Editor's Picks

Bits on Baluns – Part 2

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Balun Performance Metrics

There are many different types of baluns and the type of balun used in microwave RF designs depends on the bandwidth required, the operating frequency, and the physical architecture of the design. Most baluns usually contain two or more insulated copper wires twisted together and wound around or inside a core, magnetic or non-magnetic. The following metrics are key in considering balun design, application, and performance.

Key specifications in determining the type of balun for a specific application include:

• > Frequency coverage
• > Phase Balance
• > Amplitude Balance
• > Common Mode Rejection Ratio
• > Impedance Ratio/Turns Ratio
• > Insertion and Return Loss
• > Balanced Port Isolation
• > DC/Ground Isolation
• > Group Delay Flatness

Phase Balance

An important performance criterion based on how close the balanced outputs are to having equal power and 180° phase, or balance, measured by how closely the inverted output is to 180° out of phase with the non-inverted output. The phase angle deviation from 180° is phase unbalance.

Amplitude Balance

This metric is determined by construction and line matching and is usually specified in dB. Amplitude balance indicates the match between output power magnitude, and the difference of these two magnitudes in dB is called amplitude unbalance.  Generally, 0.1 dB improvement in amplitude balance will improve the common-mode rejection ratio (CMRR) by the same amount as a 1° improvement in phase balance.

Common Mode Rejection Ratio (CMRR)

When two identical signals with identical phase are injected into the balanced ports of the balun, they will be either reflected or absorbed. CMRR, specified in dB, is the amount of attenuation this signal will experience from the balanced to unbalanced port. The vectorial addition of the two signals determines the CMRR which is dependent on the amplitude and phase balance of the balun.

Impedance Ratio/Turns Ratio:

The ratio of the unbalanced impedance to the balanced impedance usually stated as 1:n. This differential impedance is between the balanced signal lines and is twice the impedance between the signal lines and ground. Turn ratio in a flux coupled balun transformer, is the ratio of primary windings to secondary windings; the square of the turns ratio with a 1:2 turn ratio gives a 1:4 impedance ratio. Flux coupled transformers can be used to design high impedance ratio baluns.

Insertion and Return Loss

The lower the differential  insertion loss and higher the common mode return loss means more of the inserted signal power passes through the balun, and hence improved dynamic range, and less distortion of 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 -∞.

Balanced Port Isolation

The insertion loss from one balanced port to the other as specified in dB. Most baluns do not offer high isolation because the even mode is reflected instead of being properly terminated with a resistive load. An exception is the 180° hybrid circuit where the even mode is output to a port that can be resistively terminated.

The post Bits on Baluns – Part 2 appeared first on Pasternack Blog.

Bits on Baluns

Pasternack Blog -

Part 1: What is a balun, what does a balun do, and when is a balun needed?

What is a balun?

A balun (mashup of balanced and unbalanced) is a three-port device, or a type of broadband RF transmission line transformer, with a matched input and differential output that is used to connect balanced transmission line circuits to unbalanced ones. A balun’s function is to make systems of different impedance or differential/single-ended compatible and are found in modern communication systems, including cell phone and data transmission networks.

What does a balun do?

Baluns have three basic functions:

1. Unbalanced to balanced conversion of current or voltage

2. Rejection of common mode currents with some configurations

3. Impedance transformation with some configurations (impedance ratios other than 1:1).

There are many types of baluns and include devices that transform impedances and to connect lines of differing impedance. Transformer baluns help with impedance matching, DC isolation, and in matching balanced ports to a single ended port. Common-mode chokes are also used as baluns and work by eliminating common mode signals. Baluns are used in push-pull amplifiers, wide-band antennas, balanced mixers, balanced frequency multipliers and modulators, phase shifters or whenever a circuit design requires signals on two lines that are equal in magnitude and 180 degrees out of phase.

When do you need a balun?

Baluns are most often used to interface an unbalanced signal to a balanced transmission line for long distance communications. On balanced transmission lines, differential signaling is more immune to noise and crosstalk, can use lower voltages, and is more cost effective than single-ended signaling on coaxial cables. Thus, baluns are used to interface local video, audio, and digital signals to long distance transmission lines. Baluns are present in:

– Telephone networks, wireless network modem/routers.

Balun Operation

Ideal S-parameters of a balun:

S12 = – S13 = S21 = – S31

S11 = -∞

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.

One of the two conductors is clearly grounded.

For example, conductors having equal and opposite potential constitute a balanced line. Microstrip and coaxial cables use conductors of different dimensions and these are said to be unbalanced. Baluns are designed to solve problems caused by this imbalance in that a balun will transition between a balanced or differential transmission line where opposite currents both travel in transmission lines and an unbalanced, or single ended transmission line, where the return current travels through the ground.

In a coaxial cable, the currents on the inner conductor and the inside of the shield are equal and opposite because the fields from the two currents are confined to space in between. Skin effect allows a different current to flow on the outside of the shield which, if significant, causes the feedline to act like an antenna, radiating a field that is proportional to this current. Since it is physically symmetrical and the currents flowing through the conductors are equal and opposite, the radiation from the line is minimal. However, several factors may cause the currents in the two conductors to be imbalanced, that is, other than equal and opposite and, if this is the case, the balanced feed line will radiate like a coaxial cable that has current on the outside of the shield. This imbalance can cause pattern distortion, interference, and loss.

Key specifications in determining the type of balun for a specific application include:

• Frequency coverage
• Phase Balance
• Amplitude Balance
• Common Mode Rejection Ratio
• Impedance Ratio/Turns Ratio
• Insertion and Return Loss
• Balanced Port Isolation
• DC/Ground Isolation
• Group Delay Flatness

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More Details on Zero Bias Schottky Diode Detectors

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The Schottky diode, named after German physicist Walter H. Schottky, is a semiconductor diode formed by the junction of a semiconductor with a metal. It has a low forward voltage drop and a very fast switching action.  Compared with the point contact diode used in similar applications, the zero bias Schottky diode has a wider dynamic range, increased thermal stability, and more accurate square law response. Zero bias Schottky diodes are used in precision testing equipment, transmitter, power, signal monitoring, missile guidance systems, and as microwave power detectors.

The zero bias Schottky diode detector is a type of RF power detector that does not need a bias voltage to operate and is widely used in RFID and other applications where no primary (DC) power is available in the standby or listen mode. Thus, power efficient or passive operation systems can use these detectors and forgo the large energy storage systems, DC bias, or low-power receiver circuitry. In this way, the zero bias Schottky diode detector is ideal for RFID tag applications where it can be combined with a simple antenna to form a receiver and, although it lacks the sensitivity of the superheterodyne receiver, it offers the advantages of reduced cost and zero power consumption. Although seemingly cost-effective, the performance of the zero bias Schottky diode detector is dependent upon its saturation current, frequency, temperature, DC bias, and ideality factor which, at both low and high temperature extremes, can lead to degradation in performance.

The zero bias Schottky diode detector has an established use in the detection of power at mm- and submm-wavelengths allowing for effective detection and mixing of electromagnetic radiation in the range through microwave to terahertz. These diode detectors can operate at ambient or cryogenic temperatures and have much faster response time when compared with room temperature detectors, such as Golay cells, pyroelectric detectors, or bolometers. When the diodes are optimized to have a low forward turn-on voltage, these detectors can achieve excellent frequency response and bandwidth, even with zero-bias.

Although the zero bias Schottky diode is less sensitive than alternative superconducting detectors, they generally do not require cooling and that makes them the devices of choice for applications where sensitivity is less of a priority. In the emerging field of terahertz technology, there is a need for cost-effective detectors for laboratory use as well as for serial compact and midsize instruments. Modern zero bias Schottky diode detectors are designed for use in power measurements, analyzing radar performance, leveling pulsed signal sources, AM noise measurements, microwave system monitoring, and in ultra-broadband and mm-Wave applications.

Commonly packaged in either inline coaxial barrel connectors, or waveguide-to-coaxial packages for millimeter-wave applications, zero bias Schottky diode detectors are typically compact and less expensive than other RF detector devices. Their simple construction also lends these devices to being relatively rugged and stable over a wide range of temperatures.

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Exploring Types of RF Microwave Detectors

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A radio frequency (RF) microwave detector, also known as RF power detector or RF responding detector, is a two-terminal device used to detect, and in some way, measure or convert an RF signal. As the receiving element, an RF detector is used in converting amplitude-modulated microwave signals to baseband (or video) signals in either a wireless or wired transmission. In RF circuits and systems, RF microwave detectors can detect the transmit power level of the RF signal in a particular frequency range. Another application of RF microwave detectors is to measure transmitter output power; as knowing the RF output power is crucial in not exceeding certain maximum transmitter power levels according to Federal Communications Commission regulations, and other international regulations.

The two main categories of detectors are peak and Root Mean Square (RMS). Peak detectors provide information on the peak power whereas RMS detectors provide information on the average power.

Peak detectors – also known as envelope detectors, capture the extreme of the voltage signal at its input. A positive peak detector captures the most positive point of the input signal and a negative peak detector captures the most negative point of the input signal. The output of the peak detector circuit tracks the input voltage until the extreme point is reached and holds that value as the input decreases. Ideally, the peak detector performs this function regardless of the speed of the input signal but is limited by the bandwidth of the input signal. A peak detector uses an amplifier, diode, and capacitor to capture and hold the peak value of the input RF signal. The signal charges the capacitor through the amplifier, and the diode prevents discharge of the capacitor. This is useful when the peak or maximum RF signal value is needed but it is not useful for determining average or any other sort of non-peak value.

RMS detectors – thermal detectors (power detectors) and square law detectors.

Thermal detectors like bolometers (e.g. thermistor or thermocoupler) convert the electrical power of the RF signal into thermal energy using a resistive component and then measure the temperature variation with respect to the ambient temperature. Advantages of this method are a very wide bandwidth and a good accuracy between measured power and real power. Examples include: Thermocoupler, a pair of dissimilar metal (Sb-Bi) wires joined at one end (sensing end) and terminated at the other end (reference end); Barretter: a short length of platinum or tungsten wire with a positive temperature coefficient of resistance; Crystal detector which uses the diode square-law to convert input microwave power to detector output voltage; and Schottky barrier or GaAs barrier diode which has high sensitivity noise equivalent power and the lowest detectable microwave signal power.

Square law detectors use the characteristics of semiconductors components (diodes or transistors) to convert a voltage into a signal proportional to the RF power which is typically low-pass filtered to realize the average operation. This kind of component is particularly useful for high frequency and low cost applications.

There are several other types of RF detectors, and detectors are also packaged in a variety of ways based on their application requirements. Inline coaxial assemblies and waveguide assemblies are the most common. There are log detectors, which convert a wide dynamic range signal into a logarithmic output, thus enabling an artificially enhanced dynamic range. There are also Zero Bias detectors, which apply diode detector devices that don’t require a bias voltage or current to operate, and are often used in low-power and passive applications (RFID). RF Threshold detectors convert an RF signal over a specific frequency range to a DC equivalent voltage. Lastly, there are Waveguide Detectors, which are simply an RF detector built into a waveguide housing.

Waveguide Detectors:

Log Detectors:

https://www.pasternack.com/50-ohm-sma-log-detector-positive-10-mhz-3000-mhz-pe8040-p.aspx

Threshold Detectors:

https://www.pasternack.com/nsearch.aspx?Category=Detectors&Rfacdt99design=Threshold&sort=y&view_type=grid

Zero Bias Detectors:

https://www.pasternack.com/nsearch.aspx?Category=Detectors&Rfacdt99design=Zero%20Bias&sort=y&view_type=grid

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Two Easy Ways To Test Multistage CIC Decimation Filters

Rick Lyon's Blog on DSPRelated -

This blog presents two very easy ways to test the performance of multistage cascaded integrator-comb (CIC) decimation filters [1]. Anyone implementing CIC filters should take note of the following proposed CIC filter test methods.

Introduction

Figure 1 presents a multistage decimate by D CIC filter where the number of stages is S = 3. The '↓D' operation represents downsampling by integer D...

Not All Gain Equalizers are Created Equal

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With RF, microwave, and millimeter-wave systems, the component and system losses, along with frequency dependent gain and losses of devices and components, lead to uneven frequency versus gain slopes in many systems. This is often undesirable, especially in high precision sensing and test and measurement applications. For example, with wideband radar technologies, a precisely controlled and flat gain across multiple frequency octaves is needed to reduce error. Gain equalizers are active or passive components that are designed to correct the uneven gain curves of a device or system, and enhance their gain flatness.

Gain equalizers are relied upon in electronic warfare (EW), signal intelligence (SIGINT), test and measurement applications. Applications that cascade several gain stages may also require gain equalizers to flatten the compounded negative gain slope that many RF amplifiers exhibit.

There are several key parameters for Gain Equalizers, including their impedance, VSWR, insertion loss, linearity, power handling, return loss, bandwidth, control capability, and packaging. For modern radar, test and measurement, and communication systems that rely on complex modulation schemes with high modulation orders, phase noise and phase balance may be additional considerations. The main specifications that define a gain equalizer is its insertion loss versus frequency. Power handling is a limiting performance parameter of many gain equalizers, as the signal energy absorbed by the gain equalizer is converter to heat, and the packaging and interconnect method must be able to handle sustained temperatures without derating.

Gain equalizer circuits can be as simple as a simple passive attenuator and inductors/capacitors with a negative sloping loss curve that compliments the negative sloping gain curve of an amplifier. More complex gain equalizers are active circuits that demonstrate a voltage variable insertion loss. Often these voltage variable gain equalizers have a negative loss slope which increases or decreases as higher DC bias/control voltages are applied. Gain equalizers are available as surface mount technology (SMT) packages, similar to chip attenuators, as inline coaxial assemblies, and as monolithic microwave integrated circuit (MMIC) devices.

Given that gain equalizers are often implemented as attenuators with a gain slope designed to complement the gain slope of a device or system, minimum insertion loss may be overlooked as a consideration. However, many systems can only afford to sacrifice enough gain in order to achieve a desired gain flatness, and any additional loss may reduce overall system performance. Hence, the minimum insertion loss can be seen as an additional loss beyond the loss necessary to equalize the gain. With radar as an example, reduced transmitter output power or receiver sensitivity from a gain equalizer with excessive insertion loss would reduce the range of the radar.

Parasitic capacitances and inductances in an adjacent circuit can also influence the loss curve of a gain equalizer, and degrade performance. This is less likely with coaxial packaged gain equalizers, as the interconnect method, such as wire bonding or die attach, for SMT and MMIC chip attenuators intrinsically add parasitic capacitance, inductance, and resistance.

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The Limits of Limiting Amplifiers

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Is many applications, amplifiers are used to increase the strength of a signal, while minimizing the distortion and noise without sacrificing efficiency. However, with most receiver systems, and other applications, achieving the highest signal power isn’t the goal. As receivers circuits are typically highly sensitive to input power, and can be desensitized or damaged if exposed to sustained signal energy that exceeds some nominal amount depending on the specs of the receiver, capping the maximum amount of signal power over a frequency range can be a desired function. This is where limiting amplifiers are used.

Unlike many amplifiers that have a maximum output power limited by the input signal, gain, design feature, bias, and available power, limiting amplifiers are equipped with circuitry that provides a hard maximum power limit at the output. Hence, over a given frequency range, a limiting amplifiers will only output a set maximum signal energy, independent of the input. Like other amplifiers, the dynamic range, gain, and gain flatness are still priority parameters of limiting amplifiers.

As limiting amplifiers are often used in applications that require high signal fidelity, such as sensitive radar receivers, fiber optic transceivers in RF/optic converters, linearity, noise, and additive phase noise performance are also considerations. For high throughput data signals, like those used with fiber optic, microwave backhaul, and 5G millimeter-wave trails, maintaining signal quality while limiting input power to highly sensitive receiver circuits often jeopardized by interference, is key. Electronic Warfare (EW) applications are another common use of limiting amplifiers where sensitive radar receivers, active electronically steered array (AESA) transmit/receiver (TR) modules, and critical communication receivers are subject to high signal energies from nearby friendly transmitters and unfriendly jamming or interference.

Another key benefit of limiting amplifiers is to present low variation input power to a receiver circuit. This function can also be used to remove AM modulation from incoming signals and act as a comparator. These features make limiting amplifiers critical in the use of instantaneous frequency measurement (IFM) receivers, directional finding, digital radio frequency memory (DRFM), and a range of signal intelligence (SIGINT) uses.

Limiting amplifiers can be realized in a variety of ways. Some of the simplest output limiting amplifiers use clamping networks, which can be as simple as a two Schottky diode circuit and current limiting resistor, or as complex as a multi-transistor, diode, and resistor network for greater precision and faster recovery. Other types of limiting amplifiers operate using successive gain stages that “compress” from the input to the output of the amplifier. With any type of limiting amplifier, design challenges include wideband power limiting, as it is desirable for many applications for limiting amplifiers to cover multiple frequency octaves, especially with EW applications. Other design considerations must account for low variation power limiting, frequency equalization/stability, thermal management/compensation, harmonics, and dynamic range. Due to the often extreme operation temperatures and harsh environments that limiting amplifiers operate, they are commonly assembled in hermetically sealed packages with rugged connectors and wide operational temperature ranges.

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Introduction to Gunn Diode Oscillators

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A Gunn diode, though very unlike other common semiconductor diodes, is a transferred electron device (TED) that presents a negative resistance after its threshold voltage is adequately exceeded. The controllable negative resistance properties of a Gunn Diode allows it to be used as a microwave oscillator, and these devices are often used in microwave communications, radar guns, and microwave sensors.

Gunn Diode Oscillators are built with only negatively doped semiconductor regions, two heavily N-doped regions with one lightly N-doped region separating the other two regions. The current/voltage (IV) curve of a Gunn Diode appears linear with increasing current and voltage, until a threshold voltage is reached. At this threshold voltage, the conductive properties of the lightly N-doped middle region change to enhanced resistance, which causes the current to decrease at higher voltages. This function enables microwave amplification, but can also be driven while unstable to produce oscillations when properly biased.

With the proper DC bias, which drives the Gunn Diode into the negative resistance region, when this negative differential resistance matches, and cancels out, the positive differential resistance of the load circuit, a zero differential resistance circuit is created. At this stage spontaneous oscillation will occur, which can be controlled by external circuitry or resonators. The geometries of the Gunn Diode Oscillator also contribute to the electrical behavior of the diode, including oscillating frequencies.

With a resonator, a Gunn Diode Oscillator will generate an oscillating signal at the resonant frequency of the resonator. Some types of resonators can be manipulated to control their resonant frequency, such as cavity resonators and yttrium-iron-garnet (YIG) resonators, and can be used to create tunable oscillators with Gunn Diodes. The type of resonator tuning limits the frequency range at which a Gunn Diode Oscillator can be tuned. The quality of the resonator will also determine the phase noise, frequency stability, and output power of the oscillator.

Gallium arsenide (GaAs) and gallium nitride (GaN) are the most common Gunn Diode materials, where GaAs Gunn Diodes can be used to a couple hundred gigahertz, and GaN Gunn Diodes can hit operating frequencies of a few terahertz. Waveguide Gunn Diode Oscillators, are often designed to take advantage of extremely reliable mechanical tuning methods that leverage precision machined cavity resonators. Given the appropriate material choices and construction, waveguide Gunn Diode Oscillators can achieve good electrical characteristics, high reliability, and stability over a wide temperature range.

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Handheld Analyzers and Cable Testers Require Rugged RF Test Cables Part 2

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In our last post, the potential benefits of rugged RF test cables for handheld analyzers and cable testers was discussed. In this post, we will focus more on the features of rugged RF test cables, and how they provide benefits for harsh environment and use applications. As was mentioned in the last blog, the main design features that enable rugged coaxial cable assemblies are UV-resistant jacketing, material construction allowing for wider operating temperatures, crush and torque resistant armor, corrosion resistant conductor plating, and mechanically sturdy and reliable stainless steel body construction.

The jacketing of an RF test cable assembly is actually very important in maintaining the electrical performance and mechanical protection of an RF cable. Exposure to UV radiation can often change the cable jacket’s mechanical and material properties, and if the cable jacket isn’t UV resistant, this could cause the cable to fall out of specification or otherwise lead to cable damage and failure. An example would be UV exposure causing the cable jacket to stiffen and eventually crack, which could reduce the cable’s phase stability during flexure and allow water and corrosive ingress into the cable and connector.

Equally as important as the jacketing, crush and torque resistant cable armor can prevent the occasional banging, crushing, or shock from causing the delicate internal coaxial cable structures from compressing, pinching, twisting, or otherwise deforming and damaging the cable. Though cable armor does increase the weight and diameter of a cable, it also can prevent excessive twisting and bending from damaging the cable, and aids with maintaining phase stability during flexure after repeated use. This can be especially useful during installation, as the forces placed on a coaxial cable while it is being pulled through metal structures, conduit, or other building structures can be significant. Cable armor also protects from punctures and gouges, which could easily end the life of a less rugged cable.

Material construction that enables environmental resilience, such as temperature and corrosive resistance, helps in prolonging cable life while used in harsh environments. Even while in storage, an RF test cable can be damaged by intense heat or cold, as the material properties of the plastic dielectrics and jacketing can be affected in extreme conditions. Though most RF test cables won’t experience the same environmental conditions as commercial airliners or jet fighters, for less environmentally resilient test cables, even short term exposure can damage a cable enough for it to be useless for prevision RF testing. Moreover, an environmentally resilient cable assembly design will ensure consistent and accurate testing over a longer lifespan than typical coaxial cable.

Another important factor with RF test cables for field applications, is to have a variety of RF coaxial connector options to choose from. Many technicians will carry several test cables and adapters, so that they can more readily use their portable analyzers with whatever connectors the installation uses. Using RF adapters at test equipment ports, and for cables that experience frequent mate/demate cycles, can help to ensure the test cables and test equipment ports are clean and in good condition. As test cables are typically cost more than adapters, this method is a convenient and cost effective approach to maintaining field test equipment.

https://www.pasternack.com/pages/RF-Microwave-and-Millimeter-Wave-Products/handheld-rf-analyzer-rugged-phase-stable-cable-assemblies.html

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Handheld Analyzers and Cable Testers Require Rugged RF Test Cables Part 1

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The most recent handheld RF analyzers and cable testers for telecommunications are multi-functional instruments rich with features, and offer near-laboratory grade performance. This is extremely impressive considering many of these testers, which operate as network analyzers, spectrum analyzers, fault testers, signal generators, power meters, modulation analyzers, amongst other features, can be slung over a shoulder and carried while climbing an antenna tower. Like with all RF test and measurement instruments, however, even these devices are limited by the quality and application requirements of the interconnect needed for them to operate. In the case of handheld testers for telecommunications installation, maintenance, troubleshooting, and field testing, this means coaxial cables, adapters, and test cable assemblies that can survive rough conditions and still maintain metrology grade performance.

Most RF test cable assemblies are designed for sterile laboratory environments with relatively constant temperatures, pressures, and very few contaminates. The greatest threat to these cables is the mishandling of a novice or a coworkers messy take-out. In the case of RF field testing, there is usually nothing sterile or gentle involved in the handling or transporting of test cables. Technicians and operators are often under substantial time pressures, in remote conditions with only their available equipment, and limited in what/how they can carry test equipment. This often translates to RF test cables being wound up, bunched, crushed into carrying cases/tool bags, and banged around along with hand tools, power tools, and other equipment. If this doesn’t sound bad enough for VSWR, phase stability, and PIM performance, the remote tower, underground, industrial, or in-building/on-building telecommunication installations often have a variety of their own hazards and threatening environmental factors. These include wind, sun, wide temperature ranges, corrosives, shock, vibration, and the occasional crushing during transportation, installation, operation.

Typical RF test cables would likely fall out of spec during such treatment, hence there are more ruggedized RF test cable assemblies available. These cable assemblies are specifically designed to offer lab-grade VSWR, phase stability, and PIM performance, while also having protective features to enable a long, and reliable, lifespan. These features include UV-resistant jacketing, material construction allowing for wider operating temperatures, crush and torque resistant armor, corrosion resistant conductor plating, and mechanically sturdy and reliable stainless steel body construction.

As it is often too optimistic to expect a technician to be able to maintain ideal conditions for RF test cables, while meeting quotas and deadlines, using ruggedized RF test cables is a practical solution to the problem. With less reliable cables, technicians may frequently find themselves testing with damaged or derated cables, and either reporting false measurements, or wasting effort troubleshooting phantom anomalies caused by damaged cables. A worst case scenario would be that a damaged cable allows for peak, or sustained, power levels that cause damage to the still-pricey RF test equipment.

Check out our next blog post, “Handheld Analyzers and Cable Testers Require Rugged RF Test Cables Part 2,” where more technical detail about the construction and performance of rugged RF test cables for field applications is discussed.

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FFT Interpolation Based on FFT Samples: A Detective Story With a Surprise Ending

Rick Lyon's Blog on DSPRelated -

This blog presents several interesting things I recently learned regarding the estimation of a spectral value located at a frequency lying between previously computed FFT spectral samples. My curiosity about this FFT interpolation process was triggered by reading a spectrum analysis paper written by three astronomers [1].

My fixation on one equation in that paper led to the creation of this...

BMA Connectors Push beyond Twist Tighten RF Connectors

Pasternack Blog -

In many applications, a threaded RF connection may be impractical, or even impossible. Some examples include panel, daughter cards, or inserts with many RF connections. In these cases the pitch between the connectors may necessarily be too small for a threaded nut, or the card/assembly may be inserted into a slot where there is no access. For these reasons, push-on type connectors were developed, which are also known as blind-mate connectors. A common blind-mate connector for military, aerospace, satcom, and radar is the Blind-mate A (BMA) connector developed in the 1980s.

Originally, this connector was designated with a trademark name, OSP, so these connectors are also often referred to as BMA or OSP connectors. The benefits of a BMA connector are that the mating, even with slight misalignment, can be done by simply inserting the jack into the plug with adequate pressure. Internal mechanical mechanisms then ensure the electrical connections are made securely and with proper alignment. This precision alignment allows for standard BMA connectors to operate to 22/26.5 GHz, with VSWR as low as 1.2:1 and typical insertion loss ~0.1 dB.

Due to the utility of BMA connectors, there are many variations for different installation types. These variations include BMA coaxial plug/jack connectors, BMA bulk connectors, BMA panel mount connectors, BMA PCB connectors, and screw-in hermetic connectors. The outer housing and mounting hardware for BMA connectors is often stainless steel, passivated stainless steel, and is sometimes gold plated stainless steel. The internal contacts for a BMA connector are often beryllium copper with gold plating, and the dielectric type is often PTFE.

As some BMA connectors have a “snap-on” feature, which is a mechanical slot or ridge that aids with retention, a BMA connector can be used in applications where forces would normally disconnect a slide-on style blind-mate connector. BMA connectors are also somewhat vibration and shock resistant, as the electrical connection within the connector is supported by 360 degree of contact.

The quality of construction and materials used on BMA connectors often enable these connectors to undergo several thousand mating cycles, which is often superior to other mechanical retention mechanisms. Hence, for test and measurement applications, and automated test applications, a BMA connector type can be a reliable connector interface that both speeds up connecting and disconnecting hardware, but also provides increased reliability and performance. Occasionally, BMA connectors are also used for high-speed digital signals, and power/signal applications, where high switching speeds and blind-mate connections are needed.

Find out more about Pasternack’s BMA Connectors here,

https://www.pasternack.com/pages/RF-Microwave-and-Millimeter-Wave-Products/bma-connectors.html

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What is so Special About the 4.3-10 Coaxial Connector?

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There are many coaxial connectors used in wireless and wired networks, including DIN 7/16, N, 4.1/9.5, and the 4.3-10 interface. Typically, the choice of a connector boils down to economics, power handling, size, and installation criteria. However, the increasing number of wireless network bands and mobile data rates pose a very stringent requirement for transmission fidelity. This is especially the case for upcoming non-standalone 5G radios (NSA 5G NR) recently specified by the 3GPP, which add frequency bands in the 3 GHz and 5 GHz portion of the radio spectrum, where performance could be impacted by PIM and other nonlinearities with 4G systems. Hence, new coaxial connector interfaces, such as the 4.3-10, are becoming increasingly necessary to meet network performance goals in the latest wireless systems.

The 4.3-10 coaxial connector exhibits some important differences when compared to previous coaxial connectors, incorporating design features that were not in the similar-sized 4.1/9.5 connector  The most important feature may be the separation  of the electrical and mechanical mating planes within the connector.  This allows the connector to achieve its transmission and PIM properties even when not fully torqued.  Additionally, the electrical contacts and electrical mating plane are protected by the outer body of the connector making the connector better-suited for field installations. Size is an important feature as well.  A 4.3-10 connector, including flange mounting can be accommodated within a 1 inch-square space. These features, collectively make the 4.3-10 connector a compelling choice for new systems and retrofits.

Expanding on the separation of the mating planes, the design of the 4.3-10 connector doesn’t couple the coaxial electrical contacts and the mechanical mating hardware. This means that the amount of front contact force necessary for 7/16, N, or 4.1/9.5 connectors isn’t necessary for the 4.3-10 connector. This is due to the radial contact, which requires much less force to ensure optimum contact points. Hence, high torque isn’t needed with the 4.3-10 connector, and screw, hand-screw, and push-pull designs are all possible without sacrificing electrical performance. Moreover, the hand-screw and push-pull style 4.3-10 connectors can allow for cable rotation during installation, easing complex routing tasks.

As stated earlier the 4.3-10 connector is designed with the electrical contact surfaces protected within the mechanical connection hardware, protecting it during handling. This design feature prevents mishandling from degrading electrical performance, which can often lead to greater insertion loss, PIM, and potential system  failures. This approach also grants the female/jack connector to universally mate with any type of male/plug coupling mechanisms, instead of having to have a separate female/jack connector for each type of male/plug.

The small size of the 4.3-10 design enables the dense interconnect pitches, down to 1 inch. The reason for this is that the hand-screw and push-pull 4.3-10 connectors can be closely packed, and don’t require a torque wrench during installation. For high density multi-input multi-output (MIMO) and antenna arrays used in the latest 4G LTE, 5G, small cells, and distributed antenna systems (DAS) applications, the high performance, reliable, and high density features of the 4.3-10 connector present modern benefits that most wireless infrastructure and mobile wireless equipment systems can’t manage without.

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Distributed Antenna System (DAS) Basics

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There are many cases, especially within buildings, tunnels, large public venues, stadiums, and etc. where the signals from a cell tower does not provide sufficient coverage, or the density of users is just too great for a typical cellular approach. In these circumstances DAS installation can be employed to enhance the coverage in these areas, or through a base transceiver station (BTS), NodeB, eNodeB, or small cell paring, offer enhanced service to a large number of users. Do to the diversity of DAS installations, there is not one single type of DAS, or DAS technology, and often multiple DAS techniques are used to enhance the wireless performance of cellular, and even WiFi, services in an area.

The simplest form of DAS, is the use of an off-air passive DAS installation, which couples wireless through a donor antenna to a cellular base station, and redirects that signal through coaxial transmission lines to targeted delivery antennas. The use of diplexers at the antennas allows for downlink and uplink, and this is typically the fastest, lowest, cost, and depending on the diplexers and antennas, can be used to serve multiple carriers, and even WiFi. In this case, the DAS performance is tied to the donor signal and RF design quality of the systems and isn’t capable of adding capacity.

Active DAS technologies also exist, where the RF signal is converted into a digital signal and sent to remote radio units (RRUs), which then convert the digital signal into an RF signal again for use with user equipment. This method reduces the amount of RF amplification and RF transmission lines necessary in passive DAS to distribute the signals. Routing complexity is also reduced as industry standard Ethernet or fiber optic cables can be used to transport the digitized RF signals, which greatly increased the length of cables that can be used, and reduces the cost and complexity of extending the system.

Active and passive DAS systems can also be combined, where certain areas of the distribution are either routed passively or actively. These hybrid systems are often used where cost considerations and routing constraints limit the use of an all Active DAS installation.

The latest developments in the DAS market are Digital DAS technologies, which use a baseband unit (BBU) to communicate digitally with a DAS master unit and RRUs without an RF interface. Though Digital DAS may be theoretically cheaper, less complex, and much more expandable that even Active DAS, it is likely that competing standards have yielding relatively little real world deployment.

DAS installations can be driven from either donor cellular sites, this is called off-air DAS, through a BTS/NodeB/eNodeB, or through a Small Cell. The difference between BTS/NodeB/eNodeB and Small Cell DAS, is that Small Cell DAS typically leverage the backhaul internet infrastructure to relay cellular, and sometimes WiFi, signals. BTS/NodeB/eNodeB-fed DAS operate much like traditional cellular base stations with distributed RF signal routing and antennas, instead of a tower, or roof-top, based broadcast style distribution. Also, due to limits with connection and capacity, a Small Cell DAS usually can’t offer the extent of capacity that is possible with BTS-fed DAS, which usually are fed by dedicated, carrier installed fiber connections to the carrier’s network.

A variety of RF, digital, and fiber optic technologies are employed in DAS systems, including repeaters, amplifiers, coaxial transmission lines, Ethernet cables, fiber optic cables, a wide variety of antennas, diplexers, RF to fiber converters, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), digital signal processors (DSPs), mixers, filters, and all the associated RF and telecommunications test equipment and infrastructure to design, install, test, and maintain a DAS.

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What are the ISM Bands, and What Are They Used For?

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The Industrial, Scientific, and Medical (ISM) frequency bands are designated radio frequency bands as defined by the ITU Radio Regulations. These frequency bands were set aside for RF use for purposes other than telecommunications. Hence, using the ISM bands for telecommunications is possible, but telecommunications devices using these frequencies must be able to withstand the interference from other RF and microwave technologies, such as microwave ovens, RF heating, and other potentially electromagnetic interference (EMI) producing devices. Though the ITU determines the international band designations, individual countries exact ISM band frequencies may differ.

Though originally set aside for non-communication purposes, many short range, low power, license-free, or unlicensed error-tolerant communications systems operate within the ISM-band. Of the most common uses outside of communications, are induction heating for industrial and home applications, microwave heating for industrial and home applications, and RF and microwave heating for medical purposes, such as diathermy, hyperthermia therapy, and RF/microwave ablation. Recently, there have also been radar systems developed that operate in the ISM bands, specifically the 2.4 GHz ISM band, due to the readily available and inexpensive wireless ICs available for these frequencies.

The most common everyday uses of the ISM bands are for low-power and short range telecommunications, such as WiFi, Bluetooth, Zigbee, wireless telephones, RFID, and NFC. Many in the US are familiar with the 2.4 GHz ISM band, as most WiFi and Bluetooth communications operate in these bands, though more recently, 5 GHz WiFi systems have become more available. There are also many common RFID and NFC systems that use 13.56 MHz in the 13.553 MHz to 13.567 MHz ISM band, and many credit cards, secure access, personnel identification, and wireless payment systems use these technologies.

Moreover, many of the latest smart home and hobbyist electronics use Zigbee technology in the 915 MHz and 2.4 GHz ISM bands for low-power and short range communications between devices. In the next few years, it is likely that 60 GHz WiFi (WiGig, known as IEEE 802.11ad), operating in the 60 GHz ISM band, will gain popularity for extremely high throughput device-to-device communication. Some examples of this application may be high definition, 4k, video streaming and extremely fast device-to-device wireless data transfers.

There have been many more modulation schemes and communications platforms targeted for the ISM bands emerging in the past few years than prior years. Some of the reasons for this could be the increased potential of The Internet of Things (IoT) and Industry 4.0 applications, which will likely rely on low-power and short range machine-type communications without users being directly involved. Examples of these new technologies include Thread, Z-wave, LoRa, and NB-IoT. Though future applications of ISM bands may include satellite communications, right now there are no available ISM bands available for current cubeSATs, nanoSATs, or other smallSATs.

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Smalls Cells are Certainly Making a Large Impact

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Small Cells are essentially a mobile wireless installation that is smaller in scope and size than a traditional macro base station. So, why are Small Cells, which provide less coverage and necessarily are composed of equipment that may not have the full capability of macro-cells, on the rise? An important note is that the term Small Cells isn’t standardized, and at times may mean anything from slightly smaller non-traditional base stations, to distributed antenna systems (DAS), to new technology systems, like a prototype 5G base station deployment. Though there has been skepticism around Small Cells, they may be part of a solution to better provide wireless services to our world’s increasingly urbanizing population and an enabling factor in the latest evolution of mobile wireless.

Small Cells are a diverse set of technologies by nature. Typically, Small Cells are composed of the same types of equipment that supports a traditional base station, but Small Cell versions are often more integrated, lighter, smaller, lower power, and have less substantial, or more integrated, filtering to further reduce size and weight. The downscaling is necessary to fit Small Cells on the limited structures available in dense areas, such as street furniture, corner rooftops, and concealed mini-towers. The next generation of Small Cells, which will support advanced 5G technologies, will also leverage new multi-input multi-output (MIMO) antenna and beam steering technologies, as well as the non-standalone 5G new radio (NSA 5G NR) sub-6 GHz frequencies. The Small Cells used for millimeter-wave (mmWave) 5G trials and early 5G deployments coming in 2019, will also feature mmWave frequency operation, though the exact frequencies are yet to be determined by the 3GPP and spectrum regulation authorities.

Though often confused, DAS and Small Cells are different approaches to providing coverage in dense, indoor, or areas otherwise suffering from adequate coverage. Where Small Cells are fully contained and miniaturized base stations requiring their own backhaul and providing services, DAS are  composed of remote antenna systems that extend the coverage of a macro-cell using transmission lines or fiber optic systems. DAS and Small Cells also differ in how they are installed and regulated. Small Cells may be a popular alternative over DAS, as Small Cells are often cheaper, and don’t require coordination with other service providers, so they can be deployed on the time frame and cost structure of a single service provider.

In a recent FCC filing, Verizon wrote, “In 2017, approximately 62% of Verizon’s wireless deployments were small cells, a figure that will only grow larger as we deploy 5G in 2018 and beyond.” With many shedding doubt on the viability of Small Cells and their adoption, it seems an interesting contrast that large wireless service providers, such as Verizon and AT&T, would herald Small Cell technologies as the current, and next, generation of their wireless deployments. The filing further stated, “Small cells are needed to meet exploding consumer demand for data, drive innovation, create new jobs, and fuel new services and capabilities such as smart communities, connected cars, smart farming, and the Internet of Things.”

Though attractive for wireless service providers who intend to reach more customers and capture/maintain their market share, Small Cells aren’t without complications. As Small Cells are new technologies, the deployments are non-traditional, and Small Cells must be set up near the trafficked areas where wireless customers like to be, the deployment characteristics for Small Cells involve many layers of regulation, zoning, siting. Many municipalities in the US don’t even have standardized processing for handling Small Cell installations, and legislation on Small Cells may currently be underdeveloped. There may also be a lack of information on the extent of this problem, as the major wireless service providers have largely been tight lipped about their Small Cell deployments. This will likely be addressed as future legislation and regulation moderates the industry in preparation of massive 5G rollouts, which will necessarily require Small Cells.

Currently, there may be some growing pains with Small Cells, as a system for deploying Small Cells isn’t yet standardized. This may slow, but will likely not prevent Small Cells being deployed, as upcoming 5G services will necessarily be deployed in Small Cells. This brings up other substantial challenges, such as installing, verifying, and maintaining the abundance of Small Cells through dense urban areas. Such an effort will require substantial amounts of portable RF test equipment, the necessary interconnect, and operators available to rapidly respond to service interruptions or regulatory issues. Moreover, the pace of Small Cell deployments will also require RF equipment providers with readily stocked components, devices, and interconnect, that are able to fulfill orders same-day and without delay.

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Did You Just Say EIRP? (Effective/Equivalent Isotropically Radiated Power)

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Effective, or equivalent, isotropically radiated power (EIRP) is a measurement of the optimum power that can be radiated from an antenna from a particular transmitter. Most often conveyed as either decibels (dB), or decibels over isotropic (dBi), EIRP is used to gauge the maximum possible radiation from an RF system, either for standards purposes or for specification. With an isotropic radiator, the radiation is emitted as a point source with a spherical radiation pattern. This means that the maximum radiated power at any point of the emission pattern of an isotropic radiator are equivalent.

There are two main factors for EIRP, the total radiated power and the gain of the antenna based on the antenna pattern. The same transmitter with antennas with different antenna gains will present different EIRP results. Conversely, transmitters with different power outputs operating on the same antenna will also present different EIRP results. For example, a 100 watt transmitter with an antenna with a gain of 1 will have the same EIRP as a 400 watt transmitter with an antenna having a gain of 4. It is important to note, with EIRP, the isotropic radiated antenna is hypothetical, and not a practically realizable antenna pattern. Hence, EIRP is a theoretical maximum.

EIRP is sometimes confused with effective radiated power (ERP), which is defined by the IEEE as the radiated power of the main lobe of a half-wave dipole antenna. A half-wave dipole demonstrates an antenna gain in the main lobe of 1.64 times that of an isotropic radiator, or 2.15 dB. Hence, EIRP and ERP are related, as EIRP will be 1.64 times, or 2.15 dB greater, than ERP for the same RF system.

Other factors can also be included in an EIRP and ERP calculations to improve the accuracy of the approximations. For example, the loss of the cabling, connectors, switches, circulators, and etc., from the transmitter to the antenna can be included. Also, the mismatch between the antenna, interconnect, and transmitter output stage could also be accounted for greater accuracy.

Polarization is not accounted for with EIRP or ERP, and a system designer must account for polarization loss when positioning real antenna systems. If not, polarization loss can be as much as 3dB for linearly polarized antenna, to theoretically 100% loss with circularly polarized antenna.

For your convenience, you can use the Pasternack EIRP Calculator found at this link:

https://www.pasternack.com/t-calculator-eirp.aspx

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