inter-modulation (at f1 + f2) is in between the two harmonic distortion products, so it is also easily filtered except in broadband systems. While these are drawn as the same power level, they are not necessarily the same

power in practice. The third order har-monics (at 2f1 - f2, f1 + f2 - f1, f1 + f2 - f2, and 2f2 - f1) are all famously in band. Two of the tones are directly on top of the desired received tones, and therefore they cannot be filtered even with an in-finitely narrow and steep filter.

Now to mixers. The addi-tion of the time varying LO increases the complexity dramatically. All of the previ-ous effects will be present, and also will all be present at the LO crossed frequen-cies. For the moment lets ignore everything but the

isolations, fundamentals, and second order distortions (both single and multi-tone). (See Fig. 3)

In this case, there are two types of second order products: direct and con-verted. The direct products are at the

Input

f

f 2f 3f 4f

Input

f1 f2

f1 f2

2f1 – f2 2f2 – f1 2f1 2f2f1 + f2

Input I RL

f1 f2

f1 f2LO

fLO

2f1 2f2f1 + f2

SecondOrder

ConvertedSecondOrder

Outputs Outputs

Fig. 1

Fig. 2

Fig. 3



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same frequency as in an amplifier, and the convert-ed second order product is between the 2IF × 1LO products. Again these are drawn as the same power level, but they may or may not be identical powers.

When will these products matter? The converted products will appear in the passband in a broadband sys-tem (with a low IF) where they cannot be filtered out. Conversely the direct second order products will matter in a different kind of broadband system, with a high IF. Specifically when fIF = 1/3 fLO, the con-verted signal and the second harmonic of the IF, and the second order distortion product, will all be at 2/3 fLO. These can be a problem, but usually no worse and closely affiliated with the 2IF × 0 LO spur.

So in both cases we see that while the second order distortion exists, it is always close to a high power spur that also must be dealt with in the frequency plan.

Now lets consider a downconversion:(see Fig. 4) Once again the converted second

order shows up, this time in between the two 2LO – 2RF spurs that usually wreak havoc on downconversion systems.

Once again the same frequency plan is needed to eliminate it. The direct second order term, however, is at f2 - f1, which becomes a significant problem when the IF frequency is similar in magnitude to the separation between the two tones. In this case the direct second order tone would lie directly over one of the tones.

The converted two tone second order intermodulation product will be an issue in the same circumstance as the 2× - 2 spur is a problem, namely when you have a low IF. If the IF is at DC (direct downconversion) then the second order intermodulation will cause significant distortion at DC. This is why the most common reference to IP2 in the literature is for the mitigation of it in direct down-conversion receivers.

One thing that is not IP2, but is some-times referred to as IP2, is the half-IF

IP2 Mixers

Input R IL

f1 f2

f1 f2LO

fLO

SecondOrder

ConvertedSecondOrder

Outputs

Fig. 4



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spur. This occurs when a signal (at Frf) is downcon-verted to a low frequency, near baseband, and there is a jamming signal at a frequency (Fj) roughly half-way between the RF and LO frequencies. The down-converted jamming signal can be filtered out by the IF filter, along with all other unwanted signals. However, the jammer signal creates a high power 2 LO x 2 RF spur, however, that will show up at or near the desired signal, and there is no physical way to filter it out. (see figure 5)

Fortunately, in either a double or triple balanced mixer structure the 2 × 2 spur will be well canceled by both the LO and RF baluns, resulting in excellent sup-pression when well balanced baluns are used. For example, the ML1-0218ISM offers a downconversion 2 × 2 suppres-sion of 58 dBc with an input of -10 dBm. The T3 circuit can offer even better sup-pression, since the proprietary T3 circuit will both prevent and suppress these spurs. Therefore the T3-18 offers a supe-rior 64 dBc suppression of the 2 × 2 spur with the same -10 dBm input.

However, this is not a two tone IP2 problem. It is simply a second order dis-tortion product. So I shouldn’t take these

authors too hard to task. As Joel Dun-smore cautions in his book, Introduction to Microwave Measurements:

There is sometimes confusion in the use of the term second-order intercept; while it is most commonly used to refer to the second harmonic content, in some cases, it has also been used to refer to the two-tone second-order intercept, which is a distortion product that occurs at the sum of the two tones. Most proper-ly, one should always use the term two-tone SOI if one is to distinguish from the more common harmonic SOI.

And that is the final point of this post; when you are talking about IP2, you al-ways need to be specific about what you mean. n


IP2 Mixers

R IL

fJ fRF

fLO

Jammer DesiredSignal

LO Input

DownconvertedJammer

fIF

IF Filter

JammingSpur

DownconvertedDesired Signal

fLO fJ fRF

Leakage Terms

Fig. 5




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CONVERTINGFREQUENCIES



By Jack BrowneTechnical Contributor,Microwaves & RF

Frequency translation is necessary within many high- frequency systems, and RF/microwave mixers are the key components providing this vital function.

WITH MICROWAVE MIXERS

Downconversion and up-conversion are performed in most high-frequency re-ceivers and transmitters by means of an often-over-looked component: the RF/

microwave mixer. Over the last 20-plus years, mixers have changed a great deal in appearance and are now avail-able in a wide variety of package styles.

Still, their main functionality has not changed: to translate the frequency of a signal (usually carrying some form of information via modulation) to a second frequency (either higher or lower than the original frequency).

An RF/microwave mixer is essential-ly a three-port component, which can be fabricated as a passive component based on diodes or an active compo-



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Converting Frequencies

FOCUS ON: MIXERS

nent based on biased field-effect tran-sistors (FETs). A mixer’s three ports are commonly known as the radio-frequen-cy (RF), local-oscillator (LO), and inter-mediate-frequency (IF) ports, with two serving as input ports and one as the output port. The LO port is always an input port, so that the RF and IF ports are the ones that switch functions, de-pending upon whether a mixer is used for frequency downconversion or fre-quency upconversion.

In downconversion, a high-frequency RF input signal is mixed with a high-fre-quency LO signalusually over a similar frequency range as the LO signalto pro-duce a lower-frequency IF output signal. In upconversion, lower-frequency IF sig-nals serve as inputs, and are mixed with higher-frequency LO signals to produce RF output signals. The latter have been translated higher in frequency than the IF input signals while maintaining the modulation information of the IF signals. Downconversion is normally part of a receiver; upconversion is typically used in a transmitter. The translation of fre-quencies follows a simple mathematical mixing function:

n(fRF fLO) = fIF

where:

fLO = the LO signal frequencyfRF = the RF signal frequencyfIF = the IF signal frequencyn = the harmonic number

Here is a simple example of downcon-version, using an RF at 2100 MHz and an LO at 2000 MHz: Fundamental-fre-quency mixing would yield IF sum and difference signals, with one IF signal at 2100 2000 = 100 MHz and one at 2100 + 2000 = 4100 MHz. If the low-er-frequency signal product is desired, the higher-frequency product can be removedfor instance, through the addi-tion of a lowpass filter at the mixer’s IF output. It should be noted that it is the difference between the mixing signals that is important, and the difference of LO RF can also be used.

Many different types of RF/microwave mixers have been developed over the years in support of a diversity of differ-ent types of communications systems. These include single-balanced mixers (which can be designed with a single diode), double-balanced mixers, tri-ple-balanced mixers, image-reject mix-ers, in-phase/quadrature (I/Q) mixers, single-sideband mixers, double-side-band mixers, harmonic mixers, and subharmonic mixers.

Traditional double-balanced mixers,



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for example, are typically based on four Schottky di-odes in a quad ring config-uration, which provides ac-ceptable performance for many applications. When certain levels of enhanced performance are needed, a pair of these diode quads are incorporated in the mix-er circuitry to form a tri-ple-balanced mixer. Mixers that can process signals with I and Q components are ideal for use in systems employing digital modulation, while harmonic mix-erswhich can extract higher harmonics from the mixing processare typically used in generating and processing milli-meter-wave signals.

Ideally, when a downconverting mixer processes LO and RF input signals to produce a lower-frequency IF signal in a receiver, the received signals are mixed with the injected LO signal. But any sig-nals appearing at a mixer’s RF port that are at that target frequency range but not the desired signalsusually referred to as “image” signalswill produce IF output signals. Receivers often employ prese-lector filters to remove any unwanted im-age signals that fall into the bandwidth of the mixer’s RF port. The alternative

is to use an image-reject mixer which has been de-signed to attenuate these unwanted image signals.

Mixers are characterized by a number of perfor-mance parameterssome of which (such as conversion loss) only apply to mixers and not to other high-fre-quency components. Other important mixer parameters include port-to-port isola-tion, VSWR, noise figure,

1-dB compression, and third-order in-tercept point. For example, isolation de-scribes the separation between ports, or how much power will feed through from one port to another. High isolation indi-cates a mixer with minimal signal leak-age between ports.

A mixer’s dynamic range is the differ-ence between the maximum amplitude of signals it can handle (as determined by the 1-dB compression point) and the lowest-level signals it can process (de-termined by its noise figure). Of course, choosing any mixer is a matter of match-ing the mixer’s overall performance to a required system frequency plan. This in-cludes whether the need is for upconver-sion or downconversion, how the IF will be handled, the available LO power, and


Ideally, when a downconverting mixer processes LO and RF

input signals to produce a lower-frequency IF

signal in a receiver, the received signals are

mixed with the injected LO signal.



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even the type of mixer package desired for printed-circuit-board (PCB) mounting.

If a mixer is employed for frequen-cy downconversion, as typically used in an RF/microwave receiver, a great deal of its performance will be dependent on the available LO signals. For example, noise in the LO signals will contribute to noise at a downconverting mixer’s IF port. But a mini-mal LO amplitude will also limit the available dynamic range of the mixer. Mixers are typically optimized for different LO signal levels, such as +7, +10, and +14 dBm; this is the power that energizes a mixer’s non-linear switching elements, whether they are diodes or transistors. At levels above the optimal LO amplitude, a mixer will start to experience compression, where an increase in LO input power no longer results in an increase in IF output power. Early signs of compression are indicat-ed by a mixer’s 1-dB compression point.

A companion parameter for deter-mining a mixer’s linearity is the third-or-der intercept point (IP3), which refers to a level of intermodulation distortion

caused by two tones at a mixer’s RF port. For mixers used in digital commu-nication systems, for example, excellent linearity is important in maintaining the accuracy of I and Q signal components

processed by the mixer, with higher third-order-in-tercept-point values repre-senting enhanced linearity performance.

Passive mixers are characterized by their conversion loss, which is caused by losses due to impedance mismatches in the mixer circuit, to diode junctions, and to other cir-cuit-connection points in a mixer design. In a mixer used for downconversion, the conversion loss is the difference in signal lev-el between the RF input

amplitude and the IF output amplitude, while in a mixer used for upconversion the difference in amplitude is between the IF input amplitude and the RF out-put amplitude. Conversion-loss values of 6 to 8 dB are not unusual in standard double-balanced RF/microwave mixers. Of course, it is also possible to achieve conversion gain in an active mixer, through the use of an amplification


1. Traditional mixer packaging includes coaxial connectors and rugged metal housings. [Photo courtesy of Spectrum Microwave (www.spectrummicrowave.com).]



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stage and active circuit devic-es in the mixer. But this gain will also require the addition of bias power for the mixer’s active circuitry, as if biasing an amplifier.

RF/microwave mixers were once fairly large components, featuring metal housings with three coaxial connectors for the ports. For some applica-tionssuch as rack-mount receivers, transmitters, and test equipmentsuch mixer packaging is still a good match (Fig. 1). But as more high-frequency designers are asked to miniaturize cir-cuits and systems, mixer packaging has followed with increasingly smaller surface-mountable and PCB-mountable packages (Fig. 2), which allow engineers to achieve frequency translation in ex-tremely small areas of a circuit (when including the LO source).

Newer mixers can be specified for broadband or narrowband use, depend-ing on the specific application. In terms of the performance levels possible in small packages, mixers such as the model SYM-63LH+ from Mini-Circuits is a double-balanced mixer based on a diode quad that can handle RF/LO sig-

nals from 1 to 6000 MHz. The same firm’s MAC Se-ries of mixers is based on low-temperature-co-

fired-ceramic (LTCC) circuit substrates for RF/LO coverage from 0.3 to 12.0 GHz in a surface-mount package that is only 0.06 in. high. And the SGS-5-17 double-bal-anced mixer from Synergy Microwave uses the company’s

SYNSTRIP multilayer circuit technology to achieve RF/LO coverage from 3 to 19 GHz in a package measuring only 0.275 x 0.200 x 0.050 in.

In addition, a growing number of in-tegrated-circuit (IC) manufacturers are fabricating mixer functions as part of entire front-end assemblies. These also include preselector filters, amplification, matching transformers, and IF filters to greatly simplify the task of RF/microwave receiver and transmitter designers look-ing for a compact, frequency-translation solution. n


2. This compact plastic surface-mount package measures just 0.38 x 0.50 x 0.23 in. (9.65 x 12.70 x 5.84 mm) but supports mixer circuits well into the microwave frequency range. [Photo courtesy of Mini-Circuits (www.minicircuits.com).]


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Documents

LIBRARY - Microwaves & RF · PDF filetwo main styles of mixers are discrete (hybrid) and integrated (monolithic) ... for active types and FETs for passive types. Discrete mixers can