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Coaxial Cable Loss due to Loss Tangent

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Electrical losses in a coaxial cable create heat in the outer and center conductors and are the two main types of coaxial cable loss, skin-effect loss and dielectric loss, respectively. This heat, or loss, can be calculated with the understanding of the following concepts.

What is Skin Effect Loss?

In coaxial cable, skin effect is the movement of an alternating electric current (AC) whereby the current density is greater near the surface of the conductor and lessens within the conductor.  Skin effect is the decline in current density and the skin depth is a measure of the depth at which the current density falls to 1/e of its value near the surface. Over 98 percent of the current flows within a layer 4 times the skin depth from the surface. At high frequencies, the skin depth becomes much smaller. Skin effect loss usually occurs at high frequencies when the signal reaches and moves along the surface of the inner conductor, which causes additional RF losses at higher frequencies.

A skin depth calculator can be found here.

Resistance per unit length is the ratio of specific resistance or resistivity to the area of cross-section of given conductor in Ohm per meter. With skin-effect loss, the resistance per unit length, Rl, and the inductance per unit length, Ll, increase with the square root of the frequency.

Skin effect losses are resistive, caused by the narrowing of the conduction path. In calculating loss, loss per unit length includes the skin effect loss and dielectric loss.

What is Dielectric Loss?

Dielectric conduction loss is caused when the insulating material inside the transmission line absorbs energy from the electromagnetic field developed between the inner and outer conductors. Dielectric loss explains the amount of dissipation of electromagnetic energy or heat of a dielectric material. It is often described in terms of either the loss angle δ or the corresponding loss tangent tan δ. Both refer to the phasor in the complex plane whose real and imaginary parts are the resistive (lossy) component of an electromagnetic field and its reactive (lossless) counterpart. The ratio of two quantities is defined in term of Tan.

What is Loss Tangent?

Loss tangent is the ratio at any specified frequency between the real and imaginary parts of the impedance of the capacitor. A large loss tangent refers to a high degree of dielectric absorption. Loss tangent is the ratio between the imaginary and real parts of the complex permittivity where the permittivity of dielectric is given by:

ε =ε_re − jε_im

When this formula is drawn on an x-y plane, the tangent of the angle between the real and the imaginary quantity is discoverable which can be described as:

tanδ = ε_im/ε_re

Which means the ratio of the imaginary part to the real part of the permittivity is found to be another quantity, i.e. the loss tangent, which is used to express the losses in a dielectric material. In other words, it is the ratio of the imaginary part to the real part or the tangent of the angle between the complex number and the real axis.  This angle is the loss angle and the tangent is called the loss tangent.  Thus, the value of the loss tangent describes how lossy a material is, such that it either represents a very lossy material or a very good conductor.

Measurement of loss using sinusoidal excitation at a particular frequency yields the product of skin effect and dielectric loss functions or the sum of the skin effect and dielectric losses in units of dB. Once the skin-effect loss is derived, the portion of loss attributable to dielectric losses may be estimable.

 

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Coaxial Cable Loss due to Dielectric Conduction

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The coaxial cable provides a transmission line with low loss characteristics that can shield from outside signals and provide a durable yet flexible line that can be used in a variety of applications with a wide range of frequencies.  At microwave frequencies, much of the loss in coaxial cable transmission lines is characterized as metal loss however, losses due to dielectric conduction can be significant for some uses.

What is dialectic conduction loss?

Electrical losses in a coaxial cable create heat in the center and outer conductors. Because most of the heat is generated at the center conductor of the cable, this heat is referred to as dielectric conduction. Dielectric conduction loss is caused when the insulating material inside the transmission line absorbs energy from the electromagnetic field developed between the inner and outer conductors. The dielectric in coaxial cable refers to materials with a high polarizability, is used to indicate the energy storing capacity of the material by means of polarization, and is expressed by a number called the relative permittivity.  A coaxial cable uses the permittivity of the material between the center conductor and shield to determine its characteristic impedance when it is used as a transmission line. Dielectric insulators polarize to oppose an applied electric field which decreases the electric field in the dielectric material and reduces internal loss in the cable.

Construction and performance of coaxial cable

The coaxial cable conducts electrical signals using a solid, stranded, or copper plated steel wire, known as the center conductor or core, surrounded by an insulating layer, the dielectric, which is enclosed by a shield and protected by an outer insulating jacket. The function of the dielectric is to maintain the spacing between the shield and the center conductor but a certain amount of signal energy is dissipated in the dielectric material itself. The ideal dielectric material does not exhibit electrical conductivity when an electric field is applied however, all dielectrics have some degree of measurable conductivity.

Temperature stability is important to maintain in order to gauge and regulate the performance of the coaxial cable. If the temperature of the coaxial cable is elevated too high or too quickly, the cable may become warped and damaged beyond repair. As any heat generated due to dielectric losses is dissipated within the dielectric, it is paramount that the construction of the dielectric material be considered when selecting the correct cable for the specific application.

The most common dielectric material is polytetrafluoroethylene (PTFE) which has lower dielectric losses than PVC. The benefit of using PTFE are that this material can tolerate temperatures ranging from -50°C to +200°C and is often combined with other materials to increase dielectric constant or improve temperature stability. The drawback of this material is that it is sensitive to moisture under voltage stress.

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Coaxial Cable Power Handling Part 3

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In the previous sections of the “Coaxial Cable Power Handling” blog series, a brief overview of coax power handling, and how cable size, cable type, signal frequency, and electron velocity through a cable, influenced the power handling capability of a coaxial cable. In this section, attenuation/loss, environmental factors, VSWR, and cable capacitance, will be touched on in respects to coaxial cable power handling.

Attenuation or Loss

One of the important constraints that limit the power handling capability of a line is attenuation. Attenuation is the loss generated by the inner and outer conductor of the coaxial cable and its dielectric medium. As most of the heat is generated in the center conductor, any heat generated as a result of dielectric losses will be dissipated within the dielectric. The construction of the dielectric is key in determining the power handling capability of the coaxial cable.

Thus, lower loss RF coaxial cables will have a higher power rating than higher loss RF coaxial cables. Additionally, there is a degree of loss as a signal travels along a coaxial cable which is proportional to the length of the cable and is specified in terms of a loss over a given length or decibels over a given length, e.g. 0.5dB / 10 meters.

Environmental Factors and VSWR

Non-optimal operating conditions may require a coaxial cable to be derated. Environmental temperature can induce derating. For example, if the coaxial cable is operating in a high temperature environment, it cannot dissipate as much heat, operating temperature will increase. Hence, a de-rating factor is applied if the coaxial cable is used at high temperatures. Altitude plays a role but only at significant heights.

Thus, if the cable is operating at a high altitude and under reduced pressure, cooling will be less effective and temperature rise within the cable will be greater. Additionally, if the coax cable is operating under conditions where the VSWR is high, de-rating may be necessary due to the variations of high and low current along the coaxial cable such that the power dissipation may cause higher levels of power to be dissipated locally.

Capacitance

The capacitance of the coaxial cable is the ability to hold a charge. Capacitance exists between the inner and outer conductors of the coaxial cable which is proportional to the length of cable as well as the dielectric constant and the inner and outer conductor diameters. The larger the capacitance, the longer it takes a signal to reach full amplitude inside the cable. Thus, a high capacitance is an undesired characteristic when determining the performance of a coaxial cable.

For a detailed look at the data used to develop recommended maximum power ratings for RF coaxial cable power handling, see Power Handling Capabilities of RF Coaxial Cables, 1971, http://www.dtic.mil/dtic/tr/fulltext/u2/912072.pdf. in which “representative coaxial cable types were subjected to high power CW and pulse tests” which were “conducted over a range of frequencies and ambient temperature and pressure attitudes.”

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Coaxial Cable Power Handling Part 2

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In the previous portion of the “Coaxial Cable Power Handling” blog series, we discussed an overview of coax power handling and how cable power handling impacts cable choice. In this portion of the blog series, we will take a deeper look into how cable size and type, as well as signal frequency and electron velocity through a coaxial cable, relate to a coaxial cable’s power handling capability.

Cable Size and Type

Different types of coax cable are used for specific applications and the specifications help to determine the recommended or required coaxial type. For example, in microwave applications where very low loss is needed, a semi-rigid coaxial RF cable with a solid copper outer sheath may be preferred as it offers superior screening compared to RF cables with a braided outer conductor, especially at microwave frequencies.

Although the power handling capability of RF coaxial cable is not a concern for many installations, when using medium or high power transmitters, the power rating or handling capability of RF coaxial cable must be considered. In applications where high power is used, ordinary polyethylene might melt and distort the cable therefore cables with an air dielectric with the center conductor held in place by a dielectric coil that runs along the length of the cable is preferred.

For medium to high power applications, coaxial cables with a Teflon dielectric are preferred as these can withstand high temperatures up to around 160°C. When considering which cable to use, it should be remembered that as the frequency increases the skin effect becomes more pronounced, and coupled with increased losses in the dielectric, this limits the power handling capacity.

Frequency and Velocity

When considering wave propagation over a cable, the creation of higher modes can degrade efficiency, produce higher reflections, and mitigate the power handling capability of the coaxial line. As power is inversely proportional to frequency, the higher the operating frequency, the lower the power capacity of the coaxial line. Length of the cable can mitigate the power handling capability and coaxial cables are often cut to a specific length to act as an impedance transformer or a resonant circuit. If resonant lengths of RF coaxial cable are used, it is necessary to know the velocity factor of the cable.

The velocity factor specifications of a coaxial cable is the speed at which the signal travels within the cable compared to the speed of the signal in a vacuum. The velocity factor specification is a figure which is less than 1. The dielectric between two conductors indicates the velocity factor; coaxial cables composed of a solid polyethylene dielectric will have a velocity factor around 0.66, and those made of foam polyethylene will have velocity factor figures ranging from about 0.80 to 0.88.

For a detailed look at the data used to develop recommended maximum power ratings for RF coaxial cable power handling, see Power Handling Capabilities of RF Coaxial Cables, 1971, http://www.dtic.mil/dtic/tr/fulltext/u2/912072.pdf. in which “representative coaxial cable types were subjected to high power CW and pulse tests” which were “conducted over a range of frequencies and ambient temperature and pressure attitudes.”

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Coaxial Cable Power Handling – Part 1

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The coaxial cable provides a transmission line with low loss characteristics that can shield from outside signals and provide a durable yet flexible line that can be used in a variety of applications with a wide range of frequencies. Power handling capability, also known as the power capacity, is one of the important criteria in evaluating the performance of the RF coaxial cable. When considering medium or high power transmitter applications, power handling is especially critical, as the incorrect type of coax cable can result in a failure of the cable and possible damage to the transmitter.

Electrical losses in a coaxial cable creates heat in the center and outer conductors and, because most of the heat is generated at the center conductor of the cable, in the dielectric core. Power handling capability is related to the ability of the cable to withstand and dissipate this heat. In power handling, the maximum allowable operating temperature of the materials used in the cable, especially the dielectric, is the ultimate limiting factor. For applications with continuous power, heat loss within the cable is the limiting factor and, when using pulsed power through the coaxial line, operating voltage must not be exceeded. Thermal conduction of the coaxial line generally relates the physical properties of the cable with temperature. Specifically, thermal conductivity (k) is characterized through the quantity of heat (Q) transmitted in time through a thickness (L) in the direction normal to the surface area (A) due to changing temperature. Thus we have the rate of heat transfer in the cable and the thermal effects of the line, also known as power rating of the line.

As a rule, the power handling capability of a specific cable is inversely proportional to its attenuation and directly related to its size. Regarding coaxial cable power ratings, correction factors for ambient temperature, altitude and VSWR must be taken into account. For example,  high ambient temperature and/or high altitude reduces the power rating of by impeding heat transfer from the cable and VSWR reduces power rating by causing localized hot spots in the cable. Which coaxial cable to use depends on the size and type of the line, the frequency of operation, the attenuation or loss, thermal conduction and capacitance. Each of these factors plays a key role in determining the power handling capacity of the line.

 

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Introduction to Coaxial Cable Losses

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Coaxial cable transfers radio frequency power from one point to another and, in the ideal world, the same amount of power would transfer along the cable to the remote end of the coax cable. However, real world conditions include some power loss along the length of the cable. Loss, or attenuation, is one of the most important features to look for when deciding what type of coaxial cable to use in a design.  Loss is defined by decibels per unit length and at a given frequency. Thus, the longer the coaxial cable, the greater the loss. Loss is also frequency dependent, generally increasing with frequency, but the loss is not necessarily linearly dependent upon the frequency. Power loss occurs in a variety of ways:

Resistive loss

Resistive losses within the coaxial cable occur when the resistance of the conductors and the current flowing in the conductors results in heat being dissipated. Skin effect limits the area through which the current flows, which leads to increased resistive losses as the frequency rises. To reduce the level of resistive loss, the conductive area is increased resulting in larger low-loss cables. Also, multi-stranded conductors are often used.  Resistive losses generally increase as the square root of frequency.

Dielectric loss

Dielectric loss is signal energy dissipated as heat within the insulating dielectric of a cable, but is independent of the size of the coaxial cable. Dielectric losses increase linearly with frequency, and the resistive losses normally dominate at lower frequencies and. As resistive losses increase as the square root of frequency and dielectric losses increase linearly, the dielectric losses dominate at higher frequencies.

Radiated loss

Radiated loss in a coaxial cable is usually much less than resistive or dielectric losses, however poorly a constructed outer braid on some coaxial cables may yield a relatively high radiated loss. Radiated power, problematic in terms of interference, occurs when signal energy passing through the transmission line is radiated outside of the cable. Leakage from a cable carrying a feed from a high power transmitter may produce interference in sensitive receivers located close to the coax cable or a cable being used for receiving can pick up interference if it passes through an electrically noisy environment. To reduce radiated loss or interference, double or triple screened coaxial cables are designed to reduce the levels of leakage to very low levels.

Of these forms of loss, radiated loss is generally the less concerning as only a very small amount of power is generally radiated from the cable. Thus, most of the focus on reducing loss is placed onto the conductive and dielectric losses, except in certain applications.

Loss over Time

Loss or attenuation of coaxial cables tends to increases over time as a result of flexing and moisture in the cable. Although SOME coax cables are flexible, the level of loss or attenuation will increase if the RF cable is bent sharply or if there is a disruption to the braid or screen. Contamination of the braid by the plasticisers in the outer sheath or moisture penetration can affect both the braid where it causes corrosion and the dielectric where the moisture will tend to absorb power. Often, coax cables that use either bare copper braid or tinned copper braid experience more degradation than those with the more expensive silver plated braids. Although foam polyethylene provides a lower level of loss or attenuation when new, it absorbs moisture more readily than the solid dielectric types. Cables with solid dielectric polyethylene are more suited to environments where the level of loss needs to remain constant or where moisture may be encountered. Even though RF coaxial cables are enclosed in a plastic sheath, many of the plastics used allow some moisture to enter thus, for applications where moisture may be encountered, specialized cables should be used to avoid performance degradation.

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Why would you want to limit an amplifier?

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Amplifiers are electronic devices that increase the power of a signal. The amount of amplification in an amplifier is measured by its gain, or the ratio of output voltage, current, or power to the input with a power gain greater than one. An RF amplifier is similar to an audio amplifier but with components suitable for RF frequencies. Because of the higher frequencies involved in RF amplification, the impedance levels of RF amplifiers are generally much lower than for audio amplifiers. Frequencies for RF amplifiers range between 20 kHz and 300 GHz and are usually coupled with an input or output impedance that is matched to the transmission line impedance or ratios of voltage to current. Often 50 Ohms and 75 Ohms are common impedance values for RF ports, and so RF amplifiers are usually specified as providing a given output power level at these impedances. Although RF amplifiers can be characterized as amplifying voltage or current, they are, technically speaking, amplifying power.

The ideal amplifier would be a linear device, though real amplifiers are only linear within limits. Linear systems, when given a sufficiently strong input signal, will reach a point where the system departs from a linear relationship between input and output. At this point, the system is said to be going into compression or beginning to saturate. Beyond this point, the linear relationship between input and output is no longer valid and the amplifier is no longer considered to be linear, or is at the point of –1dB departure from linearity. The output power of an amplifier cannot increase indefinitely and, when an increase in input power generates no discernible increase in output power, the amplifier is said to be saturated, and the output is no longer proportional to the input signal. This value is often referred to as Psat on a datasheet.

An amplifier increases the power of both the signal and the noise present at its input. Large amplification requirements can make a circuit susceptible to noise, distortion, and other non-linear effects. Spurious signal products are non-signal related and may be caused by a low level instability in the amplifier or introduced into the amplifier from external sources via the power supply, radiated interference, or other sources of intermodulation. When the signal drive to an amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output, which results in distortion.

A low-noise amplifier (LNA) amplifies a very low-power signal without significantly degrading its signal-to-noise ratio and are designed to minimize additional noise. LNAs are found in communications systems, medical instruments, and electronic equipment. LNAs are designed for low noise figure and are very small devices which cannot tolerate handle high input power. Although LNAs are manage low frequency signal slightly above the noise floor, they must also mitigate the presence of larger signals that cause intermodulation distortion. The RF limiter is used to protect a LNA in a receiver chain. For example, where the input power is within the limits of the LNA, the limiter, positioned before the LNA, is in a low loss state whereby it passes the RF signal to the input of the LNA with marginal insertion loss. When a more powerful, possibly damaging, signal incident occurs at the input of the LNA, the limiter attenuates the signal level to ensure that the LNA or other components in the chain are not damaged.

Specifically in receiver circuits, though often in other portions of an RF/microwave circuit, there are limits to the amount of power the circuit elements can handle. Often, electronic warfare (EW), signal intelligence (SIGINT), radar, and wireless communication systems that are interference limited have highly sensitive receivers that are susceptible to desensitization and damage from excessive signal power. In the environments these receivers are deployed, they are also prone to exposure to interference, jamming, and environmental factors that could threaten the operation of the receiver. This is where a limiting amplifier is able to ensure that the total signal power of a specified frequency range remains below a desired threshold regardless of the input signal power.

Though there are several different typologies, the two main types of limiting amplifiers either shunt excessive signal energy away from the sensitive circuitry, or rely on successive gain compression stages to deliver a consistent level of output signal energy. As with the introduction of any additional element in a signal chain, the linearity and frequency performance of a limiting amplifier are key when selecting such an amplifier for an application.

To find out more about Pasternack’s line of high performance limiting amplifiers, visit this link: https://www.pasternack.com/nsearch.aspx?keywords=limiting%20amplifier&Category=Limiting%20Amplifiers&view_type=grid

 

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Bits on Baluns – Part 3

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Basic Baluns

The type of balun used in microwave RF designs depends on the bandwidth required, the operating frequency, and the physical architecture of the design. As differential power dividers, baluns can be transformers, capacitively and/or magnetically coupled transmission lines, hybrid couplers, or used in a combination of power divider and inverter. Baluns are used in many applications from creating transitions between single ended and differential signals and in canceling mode noise and signals. The baluns most important property is in power balance and phase balance.

The flux coupled balun transformer is the most common type of balun which is basically two separate wires wound around a magnetic core with one side of the primary winding grounded thus creating an unbalanced condition on the primary side and a balanced condition on the secondary side. The secondary side can have an arbitrary ratio of turns to the primary side creating the arbitrary impedance ratio. The flux coupled balun transformer will induce an AC voltage in the secondary of n times the voltage in the primary, while the current will be n times smaller than in the primary, giving an output impedance of n2 as stated above, where n is the ratio of turns in the secondary to turns in the primary.

Wire wound flux coupled transformers often have a center tap in the secondary winding with a ground – connecting this point to the ground of the secondary system can improve the balance of the output.

For example, a flux coupled transformer is best suited to frequencies below 1 GHz but often experiences coupling loss at higher frequencies; magnetic materials have a large loss tangent which leads to high signal losses at microwave frequencies. Thus, the capacitive coupled transmission line balun, most often with a bifilar transmission line wrapped around a magnetic core, solves the high frequency problem with the low frequency magnetic coupling and the high frequency capacitive coupling as seen in the Guanella balun.

 

 

 

 

 

 

 

 

 

 

A balun that is often used in microwave applications is the Marchand balun. The video, Types of Spiral Baluns describes spiral baluns, Interwound, Symmetrical, and Marchand, and the design and simulation of GaAs MMIC planar spiral balun.

 

Classical Transformer Balun

In classical transformers, or isolation transformers, there are two separate windings of wire coils around the transformer core which can be empty or air, a magnetically neutral material like porcelain, or a magnetic conductor, or soft iron. The primary winding receives the input signal and the secondary winding sends out the converted signal. For ideal transformers, although the ratio of voltage to current will change in exact proportion to the square of the winding ratio, the power, measured in watts, remains identical.

Advantage: electrically separate windings for input and output allow these baluns to connect circuits whose ground-level voltages are subject to ground loops or are electrically incompatible.

Autotransformer or Voltage Balun

An autotransformer balun has one coil or two or more coils that have an electrical connection also wound on a ferrite rod or toroid. A single winding must have at least one extra electrical connection, or tap point, between the ends of the winding. The current sent into the balun through one pair of connections acts as a primary coil and magnetizes the core.

Advantage: unlike transformer-type baluns, an autotransformer balun provides a path for DC current to ground from every terminal.

Transmission-line Transformer or Choke Balun

Sometimes called a current balun, this type of balun ensures equal current on both sides of its output, but not necessarily equal voltage. The currents inside coax are equal and opposite so the magnetic fields are also equal, opposite, and mostly cancel. When a transformer balun is combined with the transmission-line transformer balun, the resulting device has very wideband operation. The Guanella transmission-line transformer is often combined with a balun to act as an impedance matching transformer.

Advantage: the choke balun prevents extra current flowing back along the transmission through inductive impedance.

Delay-line Balun

Using connected transmission lines of specific lengths with no transformer element, these delay-line baluns are usually built for narrow frequency ranges where the connected transmission line lengths are a multiple of a quarter wavelength of the intended frequency in the transmission line medium as seen in a coaxial connection to a balanced antenna.

Advantage: causes 180° phase shift and provides a balanced input.

Self-resonance Balun

Transformers made of real materials have a small capacitance between the primary and secondary windings, as well as between individual loops in any single winding, forming unwanted self-capacitance or parasitic capacitance.  When the electrical reactance of the self-inductance and self-capacitance in the balun are equal and opposite, resonance occurs. A balun of any design type operates poorly at frequencies at or above its resonance.  Design considerations for baluns are for the purpose of making the resonant frequency as far above the operating frequency as possible. As frequencies rise, the impedance of the parasitic capacitance drops until it’s magnitude equals that of the ideal inductance known as Self Resonant Frequency (SRF).

 

 

 

 

The inductance acts like an inductor up to its SRF then the impedance becomes very high and the inductor can be used as a choke to attenuate signals near the SRF.

To learn more about Pasternack’s line of baluns, visit: https://www.pasternack.com/nsearch.aspx?Category=Baluns&sort=y&view_type=grid

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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.

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Bits on Baluns

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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:

– Radio and baseband video

– Radars, transmitters, satellites

– 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.

To learn more about Pasternack’s coaxial and waveguide Zero Bias Schottky Diode Detectors, click here

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Exploring Types of RF Microwave Detectors

Pasternack Blog -

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.

Learn More about Pasternack’s RF detectors:

Waveguide Detectors:

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

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

Pasternack Blog -

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

Pasternack Blog -

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.

To learn more about Pasternack’s line of Limiting Amplifiers, visit this link  https://www.pasternack.com/limiting-amplifiers-category.aspx

 

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Introduction to Gunn Diode Oscillators

Pasternack Blog -

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.

Learn more about Pasternack’s Waveguide Gunn Diode Oscillators: https://www.pasternack.com/pages/rf-microwave-and-millimeter-wave-products/waveguide-gunn-diode-oscillators.html

 

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