Fiber optic amplifier

Optical fiber transmission of information has played a key role in increasing the capability of communication system to transmit information. Fiber optic communication utilizes optical transmitters, optical receivers and optical fiber, among other components, to transmit light signals through the fiber. A fiber optic amplifier is an optical device for amplifying a plurality of channels of signals so as to compensate for their loss when they propagate through an optical transmission line in an optical communication system. In general, a fiber optic amplifier comprises an optical fiber for amplification, doped with a rare-earth element, and a pumping light supply system for supplying pumping light to the optical fiber for amplification. The pumping light supply system usually includes a semiconductor laser and an optical coupler for guiding the pumping light into the optical fiber for amplification. In fiber-optics communication systems in practice today, repeaters are inserted in the transmission line at regular intervals to compensate for attenuation of the optical signal due to loss in the optical fiber. In a repeater, an optical signal is converted into an electrical signal by a photodiode and amplified by an electronic amplifier, and then converted into an optical signal to be delivered into the fiber-optic transmission line again. Erbium-doped amplifiers are made by doping a segment of the fiber with erbium and then exciting the erbium atoms to a high energy level through the introduction of pumping light. The energy is transferred gradually to signal light passing through the fiber segment during excitation, resulting in an amplification of the signal light upon exit from the amplifier. Fiber optic amplifiers can amplify signal light including one or more wavelengths within a predetermined wavelength band without converting them into electricity.

Operational amplifier

Operational amplifiers (op-amps) are high-gain DC coupled amplifiers with two inputs and a single output, and have been used as comparators, audio amplifiers, filters, etc. An operational amplifier is characterized by having two inputs, that is, an inverting or negative input and a non-inverting or positive input. The operational amplifier includes an output, that is, a single-ended amplifier, or two outputs, that is, a double-ended amplifier which is also known as a fully differential amplifier. Operational amplifiers are used in many electronic circuits to condition, manipulate and amplify signals. An typical operational amplifier amplifies a voltage difference on the inputs to generate a desired output voltage. The operating characteristics of a particular operational amplifier are dependent upon its circuit topology. Generally, the operational amplifier consists of a number of stages, each containing internal sub-stages. The operating class of the operational amplifiers is defined according to the polarization of the active elements that supply power to the load and can be divided, among the various ones existing, into class A, in which the active elements always operate in a conduction zone and are polarized at about the center of it, class B, in which the active elements are polarized at the locking limit of the conduction zone, class AB, in which the active elements are weakly polarized within the conduction zone, and in class C, in which the active elements operate far from the conduction zone. Single stage differential amplifier circuits are used in many electronic applications, such as programmable logic arrays. For programmable logic arrays differential amplifier circuits are designed to vary the common-mode gain and common-mode rejection ratio utilizing more than one amplifier stage and/or with additional complex electronic circuitry. Two-stage operational amplifiers typically include a first gain stage connected to inputs of the amplifier and a second gain stage driven by the first gain stage. The second gain stage provides the output of the amplifier. Both the first gain stage and the second gain stage are operated at respective bias currents. In metal oxide semiconductor (MOS) amplifiers, the first gain stage is typically operated at bias currents which are comparable in magnitude to the bias currents of the second gain stage so that maximum gain and bandwidth may be achieved. Multiple-stage operational amplifiers typically include a cascade of one or more gain stages and an output driver stage for driving an output load. The output stage is, for example, a Class AB amplifier that provides high low-frequency gain. To achieve an overall high open loop gain (e.g. greater than 150 dB), a multiple-stage opamp normally requires three or more gain stages.

Variable gain amplifier

A variable gain amplifier (VGA) is a device having a control input that can vary the gain of the device. In the wireless communication industry, particularly for wireless communications, variable gain amplifiers are well known as being used to provide amplification of either intermediate frequency (IF) or radio frequency (RF) signals. Variable gain amplifiers are frequently used in modern radio receivers to amplify or attenuate incoming signals to properly drive an associated analog-to-digital converter (A/D). Typically, the variable gain is distributed among radio frequency (RF), intermediate frequency (IF), and low-frequency or baseband circuits. Radio receivers, or tuners, are widely used in applications requiring the reception of electromagnetic energy. Applications can include broadcast receivers such as radio and television, set top boxes for cable television, receivers in local area networks, test and measurement equipment, radar receivers, air traffic control receivers, and microwave communication links among others. Transmission of the electromagnetic energy may be wirelined over a communication media or wireless by electromagnetic radio waves. In a radio frequency (RF) transceiver, the received signal typically has a high dynamic range. In order to supply a signal of constant amplitude to a baseband section of the transceiver, a variable gain amplifier (VGA) with equivalent or better dynamic range is required. In a variable gain amplifier, a control unit will provide a gain signal to the variable gain amplifier, and, based upon the gain signal, the variable gain amplifier will accordingly amplify an input signal by an amount corresponding to the gain signal, to obtain an amplifier output signal. In order for a signal of a constant level to be supplied to a base band terminal of the received signal, the variable gain amplifier must also have a high dynamic range. Variable gain amplifiers may be based on voltage, current or charge. Voltage mode amplifiers are probably the most widely used. Examples of such include complex circuits where the amplification is provided by discrete transconductance stages. Charge mode amplifiers are one alternative. However, such a circuit utilizes a discrete time technique that is not suitable for high-speed operation. In contrast, current mode amplifiers are less constrained by reduced power supplies and are able to operate at very high speeds.

Preamplifier and Servo amplifier

Preamplifier:

A preamplifier is an electronic component that is connected to a low-level signal source for providing suitable impedances and gain in an amplified signal. A singled ended preamplifier amplifies a single ended input signal by a gain factor such that the output signal is equal to the input signal multiplied by the gain factor. Differential preamplifiers are a particular type of preamplifier wherein the differential input signal comprises a positive rail component and a negative rail component. Preamplifier circuits are used in numerous applications. Typically, sound engineers use preamplifiers to amplify and process sound signals to achieve volume boosting while at the same time manipulating certain frequencies. High performance microphones require immediate preamplification of the signal generated by the microphone capsule. Microphones have low-level outputs, but microphone preamplifiers are generally designed to eliminate hum pick-up through the use of balanced-input circuitry, which electronically cancel any noise that is induced on both leads coming from the microphone. The ability of the output signal of a preamplifier circuit to faithfully reproduce the input signal is a function of many factors, including the bandwidth of the preamplifier, the frequency of the input signal, the impedance of the input system and transmission line impedance that provides the input signal, and the input impedance of the preamplifier.

Servo amplifier:

Servo amplifiers are used to drive servo motors in positioning devices. Multiple, servo-controlled DC motors which are reversible to provide a driving force in either of two rotational directions, are employed in various applications, such as magnetic disk device, for example. The motors are controlled by amplifier circuits which are pulse width modulated to provide precise motor control. In a magnetic disk device, the magnetic head is moved in two seek modes, a forward seek mode and a reverse seek mode, each corresponding to the direction of the motion of the magnetic head, i.e., corresponding to which side of the target track position the magnetic head exists when the target track position is commanded. In a servo system for a magnetic head in a magnetic disk device, a servo amplifier circuit having an amplifier portion and an inverting portion which inverts the polarity of the output of the amplifier, is used. Among the numerous general kinds of amplifiers, a common type is a straight DC amplifier with suitable phase-lead circuits to provide system damping, and output stages with the power capability to drive the motor in either direction. Pulse width modulated servo amplifiers are widely used in control systems for aircraft, marine, industrial and computer applications. Such amplifiers are protected against damage from accidental overload only by fuses or circuit breakers which must be replaced or reset before operation can be resumed after a malfunction. Many servo amplifiers are provided with resistors which generate large amounts of heat such as regenerative resistors which function to consume regenerative current. When installing a resistor which generates large amount of heat in a servo amplifier, the resistor is mounted at location separated from the internal components of the amplifier as well as at the rear of the amplifier in order to eliminate the effects of heat on the internal components of the amplifier.

Power amplifier and low noise amplifier

Power amplifier:

Amplifiers produce from an input signal, an output signal having an increased magnitude (i.e., gain). Essentially, an amplifier produces a constant output power at a higher level. Radio frequency (RF) power amplifiers are commonly used in numerous applications, such as base stations used in wireless communication systems. Modern wireless communication base stations transmit and receive radio frequency (RF) signals through the use of RF power amplifiers. RF power amplifiers are generally designed to provide maximum efficiency at the maximal output power. A typical radio transmitter uses a radio frequency (RF) power amplifier to amplify outbound signals for transmission by an antenna. A radio frequency power amplifier is typically constructed using a printed circuit board, with various components of the radio frequency power amplifier circuit installed on the printed circuit board. The RF amplifier circuit typically includes an input, an active element, a bias circuit element, an output matching network, and an output. RF power amplifiers characterized by a plurality of operating performance characteristics responsive to a quiescent operating point established by a direct current (DC) bias current. The linear power amplifier is driven by a direct current (DC) input voltage, provided for example by a battery in the transmitter, and the efficiency of the power amplifier is given by the ratio of the output power to the DC input power.

Low noise amplifier:

A low noise amplifier (LNA) is utilized in various aspects of wireless communications, including wireless LANs, cellular communications, and satellite communications. A typical receiver for a radio frequency signal (RF signal) comprises a combination of an amplifier and a mixer for signal amplification and frequency conversion. The amplifier, usually a low-noise amplifier (LNA), receives the RF signal, amplifies the RF signal and feeds the amplified RF signal to the mixer which in addition receives a local signal from a local oscillator (LO). A critical building block in a radio receiver is the low noise amplifier (LNA). The LNA amplifies the received signal and boosts its power above the noise level produced by subsequent circuits. An LNA provides a steady gain over a specified frequency bandwidth. One common application is the use of a LNA as the input stage of a receiving circuit, such as in a mobile communication device. In a radio frequency (RF) signal receiving apparatus such as a cellular phone and a base station of a wireless communication system, a received signal has very weak intensity and includes considerable noise mixed therein. As such, the performance of the LNA greatly affects the sensitivity of the radio receiver. The low noise amplifier is capable of decreasing most of the incoming noise and amplifying a desired signal within a certain frequency range to increase the signal to noise ratio (SNR) of the communication system and improve the quality of received signal as well. Depending on signal frequency, an LNA can be implemented as an open loop or closed-loop amplifier and may also have a requirement to match a specific source impedance.

power divider and filter

Power divider:

A large class of microwave components can be formed by combining two phase shifters and two fixed power dividers (combiners). A power divider and a power combiner may be made to operate over broad frequency bands at relatively high RF power levels. In the field of RF transmission, power requirements may exceed practical levels for a single amplifier, making it desirable to amplify a signal with multiple RF power amplifiers in parallel. An input power divider circuit is used to divide the signal and provide it to the inputs of the multiple amplifiers, while an output combiner provides the combined outputs of the multiple amplifiers to the next stage load. Many applications involve circuits requiring two input power signals to be combined into one output signal to be operated on by following circuits. Radio frequency power combiners are used for combining a number of RF inputs into a single output in a transmission system. The combiner also prevent a high current flow in a short circuited failed radio frequency power source, this current being derived from the remaining operative radio frequency power sources. Power combining techniques for radio frequency signals, including millimeter wavelength signals, have been accomplished in either a waveguide circuit or in a microstrip circuit.

Filter:

A radio frequency (RF) filter is designed to be tuned to pass energy at a specified resonant frequency. In microwave communications systems, it is often necessary to filter a relatively broadband microwave signal into its component sub-bands. In order to combine a number of RF transmitters, the RF signals from each transmitter must be isolated from one another to prevent intermodulation and possible damage to the transmitters. RF filters of the air-filled cavity type may be utilized to provide isolation between the RF transmitters. High-frequency filters, such as RF filters, are used to implement high-frequency circuits in the base stations of mobile networks, mobile phones and other radio transceivers. Other RF filter applications include the adapter circuits and filter circuits of transmitter and receiver amplifiers. RF cavity filters may be used in linear power amplifiers and radio equipment such as cellular base stations, among other things, to, for example, reduce undesired frequencies in an RF signal, or to delay an RF signal by a predetermined amount of time. Radio frequency (RF) filters and multiplexers having filters typically employ a plurality of resonators.

RF Microwave oscillator and microwave connector

MIcrowave oscillator:

Oscillator circuits generate periodic electrical signals by converting a fraction of the constant polarity power supply thereto into a periodic signal output without requiring a period signal input. Radio frequency (RF) oscillators are widely used for generating, tracking, cleaning, amplifying, and distributing RF carriers. Microwave oscillators are employed in wireless telecommunication equipment, such as for instance radio links or satellite transponders, as local oscillators for frequency converters. In particular, voltage-controlled oscillators with phase-locked loops are used for clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing. A microwave VCO is a microwave oscillator whose oscillation frequency is controllable by means of a voltage. RF oscillators and modulators typically must meet certain requirements in power and frequency output.

Microwave connector:

Electrical and electronic systems make use of connectors for coupling different portions of the system together. Such connectors are capable of being engaged and disengaged so that the different parts of the system may be separated. Radio frequency (RF) connectors are generally used to connect various components of RF equipment. Such RF connectors interconnect various components including coaxial cable and printed circuit boards. Connectors associated with RF communication systems typically use coaxial transmission line systems to conduct RF signals from one point to another. Coaxial radio frequency cables having hollow center conductors are generally used for many applications including land mobile, microwave broadcast and radar band frequencies. These coaxial transmission line systems employ connectors at their ends to connect the transmission line system to additional coaxial transmission line systems or various RF circuit assemblies. The technological advancement has been calling for broader bandwidths for the radio frequency and microwave equipment. As a result, the RF coaxial connectors, either on the coaxial cable ends or on the PC boards of signal devices, play a more and more important role in signal input and output. The RF connector has an inner conductor and tube-shaped outer conductor, which connect to the respective conductors of the cable. Radio frequency connectors typically comprise a solid, straight center pin, an extruded dielectric material, and a matable housing.

Dielectric resonator

Dielectric resonators are key elements for filters, low phase noise oscillators and frequency standards in current microwave communication technology. Applications of dielectric resonators in filter design have become more and more popular due to impressive advantages, such as small size, low weight, low loss (high Q), and common commercial availability. Dielectric resonators are used in microwave circuits for concentrating electric fields. They can be used to form filters, oscillators, triplexers and other circuits. The higher the dielectric constant of the dielectric material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Dielectric resonators possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size. Microwave oscillators are used in transmission systems and more particularly close to the antenna in order to carry out a frequency transposition between an intermediate frequency band and a transmission frequency band. Dielectric resonator oscillators are commonly used in high-precision RF and microwave systems to generate high-frequency signals of extremely good spectral purity. A typical dielectric resonator for use in the microwave band is formed using a rectangular or cylindrical dielectric block having a coaxial through-hole wherein an inner conductor is formed on the inner surface of the through-hole and an outer conductor is formed on the outer surface of the dielectric block.

RF circulator

A radio frequency (RF) or microwave circulator is used to pass RF signals and block returned signals. The circulator is a three-terminal device which passes signals input to one port to the next port in a rotational fashion without allowing signals to pass in the opposite rotation. Circulators generally contain a microwave circuit comprising an arrangement of conductors and ferrite blocks, and a magnetic circuit providing a magnetic biasing field applied to the ferrite blocks that act as a non-reciprocal media for propagating radio frequency signals throughout the device. RF circulators are suitable for essentially any radio frequency (RF) application, including communications. RF circulators are also useful as isolators, easily made by tying the third circulator port to ground through a resistor. RF circulators can be built in resonant structures such as radio frequency resonant cavities and in waveguide at higher frequencies. Circulators may also be realized in planar configuration using stripline or microstrip technology which employ a planar resonating element between two ground plane conductors (stripline) or coupled to a single ground plane conductor (microstrip). Radio frequency and microwave circulators employ a DC-biasing magnetic field generated in ferrite material enveloping a conductor to provide at least one non-reciprocal transmission path between signal ports on a network. In a radar or communications system it is often advantageous to couple multiple devices, such as a transmitter and a receiver, to the same antenna. A common approach uses a radio frequency circulator to isolate the transmitted signal from the received signal to avoid overloading the receiver front end. An array antenna can include a plurality of radio frequency (RF) circulators disposed in an array in a manner in which RF signals can be received from or transmitted to the same individual radiator.

Circulator

A circulator is a passive non-reciprocal three- or four-port device, in which microwave or radio frequency power entering any port is transmitted to the next port in rotation (only). Thus, to within a phase-factor, the scattering matrix for an ideal three-port circulator is

When one port of a three-port circulator is terminated in a matched load, it can be used as an isolator, since a signal can travel in only one direction between the remaining ports.[1]

There are circulators for LF, VHF, UHF, microwave frequencies and for light, the latter being used in optical fiber networks. Circulators fall into two main classes: 4-port waveguide circulators based on Faraday rotation of waves propagating in a magnetised material, and 3-port "Y-junction" circulators based on cancellation of waves propagating over two different paths near a magnetised material. Waveguide circulators may be of either type, while more compact devices based on striplines are of the 3-port type. Sometimes two or more Y-junctions are combined in a single component to give four or more ports, but these differ in behaviour from a true 4-port circulator.

In radar, circulators are used to route outgoing and incoming signals between the antenna, the transmitter and the receiver. In a simple system, this function could be performed by a switch that alternates between connecting the antenna to the transmitter and to the receiver. The use of chirped pulses and a high dynamic range may lead to temporal overlap of the sent and received pulses, however, requiring a circulator for this function.

Radio frequency circulators are composed of magnetised ferrite materials. A permanent magnet produces the magnetic flux through the waveguide. Ferrimagnetic garnet crystal is used in optical circulators.

There have also been investigations into making "active circulators" which are based on electronics rather than passive materials. However, the power handling capability and linearity and signal to noise ratio of transistors is not as high as those made from ferrites. It seems that transistors are the only (space efficient) solution for low frequencies.

A waveguide circulator used as an isolator by placing a matched load on port 3. The label on the permanent magnet indicates the direction of circulation

Microwave Circuit Fabrication

The new £1M microwave circuit fabrication laboratory is targeted at providing fabrication facilities for system-in-package solutions (inc LTCC) for communications and sensing applications. The major items of equipment are a laser prototyping system, automatic screen printer and isostatic laminator. They are supported by a full range of standard fabrication equipment such as a spinner, etching and developing tanks, UV exposure system, digital measuring microscope, laminar flow cabinet, etc.

Thin-film inductors

Thin-film inductors offer many practical advantages over air wound cores (though of course they can not achieve the same Q factors). Thin-film inductors are easier to pick and place in a SMT process than air core inductors. They can easily be processed in the IR, vapor phase and wave processes typically used in today’s assembly. Further, they will maintain value through those processes, as well as through handling and through high-vibration environments. Though they can not be tuned in circuit like air cores can, thin-film inductors can be used to replace air cores once an exact inductance value is determined for proper circuit function (assuming the Q factor is acceptable).

As in the case of thin-film capacitors, ESR and loss are dramatically reduced due to line width control and dielectric laydown quality/accuracy. This results in an end product which can be as small as an 0402 package with virtually any inductance value imaginable plus tolerance accuracy as close as 0.05 nH. Further, consistent metalization allows relatively high-current-carrying capability in thin-film inductors – up to 1,000 mA depending on device selected.

Other thin-film structures:

A variety of other structures have been created with the knowledge and process capability gained in manufacturing thin-film capacitors and inductors. Among these are couplers and harmonic low pass filters.

Miniature SMT couplers are a welcome addition to the designer’s toolbox. These devices provide high directivity, repeatable coupling with low insertion loss. They handle large amounts of power in a PCB footprint as small as 0402 with low profiles. As in the case with other thin-film components their electrical response and consistency on a lot-to-lot and inter lot basis is unmatched.

An example of thin-film inductor implementation might be in frequency compensation on broadband amplifiers. Previously, a resistor/inductor combination was used. As in the case of thin-film capacitors, the use of a thin-film inductor can reduce the number of components used in the circuit thereby saving size, weight, assembly and cost as well as improving reliability.

Just as thin-film capacitors, thin-film inductors are limited in maximum value.

In particular, a thin-film inductor provides designers with a good solution at extremely high frequencies. A common example is in multi-gigahertz oscillators. At high frequencies, wire-wound inductors may simply not be available, due to the absence of cost effective manufacturing techniques to build such low value wire-wounds.

At this point the designer is left with the choice of creating a low value inductor with serpentine PC board trace designs or choosing a miniature SMT thin-film inductor.

Though a PCB-based solution can be considered low cost, it uses valuable board space, and can vary based upon the PCB supplier. The thin-film inductor will have the same extremely repeatable and consistent frequency response on a lot-to-lot basis and on an inter lot basis as that of thin-film capacitors.

Band-reject filters

A real-world example of this is band-reject filters. A band-reject filter is a circuit designed to block the passage of signals in a specific spectrum of RF frequency while allowing other signals to pass un-attenuated. Other common names are notch filters, bandstop filters or band suppression filters. A common implementation of a band-reject filter is between a power amplifier and matching circuit prior to an antenna.

For example, in a typical application, the narrow notch filter is used to attenuate noise from heterodynes and harmonics unintentionally generated by complex, multiband, wide coverage receivers. The use of a single high-quality thin-film capacitor can essentially replace the use of six components in a twin T design due to thin film’s near-ideal characteristics.

Thin-film capacitors (see Fig. 1) have an additional performance advantage not discussed earlier: a single resonant point response due to the fact that the devices use a single-layer dielectric design packaged as a multilayer ceramic capacitor (MLCC). A few of the thin-film capacitor’s S21 forward transmission loss characteristic curves are shown in Fig. 2

Fig. 2. S21 forward transmission loss characteristics curve.When using a thin-film capacitor, manufacturers can reap the electrical benefits of a single-layer capacitor while being rewarded with the processing ease of an MLCC-type component. A thin-film capacitor’s consistent performance impact on electrodes and oxide thickness and quality impact on dielectric K is shown in Fig. 3.

Fig. 3. A thin-film capacitor has an extremely repeatable frequency response compared to MLCCs.

It is also important to realize the limitations of thin-film capacitors used as band-reject filters. Since thin-film capacitors are typically only available in low capacitance values they are limited to relatively high-frequency band-reject filter designs. If dealing with low frequency designs, other filter methods must be utilized typically using high-Q multilayer RF capacitors.

RoF-Predistortion circuit

Radio-over-Fibre (RoF) networks performances are typically influenced by the non-linearities due to the modulating devices: lasers in direct modulation schemes, electro-optical modulators in external modulation schemes. A completely analog predistortion circuit has been developed to compensate the non-linearities generated by directly modulated semiconductor lasers. This low-cost circuit, of industrial interest, has a multi-decade correction capability, from 300 MHz up to 2 GHz; thus, it can be used effectively to reduce second and third order harmonic (HD2 and HD3) and intermodulation (IM2 and IM3) distortions generated inside the CATV, GSM/DCS, GPRS and UMTS bands. An average broadband compensation of at least 10 dB has been recorded during measurements; the compensation magnitude can be further enhanced by specifically tuning the circuit for CATV or cellular bands, alternatively.

GaAs MMIC prototypes and Radio-over-Fibre modulating devices

GaAs MMIC prototypes:

Simple GaAs MMIC prototypes have been designed and experimented with the purpose to set up and refine both the design tools and the test and measurement facilities. The chips have been manufactured by the Gec-Marconi foundry using the F20 process.

Radio-over-Fibre modulating devices

Radio-over-Fibre (RoF) networks integrate optical fibre backbones and wireless cellular networks to provide broadband services to mobile, nomadic and fixed users. To generate and distribute RFmodulated optical signals, two schemes are used: direct modulation schemes, where a semiconductor laser is directly intensity-modulated by the RF-signal, and external modulation schemes, where the optical carrier generated by the laser is subsequently modulated by the RFinformation through an electro-optical modulator. The main modulating devices of such schemes have been analysed and modelled:

• the optical behaviour of an electro-absorption modulator has been analysed by using the Compact-2D-FDTD method;
• a circuit model has been developed and used to study the non-linear behaviour of directly modulated lasers.

SPDT switch and Quasi-optical frequency multipliers

SPDT switch:

A millimeter-wave “Single Pole, Double Throw” (SPDT) switch, realized with a Si-MMIC microstrip technology has been studied. The analysis has been performed with both commercial, equivalent circuit based simulator and mixed electromagnetic-device simulator. The sinusoidal RF signal (ampl. 0.2 V, freq. 76 GHz) is injected at one end of the structure, and then forwarded along either output branches, depending on the bias supplied by the square-wave generator (ampl. 2.5 V, freq. 214 MHz). Microstrip stubs and matched loads complete the structure. To isolate the inactive

branch, {p-i-n} diodes have been used, due to their high impedance in the off-state, and to the compatibility with the high-resistivity silicon, planar process. Feasibility of such Si-MMICs, suitable for operating frequencies up to 100 GHz, has been demonstrated.

Quasi-optical frequency multipliers:

The basic quasi-optical frequency doubler is shown in Fig.6a. In this device the frequency multiplication is achieved by a diode bridge connected to the center of two /2-dipoles. These dipoles are placed in a cross configuration (referred to as crossed dipole): the longest one receives the incoming power at the fundamental frequency (3.5 GHz in our prototype), while the shortest one transmits the generated power at the doubled frequency (7.0 GHz in our prototype). The advantages of such structure are the following. First, a good isolation between input and output signals is achieved since incoming and outgoing waves are orthogonally polarized. Second, the symmetry of the structure has been exploited to obtain a balanced topology which is characterized by a good conversion efficiency (only even harmonics are generated within the multiplier). However, the conversion efficiency of the above structure is intrinsically limited by the omnidirectional nature of both receiving and transmitting antennas. To overcome the above mentioned problem, the front-to-back ratio (and thus the directivity) of both receiving and transmitting dipoles has been increased by additional layers with parasitic elements. The resulting structure is essentially constituted by two Yagi-Uda antennas working at orthogonal polarizations. The new, multi-layer structure can be better understood with the example of Fig. 6b. In this case only two parasitic elements are adopted, one for each antenna of the frequency doubler. The longest element (left of the photograph) is used as a reflector for the fundamental frequency dipole; the shortest element (right of the photograph) is used as reflector for the doubled frequency dipole.

Coplanar waveguide to slotline transitions

The uniplanar configuration has having a growing popularity in the area of microwave integrated circuits. Fundamentals uniplanar configurations are the coplanar waveguide (CPW) and the slotline(SL). Well known advantages with respect to the microstrip line configuration are: easy shunt and series mounting, elimination of via holes, reduced radiation loss (for CPW), use of thicker substrates overcompensate the necessity for air bridges to suppress the spurious slotline mode in the CPW. The lack of accurate design tools however has prevented an extensive application of uniplanar technology, of which the transition between CPW and SL is a key element. The FDTD method has been employed to analyze several types of CPW-SL transitions including the effects of the coaxial connectors, air bridges, shielding effects as well as interactions between discontinuities. Theoretical simulations show excellent agreement with experimental data appeared in the literature.

Hybrid integrated circuit bonding:

The bonding wire interconnection is a key element for the fabrication of hybrid integrated circuits. It is employed to connect solid state devices to passive circuit elements as well as multichip modules. In spite of its small physical length, when millimeter-wave operations are required, the discontinuity introduced by the bonding wire can significantly affect the performance of the whole circuit. At the University of Perugia, the bonding wire interconnection has been studied from the point of view of its modeling and electrical characterization. In particular, two electrical models of the bonding wire have been developed. First, the Finite Difference Time Domain (FDTD) method has been adopted to rigorously analyze several bonding wire configurations (including multi-chip, single- and double-wire structures) and to produce reference results. Then, a quasi-static model of the bonding wire has been derived. This model is based on the representation of the structure with four uniform transmission line sections. Such an approach is suitable for commercial microwave CAD tools since the model parameters can be evaluated analytically from the dimensions of the structure.

Packaged microwave integrated circuits

Because of the increasing operating frequency and complexity of the circuits, the prediction of the package behavior and of the interaction between the package and the enclosed circuit itself is becoming more and more significant. In most of the commercial microwave simulators, the presence of the package is taken into account by simply considering the reactance introduced by the electric walls close to the circuit; unfortunately, when the package supports resonant modes and when electromagnetic couplings between different parts of the circuit are present, this approach is not valid anymore. To overcome this limitation the full-wave FDTD simulator has successfully been used to investigate packaged MMIC circuits. In particular, the behaviour of packaged single and coupled MMIC via-hole grounds have been investigated; the theoretical analysis has been compared with experimental results showing excellent agreement. Moreover, since the package introduces resonances, we have investigated several different possibilities to choke off these resonances. It is shown that the common practice of inserting a damping layer just below the lid is often not effective. In particular, the importance of placing damping layers also on the side walls is demonstrated.

MMIC commercial package:

The behaviour of a commercial MMIC package operating in the range 0-40 GHz has been investigated. It consists of a mechanical support of kovar with a fused quartz substrate above it. The area where the MMIC must be placed is enclosed between glass side walls and a top metallic lid. The latter is connected to a ground plane by via-holes through the glass walls. Bias lines and metal backed CPW terminations are also present. It is interesting to note that this package is substantially an open structure. Within the operating frequency range, the electromagnetic shielding is realized by metallic via-holes connecting the lid with the ground plane. In order to compare theoretical and experimental results, a simple microstrip transmission line, inserted between the input and output ports has been simulated.

What is...

What is the Microwave Circuit Designer’s Duty for Wireless Systems?

• Designing filters, mixers, amplifiers, oscillators, matching networks, packaging, and system level design of the Analog and Digital Systems
• Designing the antennas and matching networks
• Propagation Affects (multipath, signal diversity)

What are the principal future challenges?

• Miniaturized and low cost microwave circuitry (applies to cellular, PCS, GPS, on-board radar).
• Direct hand held unit communication with low earth orbit and mid earth orbit satellites
• Direct high-speed digital communication with low earth orbit and mid earth orbit with portable computers.
• Miniature high-power amplifiers with low signal to noise ratios, miniature high-Q filters, novel printed antennas, new packaging technology for combining RF and digital circuitry, reducing parasitics, improved modeling and analysis capabilities

Microwave Circuit Design.

• Microwave Circuits are composed of distributed elements with dimensions such that the voltage and phase over the length of the device can vary significantly.
• By modifying the lengths and dimensions of the device, the line voltage (and current) amplitude and phase can be effectively controlled in a manner to obtain a specifically desired frequency response of the device.
• Microwave Circuits are used to design microwave amplifiers, oscillators, filters, power dividers/combiners, multiplexers, antennas and mixers.
• The necessary tools for the analysis and design of microwave circuit devices require an understanding of: transmission lines, two-port networks (Z, Y, ABCD Parameters), network theory (S-parameters), impedance matching, and filter design. Much of the course will develop a deeper understanding of these fields while integrating specific applications of microwave circuit design.

Microwave circuits find their use in a plethora of applications, including:

1. Wireless Communications (cellular, PCS)
2. Wireless Networking
3. Digital Communications (Ground-Ground, Satellite- Ground, Satellite-Satellite)
4. Radar Systems (ground based, airborne, personal vehicles) – Target detection and identification, imaging (e.g., SAR)
5. Deep Space Communications
6. Medical Imaging and treatment
7. Radio Spectrometry

videos

RADIO AND MICROWAVE FREQUENCY BANDS.

Frequency Multipliers

Hz (hertz) cycles per second 1 Hz

kHz (kilohertz) one thousand hertz 1,000 Hz

MHz (megahertz) one million hertz 1,000,000 Hz

GHz (gigahertz) one billion hertz 1,000,000,000 Hz

THz (terahertz) one trillion hertz

1,000000,000,000 Hz

Radio Band Designations

Frequency Wavelength Radio Band designation

30 - 300 Hz 10 - 1Mm ELF (extremely low frequency)

300 - 3000 Hz 1000 - 100 km ULF (ultra low frequency)

3 - 30 kHz 100 - 10 km VLF (very low frequency)

30 - 300 kHz 10 - 1 km LF (low frequency)

300 - 3000 kHz 1000 - 100 m MF (medium frequency)

3 - 30 MHz 100 - 10 m HF (high frequency)

30 - 300 MHz 10 - 1 m VHF (very high frequency)

300 - 3000 MHz 100 - 10 cm UHF (ultra high frequency)

3 - 30 GHz 10 - 1 cm SHF (super high frequency)

30 - 300 GHz 10 - 1 mm EHF (extremely high frequency)

IEEE Radar Band Designations

Frequency Wavelength IEEE Radar Band designation

1 - 2 GHz 30 - 15 cm L Band

2 - 4 GHz 15 - 7.5 cm S Band

4 - 8 GHz 7.5 - 3.75 cm C Band

8 - 12 GHz 3.75 - 2.50 cm X Band

12 - 18 GHz 2.5 - 1.67 cm Ku Band

18 - 27 GHz 1.67 - 1.11 cm K Band

27 - 40 GHz 11.1 - 7.5 mm Ka Band

40 - 75 GHz V Band

75 - 110 GHz W Band

110 - 300 GHz mm Band

300 - 3000 GHz u mm Band

Satellite TVRO Band Designations

Frequency Wavelength Satellite TVRO Band

1700 - 3000 MHz S-Band

3700 - 4200 MHz C-Band

10.9 - 11.75 GHz Ku1-Band

11.75 - 12.5 GHz Ku2-Band (DBS)

12.5 - 12.75 GHz Ku3-Band

18.0 - 20.0 GHz Ka-Band

Military Electronic Countermeasures Band Designations

Frequency Wavelength IEEE Radar Band designation

30 - 250 MHz A Band

250 - 500 MHz B Band

500 - 1,000 MHz C Band

1 - 2 GHz D Band

2 - 3 GHz E Band

3 - 4 GHz F Band

4 - 6 GHz G Band

6 - 8 GHz H Band

8 - 10 GHz I Band

10 - 20 GHz J Band

20 - 40 GHz K Band

40 - 60 GHz L Band

60 - 100 GHz M Band

Traffic Radar Frequencies

Traffic Radar Frequency Bands

Band Frequency Wavelength Notes

S 2.455 GHz 4.8 in

12 cm obsolete

X 10.525 GHz ±25 MHz 1.1 in

2.8 cm one 50 MHz channel

Ku 13.450 GHz 0.88 in

2.2 cm no known systems

K 24.125 GHz ±100 MHz 0.49 in

1.2 cm one 200 MHz channel

Europe and some US systems

K 24.150 GHz ±100 MHz 0.49 in

1.2 cm one 200 MHz channel

Ka 33.4 - 36.0 GHz 0.35 - 0.33 in

9 - 8.3 mm 13 channels; 200 MHz/ch

IR -- Infrared 332 THz 904 nm Laser Radar

Frequency Band Designations

Military Radar Bands

Military radar band nomenclature (L, S, C, X, Ku, K and Ka bands) originated during World War II as a secret code so scientists and engineers could talk about frequencies without divulging them. After the war the codes were declassified, millimeter (mm) was added, and the designations were eventually adopted by the IEEE -- Institute of Electric and Electronic Engineers. Military radar band nomenclature is widely used today in radar, satellite and terrestrial communications, and electronic countermeasure applications, both military and commercial.

Military Radar Bands

Radar Band Frequency Notes

HF 3 - 30 MHz High Frequency

VHF 30 - 300 MHz Very High Frequency

UHF 300 - 1000 MHz Ultra High Frequency

L 1 - 2 GHz

S 2 - 4 GHz

C 4 - 8 GHz

X 8 - 12 GHz

Ku 12 - 18 GHz

K 18 - 27 GHz

Ka 27 - 40 GHz

mm 40 - 300 GHz millimeter wavelength

Military HF, VHF, UHF same as Radio Band HF, VHF, UHF respectively.

The International Telecommunications Union (ITU) specifies bands designated for radar systems as described in the table below. The ITU bands are sub-bands of military designations.

International Telecommunications Union Radar Bands

ITU Band Frequency

VHF 138 - 144 MHz

216 - 225 MHz

UHF 420 - 450 MHz

890 - 942 MHz

L 1.215 - 1.400 GHz

S 2.3 - 2.5 GHz

2.7 - 3.7 GHz

C 5.250 - 5.925 GHz

X 8.500 - 10.680 GHz

Ku 13.4 - 14.0 GHz

15.7 - 17.7 GHz

K 24.05 - 24.25 GHz

Ka 33.4 - 36.0 GHz

VHF -- Very High Frequency

UHF -- Ultra High Frequency

Radio Bands

Radio band designations are summarized below. Note that the radio band chart includes wavelength. In the early days of radio it was easier to measure wavelength than frequency.

Radio Frequency Bands

Band Nomenclature Frequency Wavelength

ELF Extremely Low Frequency 3 - 30 Hz 100,000 - 10,000 km

SLF Super Low Frequency 30 - 300 Hz 10,000 - 1,000 km

ULF Ultra Low Frequency 300 - 3000 Hz 1,000 - 100 km

VLF Very Low Frequency 3 - 30 kHz 100 - 10 km

LF Low Frequency 30 - 300 kHz 10 - 1 km

MF Medium Frequency 300 - 3000 kHz 1 km - 100 m

HF High Frequency 3 - 30 MHz 100 - 10 m

VHF Very High Frequency 30 - 300 MHz 10 - 1 m

UHF Ultra High Frequency 300 - 3000 MHz 1 m - 10 cm

SHF Super High Frequency 3 - 30 GHz 10 - 1 cm

EHF Extremely High Frequency 30 - 300 GHz 1 cm - 1 mm

ECM Bands

The electronic countermeasures (ECM) industry occasionally refers to band designations as described below.

ECM Bands

Band Frequency

A 30 - 250 MHz

B 250 - 500 MHz

C 500 - 1,000 MHz

D 1 - 2 GHz

E 2 - 3 GHz

F 3 - 4 GHz

G 4 - 6 GHz

H 6 - 8 GHz

I 8 - 10 GHz

J 10 - 20 GHz

K 20 - 40 GHz

L 40 - 60 GHz

M 60 - 100 GHz