BALUN
`BASICS
`PRIMER
`
`A Tutorial on Baluns,
`Balun Transformers, Magic-Ts,
`and 180° Hybrids
`By: Doug Jorgesen and Christopher Marki
`
`Ex.1022 p.1
`
`

`
`INTRODUCTION
`
`The balun has a long and illustrious history, first documented in the literature as a device to feed the television transmitting
`antenna for the Empire State Building1 in 1939. Since then designs have evolved dramatically, and applications have evolved
`beyond driving differential antennas to include balanced mixers, amplifiers, and signaling lines of all types. Baluns have long
`been ubiquitous in low frequency audio, video, and antenna driving applications. The need for high speed, low noise data
`transfer has driven the advancement of the balun to higher frequencies and superior performance.
`
`Despite these advancements, information about baluns remains scattered and confusing; this application note seeks to resolve
`this problem by clarifying the basic characteristics of baluns. First we will define what a balun is, what it does, and how it is
`different from other components. Next we will define generic balun specs, which we then use to discuss the different types
`of baluns and their properties. Finally we will discuss the applications of baluns and how to determine what kind of balun is
`required for several different purposes.
`
`I. WHAT IS A BALUN?
`
`A balun is any three port device with a matched input and differential outputs. It is most succinctly described by the required
`(ideal) S-parameters:
`
`S12 = - S13 = S21 = - S31
`S11 = -∞
`
`Note what is implied by this:
`•
` A balun is a three port power splitter, similar to a Wilkinson or resistive power divider.
`•
`The two outputs will be equal and opposite.
`
` - In frequency domain this means the outputs have a 180° phase shift.
`
` - In time domain this means the voltage of one balanced output is the negative of the other balanced output.
`•
`The unbalanced input is matched to the input transmission line impedance (usually 50 Ω).
`•
`Unlike an isolator or circulator, a balun is a reciprocal device that can be used bidirectionally.
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`2
`
`Ex.1022 p.2
`
`

`
`Digital Data
`
`Digital Data
`
`Single Frequency
`
`Vunbalanced
`
`Zunbalanced
`
`Single Frequency
`
`Unbalanced/
`Single Ended/
`Common
`
`Positive/0°/
`Non-inverted
`Balanced/Differential
`Negative /180°/
`Inverted
`
`𝐼𝑚𝑝𝑒𝑑𝑎𝑛𝑐𝑒 𝑅𝑎𝑡𝑖𝑜
`=𝑍𝑢𝑛𝑏𝑎𝑙𝑎𝑛𝑐𝑒𝑑
`𝑍𝑏𝑎𝑙𝑎𝑛𝑐𝑒𝑑
`= 𝑇𝑢𝑟𝑛𝑠 𝑅𝑎𝑡𝑖𝑜 2
`
`Vunbalanced
`
`Vbalanced
`
`Amplitude
`Balance
`
`Isolation
`
`Zbalanced
`
`Phase
`Balance
`
`Vbalanced
`
`Digital Data
`
`Single Frequency
`
`Fig. 1: Baluns convert between unbalanced and balanced signals
`
`Also note what is not implied by this:
`•
` The two outputs are not necessarily matched.
`•
` The outputs of the balun may or may not be the same impedance as the input.
`•
` There is no constraint on S23, so the outputs may or may not have isolation.
`•
` Therefore there may be a different return loss on the outputs for differential and common mode signals.
`The term balun is a portmanteau of balanced and unbalanced, indicating that a balun will transition between a balanced
`(also called ‘differential’) transmission line (where opposite currents both travel in transmission lines) and an unbalanced (also
`called ‘single ended’) transmission line (where the return current travels in the ground). However, this description obscures
`the simplicity of the balun. A balun has equal power outputs just like a Wilkinson power divider, resistive power divider, or
`quadrature hybrid coupler. However, it has a 180° phase difference between outputs, while the power dividers have 0° phase
`difference and quad hybrids have 90° phase difference.
`
`At low frequencies, the terms balun and transformer are often used interchangeably because low frequency baluns are almost
`always implemented using flux coupled transformers. For this reason it is often said that a balun is a type of transformer, but
`it is more accurate to say that a transformer can sometimes be used to implement a balun. Many other structures can also be
`used to implement balun functionality, as we will discuss in section IV. Before discussing the virtues of different types of balun
`structures, we need to define what performance specs are important for baluns.
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`3
`
`Ex.1022 p.3
`
`

`
`II. BALUN PERFORMANCE SPECS
`
`Frequency coverage: As with all RF/microwave circuits, each performance metric is only valid across some specified
`bandwidth. Increasing the bandwidth from octave, to decade, to multi-decade without sacrificing performance is a major
`challenge. In general Marki baluns can be divided into two types. Those with magnetic coupling perform below 10 MHz,
`while those with only capacitive coupling have low end performance limited to about 1 GHz, but can operate up to millimeter
`wave frequencies.
`
`Phase Balance: The most important performance criterion is how close the balanced outputs are to having equal power and 180°
`phase, called balance. Phase balance is the measure of how closely the inverted output is to 180° out of phase with the non-inverted
`output, usually given in degrees. It is the most critical parameter for many balun applications. In addition to the quality of the balun
`structure, how closely matched the lengths of the output lines are determines the balance. Typical phase balance for standard
`microwave baluns is ±15° max and ±10° typical, while high performance Marki baluns approach ±5° max and ±2° typical.
`
`Amplitude Balance: Related to phase balance, amplitude balance is also determined by construction and line matching.
`Although it is called amplitude balance, it is usually specified in dB and actually gives the match between output power
`magnitude. Low performance baluns have amplitude balance of ±1.5 dB max and ±1 dB typical, while Marki products
`approach ±.5 dB max and ±.2 dB typical.
`
`Common Mode Rejection Ratio: If two identical signals with identical phase are injected into the balanced ports of the balun
`(called ‘common mode’ or ‘even mode’ signals), they will be either reflected or absorbed. The amount of attenuation this signal
`will experience from the balanced to unbalanced port is called common mode rejection ratio (CMRR) and is expressed in dB. It
`is determined by the vectorial addition of the two signals, and therefore is dependent on the amplitude and phase balance of
`the balun. The relationship between amplitude balance, phase
`balance, and CMRR is shown in Fig. 2. As a rule of thumb, a 0.1
`dB improvement in amplitude balance will improve the CMRR
`by the same amount as a 1° improvement in phase balance.
`A low performance balun will have 15-20 dB of CMRR, while
`Marki baluns can achieve 25-55 dB of CMRR.
`
`Common Mode Rejection Ratio (dB)
`
`14
`
`12
`
`10
`
`8
`
`−15
`
`−15
`
`−20
`
`−25
`
`−19
`
`6
`
`4
`
`2
`
`−23
`
`−27
`
`−30
`
`−35
`
`−40
`
`0.5
`
`1
`Amplitude Balance (dB)
`
`1.5
`
`2
`
`Fig. 2 Common Mode Rejection Ratio (in dB) as a
`function of amplitude and phase balance
`
`Phase Balance (degrees)
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`4
`
`Ex.1022 p.4
`
`

`
`Impedance Ratio/Turns Ratio: While the unbalanced impedance of a balun is matched to the input transmission line, the balanced
`impedance can be any value. The ratio of the unbalanced impedance to the balanced impedance is the impedance ratio, and is
`usually stated as 1:n (i.e. 1:1, 1:2, 1:4). Note that the differential impedance is between the balanced signal lines. This is twice the
`impedance between the signals and ground. A related value is the turns ratio, which for a flux coupled balun transformer is the
`ratio of primary windings to secondary windings. The impedance ratio is the square of the turns ratio (i.e. a 1:2 turn ratio gives a 1:4
`impedance ratio). A higher output impedance will provide increased voltage at a reduced current, which is desireable for matching
`into high impedance semiconductor devices. High impedance ratio baluns are easy to design using flux coupled transformers, but
`much more difficult for transmission line transformers and other high frequency constructions.
`
`Insertion and Return Loss: A lower insertion loss and higher return loss will mean more power available for downstream functions,
`an improved dynamic range, and less distortion of signals in previous stages of the system. In a balun without isolation, as in a
`reactive splitter, the return loss of balanced ports will be different for common mode and differential mode signals. In an ideal balun
`without isolation, the common mode signal would be perfectly reflected, with a return loss of 0 dB, while the differential signal
`would pass through completely with a return loss of -∞. To properly characterize this effect one can use mixed-mode S-Parameters
`instead of standard S parameters to determine how the device will operate with differential inputs2.
`
`Balanced Port Isolation: Usually referred to simply as isolation, this has the same meaning as in other power dividers and
`couplers, namely the insertion loss from one balanced port to the other in dB. Most baluns do not offer high isolation because
`the even mode is reflected instead of being properly terminated with a resistive load. The exception is 180° hybrid circuits,
`where the even mode is output to a port that can be resistively terminated.
`
`DC/Ground Isolation: Different from the balanced port isolation, DC isolation is whether the unbalanced port has a DC
`connection to one of the balanced ports. Ground isolation is whether there is a connection between the unbalanced ground
`and the balanced signals or grounds.
`
`Group Delay Flatness: For data transmission applications, a flat group delay will ensure a minimal amount of distortion.
`Group delay flatness is the difference from the average delay across frequencies. This parameter can most easily be evaluated
`by either measuring directly on a VNA or examining the output eye diagrams from an input amplitude shift keyed signal.
`Unwanted group delay ripple is related to poor broadband matching. Baluns with superior return loss will have superior group
`delay flatness.
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`5
`
`Ex.1022 p.5
`
`

`
`III. TYPES OF BALUNS
`
`The most common type of balun by volume is the flux coupled balun transformer. This is a balun created by winding two separate
`wires around a magnetic core (the same as any transformer), and grounding one side of the primary winding. This creates an
`unbalanced condition on the primary side, and a balanced condition on the secondary side. In addition, the secondary side can
`have an arbitrary ratio of turns to the primary side, creating an arbitrary impedance ratio (the theory of a transformer is explained in
`many introductory electrical engineering texts).
`
`Magnetic Core
`
`Magnetic
`Coupling
`
`Unbalanced
`Input
`
`Balanced
`Output
`
`Unbalanced
`Input
`
`Bifilar
`
`Balanced
`Output
`
`a
`
`Capacitive
`Coupling
`
`b
`
`Unbalanced
`Input
`
`c
`
`Balanced
`Output
`
`Capacitive
`and
`Magnetic
`Coupling
`
`Fig. 3 Construction of a) a flux coupled balun transformer using two wires wrapped around a common magnetic core,
`b) a transmission line balun consisting of a bifilar coil of two wires wound around each other, with one end connected to ground,
`and c) a transmission line balun using magnetic core for additional low frequency coupling
`
`The flux coupled balun transformer will induce an AC voltage in the secondary of n times the voltage in the primary, while the
`current will be n times smaller than in the primary, giving an output impedance of n2 as stated above, where n is the ratio of turns in
`the secondary to turns in the primary. The circuit symbols typically used for a balun transformer are shown in Fig. 4. Circuit diagrams
`typically use a dot convention to indicate which side corresponds to the input polarity. Wire wound flux coupled transformers will
`often have a center tap in the secondary winding. In the middle of the secondary winding a virtual ground exists, and connecting
`this point to the ground of the secondary system can improve the balance of the output.
`
`+
`
`+
`
`Center Tap
`
`-
`Balanced
`
`Unbalanced
`
`Fig. 4 Circuit symbol for a flux coupled balun
`transformer, showing the dot convention and center tap
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`6
`
`Ex.1022 p.6
`
`

`
`Ideally, a flux coupled transformer could be used whenever balun functionality is required. It is well understood, relatively simple
`to build, provides an arbitrary impedance ratio than can be easily tuned, and provides both DC and ground isolation. Unfortunately
`they are generally limited to frequencies below 1 GHz. At higher frequencies the dipoles in the magnetic material cannot switch
`fast enough, and the balun loses coupling. The parasitic capacitance between wires causes high frequency signals to travel directly
`to ground without coupling through the magnetic material. Magnetic materials also always possess a large loss tangent, leading to
`high signal losses at microwave frequencies.
`
`Because of these difficulties, the capacitively coupled transmission line balun was developed. This is a set of coupled lines with one
`end grounded, such that the coupling will induce equal and opposite signals in both lines. Converting the ground to a transmission
`line allows the signal to be used differentially. This can be done in many ways, most often with a bifilar transmission line wrapped
`around a magnetic core to take advantage of the low frequency magnetic coupling as well as the high frequency capacitive coupling
`(Fig.3c). This basic structure can be connected in many different ways; the more common forms include the 1:4 impedance ratio
`Ruthroff balun and 1:4 Guanella balun3.
`
`+-
`
`+-
`
`+ -
`
`Fig. 5 Circuit diagram of a 1:1 transmission line balun (left),
`1:4 Guanella balun (center), and one type of 1:4 Ruthroff balun (right)
`
`This functionality can also be performed with a microstrip transmission line where
`the ground plane is simply tapered into a bottom transmission line. This structure,
`called a tapered balun (also called microstrip-to-balanced stripline balun, Fig. 6),
`has the advantage of high frequency operation but the disadvantages of no low
`frequency capability and a difficult-to-implement geometry. The tapered balun, in
`turn, is very similar to other type of coupled line baluns such as the Marchand balun
`(Fig. 7), coplanar waveguide balun, coaxial balun, planar transformer (spiral) balun,
`and many other types of coupled line circuits that can be used as baluns, and will not
`be reviewed here.
`
`Fig. 6 Tapered microstrip balun
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`7
`
`Ex.1022 p.7
`
`

`
`+ -
`
`-
` Fig. 7 Circuit diagram of a capacitively coupled transmission line balun (left) and basic Marchand Balun (right)
`
`+
`
`These types of baluns are all based on quarter wavelength sections of transmission lines, which means that they need to be
`longer (and higher loss) to operate at lower frequencies. This limits the practical low end of the frequency band to around
`500 MHz to 1 GHz. On low dielectric substrates the parts will be larger (and usually lower loss), while on higher dielectric
`substrates they will be smaller (and higher loss). For example a quarter wavelength section at 1 GHz on a 2.2 dielectric
`substrate will be 2.15” long, therefore the minimum length of a Marchand balun would be 4.3” long. Conversely at 10 GHz,
`the same balun would be 0.430” long and easily printed using standard fabrication methods.
`
`All the previously mentioned baluns are of one type, where coupling of some sort is used to float the ground of an unbalanced
`transmission line, creating a balanced transmission line. Another type of balun is one where an in phase power division
`is performed first, and then a 180° phase shift is applied to one
`of the outputs, creating a balanced output. This structure is not to
`be confused with the 180° power divider, which is discussed
`below. This phase shift can be narrow band, such as a half wave
`transmission line, or a broadband phase shift such as an inverter
`(Fig. 8). This technique is commonly used to create higher frequency
`baluns for test and measurement. A half-wave balun uses a
`ladder of quarter wave transformers combined with half wave
`transmission sections to expand the bandwidth of a simple single
`frequency balun. Another method is to use a coupler 90° phase shift
`on one arm and a coupler with a -90° phase shift on the other arm.
`These baluns can achieve multi-octave bandwidth if they use interdigital Lange couplers.
`
`32
`
`Matched
`Delay
`
`1
`
`Power
`Divider
`
`Inverter
`
`Fig. 8 Phase shift balun
`
`The final type of balun is the 180° power divider, which is a balun with isolation between the outputs. These are implemented
`using 180° hybrid junctions (Fig. 9). These are similar in function to 90° hybrid couplers, but they have a phase shift of
`180° between the non-isolated ports. They have the property that from two inputs, the common or even mode will output from
`one port (the Σ or sum port), while the differential or odd mode will appear at a different port (the Σ or difference port). A
`180° hybrid coupler can be made into a 180° power divider by terminating the sum port with a 50 Ω load. These types of
`circuits suffer from the same quarter wavelength length requirements as capacitively coupled baluns. Common examples of
`180° hybrid couplers include the rat race coupler, the asymmetric tapered coupled line coupler, and the magic-T (Fig. 10).
`Interestingly, a Wilkinson power divider is actually a type of 180° hybrid where the sum port is terminated with a lumped
`resistor, called the isolation resistor.
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`8
`
`Ex.1022 p.8
`
`

`
`Fig. 10 Rat Race Coupler (left), asymmetric tandem
`coupler (center), and Waveguide Magic-T (right)
`
`3 4
`
`4 Port
`Hybrid Junction
`
`1 2
`
`Port 1
`
`Input
`
`Port 2
`
`Isolated
`
`Isolated
`
`Input
`
`Port 3
`
`Input/2
`
`Input/2
`
`Port 4
`
`Input/2
`
`-Input/2
`
`Fig. 9 Schematic and I/O table for a 4-port hybrid junction
`
`Balun Type
`
`Max BW
`Ratio
`
`Flux Coupled Balun
`Transformer
`Wire-wound
`Transmission Line Balun
`
`Capacitively Coupled
`Transmission Line Balun
`Power Divider - Inverter
`Balun
`Half Wave Balun
`Rat Race Coupler
`Asymmetric Tandem
`Coupler
`Waveguide Magic T
`
`106:1
`
`105:1
`
`102:1
`
`106:1
`
`<2:1
`<2:1
`10:1
`
`<2:1
`
`Practical
`Frequency
`Range
`20Hz –
`1 GHz
`500 kHz –
`10 GHz
`
`.5 – 65 GHz
`
`200 KHz –
`65 GHz
`.5-60 GHz
`.5-50 GHz
`.5-40 GHz
`
`1-146 GHz
`
`Balance
`
`Isolation
`
`Fair -
`Excellent
`Fair -
`Excellent
`
`Fair -
`Excellent
`Fair
`
`Good
`Fair
`Fair
`
`Fair
`
`With center tap
`
`No
`
`No
`
`Depends on
`Power Divider
`No
`Yes
`Yes
`
`Yes
`
`Common
`Impedance
`Ratios
`Arbitrary
`
`1:1, 1:4
`
`1:2
`
`1:2
`
`1:2
`1:2
`1:2
`
`1:2
`
`Applications
`
`Balanced Transmission Lines,
`Differential Antennas
`Interface to Differential ICs
`(ADC/DACs)
`
`Balanced Mixers, Push-Pull
`Amplifiers, Signal Combining
`Test Instruments
`
`Mixers
`Mixers, Duplexers
`Mixers, Duplexers, Amplifiers,
`Antenna Arrays
`Mixers, Duplexers, Amplifiers
`
`IV. APPLICATIONS OF BALUNS
`
`The most common application of baluns is to interface an unbalanced signal to a balanced transmission line for long distance
`communications. Differential signaling on balanced transmission lines is more immune to noise and crosstalk, can use lower
`voltages, and is lower cost than single-ended signaling on coaxial cables. Hence, it is used for most common intermediate
`and long distance transmission lines such as RS-422, RS-485, Ethernet over twisted pair, PCI Express, DisplayPort, HDMI,
`and USB. Therefore, baluns are used to interface local video, audio, and digital signals to long distance transmission lines. In
`these applications the most important characteristic is common mode rejection ratio. The second most important application of
`baluns is for driving differential antennas. There is an abundance of literature from amateur ham radio operators on techniques
`to build and operate baluns for various antenna patterns to maximize the antenna gain4. As in differential signaling, the
`rejection of common mode current is the most important metric for an antenna feed balun, although performance also requires
`proper impedance ratios and matching to the antenna.
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`9
`
`Ex.1022 p.9
`
`

`
`Another extremely high volume application is the use of baluns to create balanced devices such as push-pull amplifiers and
`balanced mixers (Fig. 11). Push-pull amplifiers work by splitting the signal into a positive and negative version with a balun,
`amplifying them, and then recombining the signals with another balun. One advantage of this scheme is that the saturated
`output power can be doubled. Alternatively the input power to each amplifier can be reduced by half for a given output
`power, significantly reducing the distortion products created by higher input powers. Another benefit is that this scheme will
`dramatically reduce, by the baluns’ CMRR, the second order distortion outputs of the amplifier. The second order and all other
`even order distortion products will be identical in both amplifiers, while the fundamental will be out of phase. Therefore all
`even order products will be canceled in the output balun, while the odd order products pass through.
`
`It is this even product cancellation that can be used to
`dramatically reduce spurious products in balanced mixers like
`the double balanced mixer shown in Fig. 11. In this structure
`not only are the even order distortion products of both the
`RF and LO canceled out, but also the fundamental of the LO
`will be canceled out traveling to the RF and IF ports. Marki
`Microwave has been using this technique to design the world’s
`best mixers for many decades. Owing to our vast experience
`designing baluns for mixer applications, Marki Microwave
`can now offer discrete baluns to meet the most demanding requirements.
`
`LO
`
`IF
`
`RF
`
`Fig. 11 Push-Pull Amplifier (left) and
`Double Balanced Mixer (right)
`
`Marki offers high performance connectorized and surface mount baluns. Our connectorized baluns are typically used for
`interfacing high speed differential chips to unbalanced signals, either from test equipment or receivers, for testing purposes.
`These units allow single ended test equipment such as synthesizers, oscilloscopes, power meters, and network analyzers to
`interface with differential devices such as cables, differential amplifiers, receivers, and transmitters. This is very important for
`differential devices, since they will generally behave quite differently when excited with a differential signal vs. when they are
`excited with a single ended signal (which can be decomposed into both differential and common mode signals). In particular,
`a 2 port VNA can be used to measure differential devices with a matched set of baluns, but special care must be taken to de-
`embed the baluns if they do not have isolation5.
`
`Our broadband, high performance surface mount baluns are most frequently used as the interface between high speed digital
`converters and heterodyne transmission systems. In this circumstance designers are replacing what was previously the final IF
`transmission stage for these heterodyne converters. In this application the most important spec for the balun is the phase balance.
`An improvement from 12 degrees of phase balance to 3 degrees of phase balance can improve the even order dynamic range of
`the ADC by more than 10 dB6.
`
`Matching a wideband analog to digital converter (ADC) to a single ended source is a difficult challenge, especially when
`using super-Nyquist sampling (at frequencies above the fundamental Nyquist zone). A differential amplifier at the front end will
`add noise and degrade linearity, while a balun will provide voltage gain without adding noise (an ADC responds to voltage,
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`10
`
`Ex.1022 p.10
`
`

`
`which will be √2 higher or more depending on
`the impedance ratio at the differential outputs of a
`balun). The input impedance of an ADC is typically
`much higher than 50Ω, generally in the kΩ range.
`This means that a higher output impedance balun
`will generally match better to an ADC input than a
`1:1 balun.
`
`BALUN
`
`100 nF
`
`15Ω
`
`ADC
`
`150Ω
`
`150Ω
`
`15Ω
`
`100 nF
`
`~1kΩ
`
`100 nF
`
`~2pF
`
`An example ADC matching circuit is shown in Fig. 12.
`It involves AC coupling capacitors, parallel resistors, and inductance to match the capacitive, high impedance ADC load to the
`transmission line input. It also includes series resistors to limit any amount of charge injection coming from the ADC’s internal
`sampling structure back into the analog system. Due to the band-limiting nature of the ADC, it is often necessary to use a balun that
`is much wider band in a pure 50 Ω system than the required system bandwidth.
`
`Fig. 12 ADC Matching Circuit using a Balun7
`
`SUMMARY
`
`Because the term ‘balun’ encompasses a wide range of devices and applications, the information on them is scattered and confusing.
`In this application note we have clarified that baluns are differential power dividers. They can be built as transformers, capacitively
`and/or magnetically coupled transmission lines, hybrid couplers, or as a combination of a power divider and an inverter. Their
`most important characteristic is how well balanced they are in power, and how close to 180° out of phase their balanced ports are.
`Baluns can be used for many applications to transition between single ended and differential signals and to cancel common mode
`noise and signals. The future of baluns lies in further improving the balance, increasing the power handling, and reducing the size,
`complexity, and cost in these critical communications applications.
`
`FOOTNOTES
`
`1
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`2
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`3
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`4
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` N.E. Lindenblad, “Television transmitting antenna for Empire
`State Building,” RCA Rev., vol. 3, pp. 387-408, April 1939.
` Bockelman, D.E., Eisenstadt, W.R., ”Combined differential and
`common-mode scattering parameters: theory and simulation”,
`Microwave Theory and Techniques, vol. 43, no. 7, 1995, pp.
`1530-1539.
` Gustav Guanella (1909-1982) was a brilliant Swiss inventor
`and manager with over 200 patents. He developed the basic
`transmission line balun, and the one that bears his name, to
`match a high impedance power amplifier to a low impedance
`antenna. His sister married Albert Hoffman, inventor of LSD.
`Clyde Ruthroff was a scientist at Bell Labs from 1946-1977.
`In his paper, ‘Some Broad-Band Transformers’, he actually
`proposes 9 different transformers that became the basis of
`hundreds of variations. We show only one here.
` Most important is Jerry Sevick, who was drafted by the Chicago
`Bears, but instead spent his career working on antennas at Bell
`Labs. He also coined the term ‘unun’ to describe an unbalanced
`
`5
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`6
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`7
`
`to unbalanced impedance transformer. See Jerry Sevick,
`W2FMI, Understanding, Building, and Using Baluns and
`Ununs, CQ Communications, 2003.
` For more information see “The Problem with Back to Back
`Baluns”, Marki Microwave Tech Notes, http://www.
`markimicrowave.com/blog/2013/11/the-problem-with-back-to-
`back-baluns/
` For more information see “Why buy a high quality balun/
`transformer for an analog to digital converter (ADC)?”, Marki
`Microwave Tech Notes, http://www.markimicrowave.com/
`blog/2013/07/why-buy-a-high-quality-baluntransformer-for-an-
`analog-to-digital-converter-adc/
` See Rob Reeder, “Wideband A/D Converter Front-End
`Design Considerations: When to Use a Double Transformer
`Configuration”,
`http://www.analog.com/library/analogDialogue/
`archives/40-07/transformer.pdf
`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`11
`
`Ex.1022 p.11
`
`

`
`© 2014 Marki Microwave, Inc. | 215 Vineyard Court | Morgan Hill, CA 95037
`P 408.778.4200 | F 408.778.4300 | support@markimicrowave.com
`www.markimicrowave.com
`
`Ex.1022 p.12