Editor's Picks

What is Antenna Polarization, and Why Does it Matter?

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Antenna polarization, much like polarization with light, and nothing to do with arctic weather, and everything to do with the transmission and reception of electromagnetic radiation based on its orientation. With light polarization, a film or glass will appear darker, and block, light which is polarized in a certain direction, while allowing correctly polarized light to pass through. This is similar with an antenna, as the polarization of an antenna will determine how well EM radiation is transmitted or received by that antenna.

Polarization is based on the plane from which the electric field component of the EM radiation is oscillating. If the polarization of the EM wave is offset rotationally from the polarization of an antenna, only a portion of the EM wave will be captured by the antenna. Hence, for optimum efficiency of a communication link, the polarization of the transmitting antenna and receiving antenna should be the same, if the antenna are referenced to the same plane. There are also other physical phenomenon that contribute to the choices of polarization for certain applications.

There are three main types of polarization, with many other possibilities. Typically, RF antenna are either linearly or circularly polarized. Linear polarized antenna are generally either vertically polarized or horizontally polarized, while circularly polarized antenna are either left-hand or right-hand circularly polarized. There is a third common type of polarization, which is a complex combination of both linear and circular polarization, known as elliptical polarization.

Depending on the angle between the polarization vectors of a linearly polarized antenna and an EM wave, the maximum polarization losses of a linearly polarized system are at 45 degrees of each other. At 45 degrees of polarization vector offset, the maximum polarization losses would be 0.5, or 3 dB. For a circularly, and elliptically, polarized system, the calculations are more complex, and the maximum polarization losses can be up to 30 dB. This is why polarization can be used to isolate signals and antenna systems that could interfere with each other. Though there is polarization loss, antennas that are polarized in a different fashion will receive signals from EM waves with different polarization. Hence, there is a limit to the amount of isolation polarization provides.

The polarization of an antenna is most often chosen based on the application requirements. Certain applications benefit from different polarization schemes. For example, vertically polarized antenna may perform better with land-mobile applications, as vertically polarized EM waves tend to travel across the curvature better than horizontally polarized EM waves. Horizontally polarized schemes may perform better when applications that rely on the ionosphere, typically with long distance communication. Lastly, circular polarization is often used in satellite communications, as the circular polarization tends to perform better in mitigating fading caused from shifts in satellite orientation.

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Massive-MIMO in a Nutshell

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It is no surprise that current and upcoming mobile wireless standards and methods are leveraging multiple technologies to increase the throughput to user equipment (UE) and the availability of high-speed data services. Some of the major challenges to overcome for high speed wireless data to be offered are limited spectrum availability, interference, and the sheer number of devices that will require servicing. Other associated challenges include energy efficiency, network infrastructure, and UE capabilities. One of the most significant enhancement technologies for mobile wireless, currently available with 4G LTE-Advanced, and targeted for future 5G applications, is multi-input multi-output (MIMO) antenna and RF front-end technology.

What MIMO enables is the use of an antenna array and intelligent processing features to create additional capacity using the optimum propagation channels, known as spatial multiplexing. Hence, a MIMO equipped base-station and UE could operate on several spatially multiplexed channels, and extending the available throughput to that device. Typical MIMO systems in handheld UE is 2 by 2, or 2×2 MIMO, which means that both the base station and UE have an antenna array with two transmit and two receive antennas–effectively doubling capacity. Some WiFi routers and other wireless systems currently have 4×4, 8×8, and even 16×16 MIMO systems, with other asymmetric combinations also existing for certain applications.

Given the future predictions of tens, hundreds, and even thousands of devices requiring high throughput data services, the concept of designing base stations with many multiples more transmit and receive antennas than with current MIMO systems has evolved into Massive-MIMO or Massive Multiuser MIMO (MU-MIMO).

The goal of Massive-MIMO is to provide base stations with a high number of transmit and receive streams, along with other network capacity augmentation technologies and methods, to improve peak downlink throughput, substantially improve uplink performance, and enhance coverage, specifically in densely populated and urban environments. Other than the obvious increase in network capacity, other benefits of Massive-MIMO could be an increase in spectral efficiency–especially for sub-6 GHz applications–, a reduction in energy consumption and battery life extension for UE, and the potential for less-complex scalability than with prior mobile wireless technologies. Moreover, Massive-MIMO may be a solution to providing the huge number of machine-type devices part of the Internet of Things (IoT) and Industry 4.0 trends with connectivity services, efficiently. Lastly, Massive-MIMO may also be a solution for ultra-reliable communication, as several physical links can be established to ensure uninterrupted communications to critical systems in aircraft, infrastructure, vehicles, and more.

Massive-MIMO provides significant improvements to spectral efficiency in low-mobility and no-mobility applications, but is less effective in high-mobility applications. The loss in spectral efficiency as a product of mobility is due to the drop in channel coherence and lower pilot availability reducing the multiplexing gain of a Massive-MIMO system servicing to the mobile UE. Other challenges with Massive-MIMO will likely be the availability of low-cost RF hardware and installation of network infrastructure that could power Massive-MIMO base stations.

Massive-MIMO is also commonly confused with large MIMO links with tens and hundreds of transmit and receive channels and full-dimension MIMO concepts, though these technologies are implementations of MIMO, they don’t serve a large number of users simultaneously.

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The Skinny on Skin Depth and Skin Effect

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The skin effect and skin depth is a rarely discussed topic, considering as it is an essential phenomenon to be aware of when working with any conductor, or semiconductor, at RF frequencies, or really anything other than DC. Essentially, the skin effect is the term to describe the way current is distributed in a conductor as a function of frequency and material properties. It can be observed that the distribution of charges in a conductor tends to travel nearer to the surface of a conductor the higher the frequency of the signal. This phenomenon affects all conductors, be it single wire, coaxial cable, microstrip, or antennas conductors.

The skin effect contributes to the RF resistive losses of the conductors, but only on the conductors which are carrying the current of the propagating RF energy. For waveguide, coaxial cables/connectors, and antennas, this typically applies to the outer surface of the inner walls of the transmission line. For some microstrip and stripline structures, this could be somewhat confusing, as the inner surfaces touching the dielectric maybe the conductors carrying the current, not the outer plated surfaces. Typically, the RF losses are greater the smaller the skin depth, as the resistance at the surface of a conductor is greater. The distribution of the current through a conductor can be calculated if a few parameters are known. The distance that the majority of the charges travel within a conductor is known as the skin depth. As the skin depth is a product of frequency, resistivity, and magnetic permeability of a conductor, the RF losses of a material over frequency differ between conductor materials. For example, copper has a resistivity of 1.678 micro-ohms per centimeter and a relative permeability of 0.999991, gold has a resistivity of 2.24 micro-ohms per centimeter and a relative permeability of 1, and nickel has a resistivity of 6.84 micro-ohms per centimeter and a relative permeability of 600. The skin depth of copper, gold, and nickel at 1 GHz, is 2.06 um, 2.38 um, and 0.170 um. Hence, the RF losses for the nickel material would be the worst, while copper, and finally, gold would be far less.

Some interesting conclusion come from this phenomenon. Firstly, that a conductor’s magnetic permeability can greatly impact the RF losses of a material. Secondly, that for much higher frequencies, the majority or charges will only be traveling in a thin layer near the surface, and much thinner conductors can be used without sacrificing insertion loss performance at those frequencies.

However, just the frequency, relative permeability, and resistivity don’t tell the whole story of the RF losses of a conductor. Surface condition also greatly influences the losses, with very rough and uneven surfaces creating a much longer path, hence higher resistive losses, for the near-surface traveling current. Which is why surface roughness is a major concern in thin-film RF, and high precision millimeter-wave applications. Or, conversely any broadband application, as the insertion loss and attenuation along a conductor or transmission line will “worsen” as the frequency increases.

Check out our handy Skin Depth Calculator

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When and where are Flexible Waveguides Useful?

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Waveguide interconnect and assemblies are used in a wide variety of microwave and millimeter-wave applications. These applications include military, aerospace, Satcom, radar, microwave/millimeter-wave imaging, industrial heating/cooking, and more. In several of the applications and specific cases, a rigid waveguide assembly, or interconnect routing, must be done in a geometry which would require a waveguide structure that is too expensive, complex, or too rigid to meet successful design criteria.

Such a scenario could occur because the geometry requires very irregular bends, difficult to reliably make or too expensive to make with rigid waveguide. Another case is that it may be desirable to provide some mechanical isolation between assemblies or structures. For these reasons, flexible waveguide interconnect was invented and is used in a wide variety of application. Though helpful in many circumstances, flexible waveguide also has its limitations, and a designer must be mindful of the tradeoffs when finalizing a waveguide routing and a waveguide assembly.

How are Flexible Waveguide Different from Rigid Waveguide?

Unlike rigid waveguide, which are constructed of solid structures and welded/brazed metal, flexible waveguide are made using tightly interlocking sections of folded metal. Some flexible waveguide are augmented with soldered to seal the seams within the interlocking metal sections of a flexible waveguide. What these interlocking sections allow for, is some minor flexure at each joint. Hence, a longer piece of flexible waveguide can flex more than a similarly constructed shorter piece. The interlocking sections are also designed to maintain as strait a waveguide channel within the waveguide as possible.

There are variants of flexible waveguide that allow for flexion at the widest wall, some that allow flexing at the short wall, and some that allow for flexure at both walls. Other variants, called “twist” waveguide, allow for the waveguide to rotate along its length. There are also waveguide that are capable of combinations of these functions.

With rigid waveguide, a designer must either base their design on available waveguide sections, or commission a custom rigid waveguide part. Flexible waveguide, however, can be purchased in standard lengths and bent/flexed for mating. For greater structural performance, some flex waveguide are manufactured with a sturdy outer jacket, and the waveguide can be “pre-formed” to the desired shape.

When/Where are Flexible Waveguide Used?

Flexible waveguide are used in applications where rigid waveguide would be prohibitively complex, expensive, or exceeds a necessary production schedule. Occasionally, redesigns are necessary, and rigid waveguide sections are replaced with readily available flexible waveguide to account for varying design changes. Prototypes are often assembled with flexible waveguide as proof-of-concepts prior to finalized design geometries.

Certain applications actually require flexible sections, as a flexible waveguide will not transfer as much mechanical energy along its structure, than would rigid waveguide. For example, if there is a junction whose relative position changes dramatically under varying environmental conditions– such as temperature variations, humidity, or under load–then a flexible waveguide section could be used to provide the extra “slack” between the shifting joints. Also, some flexible waveguide could also provide some shock and vibration isolation, though such use may also reduce the lifespan of the flexible waveguide.

When is it a bad idea to use Flexible Waveguide?

Flexible waveguide are typically less rigid and less physically robust than rigid waveguide structures. Where some rigid waveguide can be used to also provide mechanical support, a flexible waveguide could be damaged and its electrical performance could be compromised if put under any significant mechanical strain or load. Excessive vibration and shock will also lead to mechanical and electrical failure of a flexible waveguide. Flexible waveguide are also not typically designed for repeated flexure, which could wear down the joints, damage the jacketing, and lead to early failure. The thinner metal, contact resistance between the sections, and non-ideal inner waveguide surface of flexible waveguide also lead to reduced electrical performance compared to some rigid waveguide. Hence, flexible waveguide typically have slightly poorer transmission characteristics and lower power handling than rigid waveguide. Flexible waveguide may also not have the same temperature range of operation as rigid waveguide, as the jacketing material and joint solder may be a critical component of the flexible waveguide structures performance, and these materials likely won’t have the same temperature performance range as rigid metal.

If a flexible waveguide isn’t jacketed or otherwise sealed, humidity and other environmental contaminants can enter the waveguide structure through tiny gaps in the joints. Though purging and desiccants can be used to minimize the amount of humidity inside a flexible waveguide, high humidity or environmentally contaminated environments can compromise the performance of flexible waveguide over time.

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What is a Magic-Tee, and Why Are They So Magical?

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A Magic-Tee is a 4-port waveguide structure similar to a rat-race coupler fabricated with striplines or microstrip waveguides. The Magic-Tee, or EH plane Tee, is able to perform an apparently “magical” function, which is to allow for energy going into the E and H plane ports to be divided between the two colinear ports while isolating the E and H plane ports and the two colinear ports from each other. This microwave component was originally developed during WWII, and was first published by W. A. Tyrell in an IRE paper from 1947. It is thought that both L. Kyhi and Bob Dicke independently discovered and created Magic-Tees near the same time frame.

Different than other power dividers or couplers, the Magic-Tee performs different functions at the E plane and H plane ports. Where the H plane port, known as the sum port, will present the same phase at both colinear ports, the E plane port presents a 180 degree phase referenced to the different colinear ports. This function of a Magic-Tee is also symmetric, and will split the energy going into a colinear port to both the E and H plane ports. Hence, signals going into the colinear ports will be simultaneously divided, summed at the H plane port, and differenced at the E plan port.

The ideal S-Parameters of a Magic-Tee are as follows:


Source: Wikipedia.org

However, these functions are only possible under ideal or theoretical cases, and a real Magic-Tee has practical limitations based on its matching, balance, and isolation. The tuning and impedance matching mechanisms, necessary for a Magic-Tee to function, also impose a frequency range limit, insertion loss, limited E-H port isolation, and limited isolation between the collinear ports. Matching is needed for both the E plane and H plane ports, which also influences the match at the colinear ports. Assembly quality also affects a Magic-Tees performance, as the E plane and H plane ports need to be completely orthogonal to each other to maintain a high level of isolation.

A Magic-Tee can be used for several applications, such as an impedance measuring tool, duplexer, and even as a mixer. A Magic-Tee can be used to measure impedance by connecting a null detector to the E plane port, a microwave source to the H plane port, and by balancing the impedance bridge created with the colinear ports. To use a Magic-Tee as a duplexer, the transmitter and receiver can be connected to different colinear ports, while the antenna is connected to the E plane port and the H plane port is terminated in a matched load. If a Magic-Tee’s E plan port is connected to an antenna, and its H plane port is connected to a local oscillator, a mixer can be created with a terminated collinear port with a mixer circuit on the remaining colinear port.

For a deeper dive into the mathematics of a Magic-Tee, see this source


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Why Are Frequency Band Designations So Confusing?

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A quick google search of “frequency bands” will often provide links to radio spectrum, cellular frequencies, spectrum bands, etc. If the searcher is so bold as to click on Wikipedia’s “Radio Spectrum” page, they will likely encounter tables with swaths of the electromagnetic spectrum designated by the International Telecommunication Union (ITU), IEEE, EU/NATO/US ECM, and waveguide bands. These designations, of course, are all different from the US military, other military, and other country frequency band designations, which are also different from other commonly used designations. When encountering all of these frequency band designations, some of which are more than a little confusing to those who didn’t create the designations in the first place, it is not uncommon to wonder why the nomenclature is so confusing.

A simple answer to the question, is that as radio technology and electromagnetism emerged, there weren’t simple terms available to define the phenomenon being observed and used. Hence the naming of devices and frequencies were often up to the scientist, engineer, or business person that pioneered their use. For example, the application of radio became a designation for the entire radio frequency industry, the engineers that work with radio frequencies, and a frequency band designation loosely constrained between a few hundred kilohertz and a few hundred megahertz. Another example, microwaves, is a little more confusing, as its exact origin is unknown, but could possibly have been used by a news reporter to describe waves that were much smaller than radio waves.

The military radar and military frequency band designations often came about in response to new technologies developed for, or encountered, on the battlefield. For example, L-band between 1 gigahertz and 2 gigahertz was originally used for long range air traffic control and surveillance radar, so the “L” is for “long”. X-band was labeled with the “x”, as it was a secret frequency band used during WWII. K-band is derived from the German word for “short”, “kurz”, and the surrounding bands, Ku-band and Ka-band, stand for “under” and “above”, respectively.

Though the origins of frequency band designations are confusing enough, several organizations, such as the IEEE, ITU, NATO, and etc. have also developed their own standardized approach for spectrum designation. This may sound orderly and a good approach, which it is, but it also has created another side effect. Many application, engineers, and media outlets still used the older designations and common names for frequency bands, so there now is a proliferation of terms to define frequency bands. Depending on the application, many different frequency band designations may be used. For some applications, there is an unspoken agreement to use a particular convention and exclude all others, such as military radar engineers having unwavering dedication to the legacy radar band designations.

For a novice, or someone who hasn’t been introduced to the general confusion of frequency bands, it can be a bit much to become familiar with. This is why frequency charts and tables of the electromagnetic spectrum and frequency bands often decorate an engineer’s desk or lab space.

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Distinctions and Types of Horn Antennas

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There are many types of antennas to serve the various RF, microwave, and millimeter-wave applications. A very common antenna for microwave and millimeter-wave systems are horn antennas, which have been in use since the early 1900s. Horn antennas are essentially rectangular or circular waveguides that reduce the size of a waveguide or coaxial port at one end. Horn antennas exhibit a low voltage standing wave ratio (VSWR), a very wide bandwidth operation (10:1 and even 20:1 horn antennas are possible), are relatively simple and inexpensive to produce, and provide moderate directivity. The gain and SWR of horn antennas is also typically flat over the bandwidth, which is why they are well suited to testing other antennas performance.

Horn antennas are commonly used as directive antennas for radar, microwave radiometers, as feed horns for large antenna structures such as parabolic antennas, and as calibration and testing scenarios for testing other antennas. Many new 5G testing platforms and proof of concept prototypes also use horn antennas for their simplicity, and horn antennas are commonly used in channel sounders and other field testing apparatus. There are several different types and structures of horn antennas to serve these various applications.

Pyramidal Horn Antenna

This type of horn antenna is a common variety that looks like a rectangular, or square, pyramidal structure terminated in a waveguide port, typically rectangular waveguide.

Sectoral Horn Antenna

These types of horn antennas are identified by having one side of the horn antenna structure aligned with a wall of the terminating waveguide port, while the other sides of the horn antenna are flared out. Depending on the orientation of the flared and flat sides of a sectoral horn antenna, the antenna could be an E-plane or H-plan horn antenna.

Conical Horn Antenna

Instead of a rectangular pyramid structure, such as with a pyramidal horn antenna, a conical horn antenna is often a circular or elliptical cone terminated in a circular or elliptical waveguide. Though some conical horn antennas are terminated in rectangular waveguide.

Scalar Horn Antenna or Exponential Horn Antenna

Unlike a typical pyramidal, sectoral, or conical horn antenna, an exponential horn antenna variant of these types has an exponentially tapered side that creates a curved surface from the opening of the antenna to the waveguide termination. This method of construction minimizes the amount of internal reflections and allows for a consistent impedance and electrical performance over a very wide bandwidth.

Corrugated Horn Antenna

A corrugated horn antenna has sidewalls of the antenna which are grooved or slotted on the inside surface of the antenna horn, and are transverse to the axis of the antenna. These design features, which are electrically small compared to the wavelength of operation, enable very low side lobes and cross-polarization levels over the bandwidth. These types of antennas are often used in satellite and radio telescope applications.

Gain Horn Antenna

Gain horn antennas are horn antennas with a high and consistent gain over a broad bandwidth. These horn antennas are often used in the testing of other antennas, for radar, and for satellite and space applications.

Feed Horn Antenna

A feed horn antenna is simply a horn antenna which is used to transmit and receive signals from the RF and microwave electronics to the parabolic reflector commonly used for satellite dishes and radio telescopes.

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What are RF Isolators and RF Circulators?

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RF circulators are three port devices designed to provide isolation between transmit and receive signals for radar, actively electronically steered antenna (AESA) arrays, satellite communications, and telecommunications applications. An isolator is a modified circulator with one port terminated with a matched impedance. Circulators and isolators are typically used to protect sensitive receiver circuitry from high powered transmitter outputs, by separating the transmitter’s signals from the received signals at the input of the antenna. For radar and antenna array applications, this is especially critical, as the number of antennas is doubled if full-duplex operation of an antenna isn’t possible.

Most RF circulators are coaxial or waveguide packaged devices based on passive ferromagnetic technologies. These types of circulators typically provide very high isolation in their frequency range of operation. The magnetic field generated within the ferromagnetic circulator during operation “forces” the RF signals in each port to follow the rotation of the magnetic field and prevents signal leakage in the opposite direction. Compared to cavity duplexers used in base stations and in-building telecommunications installations, RF circulators are more compact, often lower cost, and provide enhanced isolation.

Circulators and isolators are used in applications from hundreds of megahertz to tens of gigahertz, and are mostly specified for operation in radar and communication bands. The frequency bandwidth of operation of a circulator is dictated by the geometry of the magnetic material used, the design of the transmission line, and the impedance matching network of the circulator. The type of ferrite and manufacturing techniques used will also influence the performance of a circulator.

Isolation, insertion loss, and bandwidth are the most critical performance parameters used to describe circulators and isolators. Many other factors may be important for a particular application, such as power handling, VSWR, interconnect technology, size, temperature range, and other environmental considerations. The power handling of a circulator is typically driven by the type of ferrite material, housing, and interconnect technology used. An isolator power rating may also be limited by the type of termination used in the 3rd port of the device.

An isolator can be made by terminating a circulator. However, manufactured isolators may have a much better matched port which enables enhanced bandwidth and isolation compared with a circulator with an off-the-shelf termination.

Though coaxial circulators and isolators are the most common, there are also drop-in and surface-mount technology (SMT) circulators and isolators. The size and interconnect with compact drop-in or SMT circulators and isolators limits the bandwidth, power, isolation, and insertion loss capabilities of these devices. Lastly, there are various techniques of creating integrated circuit compatible circulator and isolator devices being researched. Some of these techniques use switching and unique material properties to create the circulation effect.

To learn more of Pasternack’s Circulator and Isolators, visit: https://www.pasternack.com/nsearch.aspx?Category=Circulators^Isolators&sort=y&view_type=grid.

If you have specific questions about RF Circulators and Isolators, you can reach Pasternack’s RF Experts here: USA & Canada (866) 727-8376 International +1 (949) 261-1920.

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An Efficient Linear Interpolation Scheme

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This blog presents a computationally-efficient linear interpolation trick that requires at most one multiply per output sample.

Background: Linear Interpolation

Looking at Figure 1(a) let's assume we have two points, [x(0),y(0)] and [x(1),y(1)], and we want to compute the value y, on the line joining those two points, associated with the value x. 

What are Signal Power Taps, or Tappers, and how are They Different than Directional Couplers?

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Signal power tappers, signal taps, power tappers, or just taps, are passive RF devices designed to draw a small portion of power from the main transmission line without causing substantial loss. Signal power tappers are similar to directional couplers, but a signal power tapper is typically a 3-port devices, but come in many additional tap port configurations, based on capacitive coupling instead of direct coupling. Multi-port signal tappers may be equal splits, or vary depending on the need. Like directional couplers, the taps ports are often set to the impedance of the main transmission line and must be correctly terminated to ensure proper performance.

Sometimes, taps are confused with power splitters/combiners, but they are not suited for power splitting or combining applications. This is due to the lack of return port isolation and limited coupling to the tap ports. Generally, power splitters are designed to evenly split power to several ports, with high isolation between each of the split ports.

Signal tappers are often used with DAS applications for their low-cost broadband behavior, high power handling, low PIM performance, and are readily available. Moreover, taps are also used with base station, 4G LTE, AWS, PCS, PMR, UMTS, TETRA, Wi-Fi, WiMAX, and other wireless infrastructure applications. Being passive components, signal tappers can be enclosed in a rugged housing and be designed to meet many indoor and outdoor environmental conditions For example, IP67 and military standards.

Compared to directional couplers, signal tappers generally have much wider bandwidths and offer greater performance than directional couplers, which become more complex and expensive at higher bandwidth and high performance regimes, comparatively. Moreover, signal tappers are bidirectional by nature, and have no directivity. This is because signal tappers leverage capacitive coupling. The tapper will not provide substantial isolation from the return path. The VSWR for directional couplers, however, is generally much higher than with a signal tapper at the coupled port. The input and output VSWR for signal tappers is generally excellent.

Signal tappers typically present slightly greater loss than directional couplers. Depending on the electronics connected to the coupled port of a signal tap, considerations of the maximum power output of the coupled port of the tap must be accounted for to prevent damage of sensitive electronics. Some RF taps are designed specifically for low-PIM operation to prevent distortion common to RF interconnect for wireless and telecom applications.

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How Noisy Is your Noise Source: Noise Sources 101

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Random Electrical and thermal noise are natural phenomenon that is produced by every devices within a RF and microwave system. Depending on the severity of the noise, the introduction of a noise source in an RF system could reduce the signal integrity, bit-error rate (BER), overload a receiver, reduce the dynamic range, and otherwise degrade system performance. So, why would an RF system designer want to introduce noise into a system? For the purpose of designing a RF system resilient to noise, an RF engineer may use a calibrated RF noise source to better understand how sound propagates through an RF system and to provide a baseline understanding of the noise performance of RF devices.

With known level of noise injected into a system, the sensitivity of the RF system to noise can be readily observed–which is a measurement known as noise figure. The determination of a component, device, or system noise figure enables comparison of RF performance. This process is often used with amplifiers, filters, switches, mixers, and other RF components and devices to determine the noise performance of each piece of an RF system. This way, the overall RF system noise performance can be predicted. It is important to note, that noise performance is often a function of frequency, power, temperature, and other phenomenon.

For these reasons, calibrated noise sources with the power level, broadband frequency performance, and interconnect capability are what distinguishes various noise sources for a particular application. The most common noise sources are inline-coaxial noise sources, but there are also bench top variable noise sources that allow for noise power level adjustment. Noise source modules that only provide an output noise are also available.

The performance of a noise source is measured as the excess noise ratio, which is the normalized measure of how much noise a source adds above thermal noise. The ENR of a noise source will likely vary over frequency, and this should be considered when evaluating a noise source. This is sometimes referred to as noise flatness, or simply flatness.

The quality of a noise source is also measured by its noise output temperature variation and noise output variation. Other factors to consider are the VSWR of an inline noise source, as this will also impact the signal passing through the device, as well as the impedance. As noise sources require external power, a bias voltage and input current may be carried along the coaxial transmission line, or powered through external connections.

Modern applications of noise sources also include the use of a traditional white Gaussian noise source as a jitter source. Such a source would be used to evaluate the performance of digitally modulated waveforms in high-speed communications application and bit-error rate performance. The compact nature of inline or terminal noise source modules enable them to be used in automated test equipment systems, or even built-in test equipment and diagnostic systems.

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What’s the Difference Between a Diplexer and a Duplexer?

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RF Diplexers and Duplexers are very common RF components in transmitters, receiver, and transceiver circuits for communications, radar, and other sensing applications. As the terms are similar, they are sometimes mistakenly used interchangeably, which isn’t appropriate. Diplexers and Duplexers perform very different functions, though they are located in similar areas in a circuit block diagram. This blog will explain the basic functions of Diplexers and Duplexers, and detail the differences in operation.


A diplexer is a 3-port RF device, which enables the use of two signal paths on the same antenna or transmission line. This is done by frequency division using filters, either high-pass, low-pass, or band-pass filters. Hence, signals at two different frequencies could be sent and received from the same antenna. For a diplexer to function well, the quality and attenuation of the filters must scale with how close the signals, what power levels they operate at, and what nonlinearities are expected.

Diplexers are commonly used in telecommunications, where multiple modulation methods and carriers may operate on the same antenna. For example, cellular base stations may need to transmit and receive CDMA, LTE, or GSM signals on the same antenna, as a cell site may have limited availability off tower space for additional antennas.


A duplexer is also a 3-port RF device, and its purpose is to separate transmit and receive signals from an antenna to two different signal paths based on direction. These transmit and receive signals may be operating at the same frequency, and hence a duplexer enables true two-way communication from a single antenna. For example, a duplexer may be used in a radar system were the high power transmitter signals need to be isolated from the sensitive receiver circuitry, but operate on the same antenna.

Either switched systems or magnetic circulators are used to create the isolation between the incoming and outgoing signals within a duplexer. Duplexers are limited by how well they can isolate the receive path from the transmit path, as well as their bandwidth of operation. With radar transmit/receive (TR) modules, the transmit and receive frequencies are typically very close, and can only reasonably be separated through duplexing. Duplexer and circulator is sometimes used interchangeably, which is a common simplification, though not always accurate.

Diplexer versus Duplexer

 Simply put, a duplexer separates a transmit and receive path based on signal direction and can be used for same frequency signals, and a diplexers separates signals based on frequency with filters. Their operation is not interchangeable, and a diplexer could not replace a duplexer in common circuits. Duplexers may become more common in telecommunications applications and future 5G technologies may benefit from having transmit and receive signals at the same frequency for greater spectrum utilization.

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Spinning Into Control with Coaxial Rotary Joints

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Coaxial rotary joints are necessary inline components when a static RF system must be connected to a rotating RF line. These rotary joints are used in military, aerospace, and commercial applications, most often with rotary antennas and radar. Specifically, coaxial rotary joints are used in Air Traffic Control, image transmissions, medical/industrial, telecommunications control, and ground-, ship-, or air-based radar. As all of these applications require high performance interconnect, reliability, and long-life operation, the same standards are applied to coaxial rotary joints.

Like all coaxial systems, the coaxial connectors used at the ports of the rotary joints limit the maximum frequency of operation. Hence, rotary joints to 18 GHz may use 2.92mm female coaxial connectors, where coaxial rotary joints to 6 GHz are likely to employ standard SMA connectors. Some specialized rotary joints may also come with PCB pin connectors, which could be used as end-launch microstrip, or stripline, waveguide to coaxial rotary converters. Other types of rotary joints include 90 degree coaxial rotary joints. Some waveguide rotary joints also incorporate a coaxial intermediate stage within the device, though the end-port may be waveguide or coaxial.

Unlike other coaxial converters or inline interconnect, coaxial rotary joints have additional mechanical considerations. These include an average rotational speed limit, in revolutions per minute (RPM). These limits are typically dictated by the bearings and mechanical assembly of the rotary system. Exceeding the average maximum RPM limit for any significant amount of time would likely lead to early failure of the rotary capability, and likely reduce RF performance as well.

Because of the additional rotary system to the inline coaxial interconnect, there may be very critical input power and peak power specifications for a coaxial rotary joint. If the power limit is succeeded, or if the temperature of operation is exceeded, then the rotary performance of the joint may be diminished, and the joint may be more easily damaged. Typically a coaxial rotary joint will have slightly lower VSWR performance than typical inline interconnect.

Lastly, the rotary nature of a coaxial rotary joint will also introduce RF performance variation based on the rotation parameters. Typically, there is a max VSWR, insertion loss, and phase variance provided with the datasheet, which is specified at the recommended RPM. Deviated from the recommended RPM could influence this variance, as would any excessive vibration or mechanical force being applied to the joint. Moreover, the variance to VSWR, insertion loss, and phase are also likely a function of frequency, and may need to be taken into consideration when making a selection.

The post Spinning Into Control with Coaxial Rotary Joints appeared first on Pasternack Blog.

When Do You Need a Bias Tee or DC Block?

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Bias Tees and DC blocks are both low frequency filters designed to pass certain wanted signals and power rails while blocking other signals and limiting the performance impact on RF/microwave circuits. Bias Tees are essentially diplexers with an extremely low crossover frequency, and DC blocks are high pass filters with cutoff frequency down to audio frequencies and DC.

DC blocks are used for enhancing signal-to-noise ratio and dynamic range on some very low frequency or wideband systems, as well as block DC and audio frequencies from testing that may require isolation from such low frequency components. DC blocks are also used for signal source modulation leakage suppression, and ground loop elimination.

Bias Tees are used to allow for DC currents and/or voltages to pass to RF devices while blocking RF/microwave signals on the same line. For example, a Bias Tee may be used to enable a power supply to a transistor or amplifier circuit, which requires a DC signal and would be disturbed by the RF content on the signal and power line. There are also pulsed bias tees, which allow for minimum distortion on current, or voltage, pulses for amplifiers and devices which require intermittent signals for biasing or power.

Bias Tees and DC blocks are both very commonly used in many RF/microwave circuits which require the conveyance of DC signals along the same coaxial or microstrip signal path as RF/microwave signals. Bias tees and DC blocks may even be used together at a node where the DC power or bias voltage/current is needed, but would be disruptive if it passed further down the RF transmission line.

Bias tees are used anywhere from cell phone amplifiers to test and measurement equipment. An example of this is with powered probes which have a power hookup at the same port as the RF signal port.

DC blocks are typically only used where powered RF transmission lines, or “hot” conductors, are used. However, DC blocks may also be used to separate a circuit from a ground place and DC and audio signals, to prevent current passing or voltage developing from that circuit node to ground. An example of this is in the instance where a voltage is injected into the source of a shunt FET, which is also grounded to the grounded housing or fixture of the assembly.

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Tips on Selecting a Frequency Divider or Frequency Multiplier

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Frequency translation is the backbone of superheterodyne communications and radar circuits, as well as many other useful RF/microwave devices. There is often confusion surrounding these nonlinear devices, specifically the role of mixers, multipliers, and dividers, and how to select the best device for an application. This post aims to briefly describe the similarities and differences, and some illumination on device selection criteria.

What is a Mixer?

A mixer is a nonlinear, three-terminal device, often comprised of a diode or transistor operating in the nonlinear region. From two input signals, a mixer will produce the sum and difference of the two signals in the output. This can be used in upconversion, downconversion, as an IQ mixer, or with different performance parameters based on configuration. Mixers are commonly used in demodulation circuitry, upconverters, and downconverters, providing frequency translation before transmission, or after reception.

What is a Multiplier?

A multiplier is a nonlinear device, which generates harmonics of higher frequencies based on an input signal’s behavior. For example, a doubler is a frequency multiplier that creates a strong 2nd harmonic. Inevitably, the input signal, higher harmonics, and noise/interference will also leak and be mixed in with the output signal. Multipliers are often used in demodulation circuitry and to raise the frequency of an oscillator or signal generator source.

What is a Divider?

A frequency divider is similar to a frequency multiplier, with the exception that the output frequencies are a submultiple of the input signal frequencies. The same considerations apply to dividers, as they do to multipliers.

What should I know when choosing Multipliers or Dividers?

As part of the many use cases of multipliers and dividers, isolation, harmonic suppression, and phase noise characteristics are important factors to consider when selecting a multiplier or divider. Isolation describes how well the signals input into the multiplier or divider are prevented from leaking into the output, while harmonic suppression describes how well the multiplier or divider design prevents harmonics of the input signal from appearing at the output. Both factors are critical, as they directly impact the usability of a multiplier or divider device. Additive phase noise and noise performance of a multiplier or divider are important for signal generation and modulation circuitry that is noise, phase noise, or interference limited, as these parameters describe with the device adds to an input signal when generating the output signal.

The amount of signal power needed to drive the multiplier or divider, or necessary signal input power for proper operation, may also be a factor to consider, as some multipliers and dividers require substantial input powers. Many test and measurement grade precision signal generators, and arbitrary waveform generators, may not produce the necessary signal strength to drive a multiplier or divider. Hence, an amplifier, which as its own distortion, noise, and phase noise, may be required.

The post Tips on Selecting a Frequency Divider or Frequency Multiplier appeared first on Pasternack Blog.

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