Radio Frequency and Microwave Design and Development

These are a few areas in radio frequency (RF) and microwave engineering that demonstrate FCL's capability and the work it has been engaged with. Please contact FCL for more information.

From Components Through to Major Systems

FCL has supported clients in many RF and microwave specialisations on projects ranging from circuit level surface mounted components through to major fixed and deployable systems.

FCL has designed and developed several PCB circuits with architectures down to discrete components and ICs. These have had numerous functions: RF, digital, mixed signal and field programmable gate arrays (FPGAs). IC functions encountered have included attenuators, phase shifters, synthesizers, IQ modulators, Ethernet interfaces, amplifiers, diplexers, switches and varieties of DACs and ADCs.

Common requirements for deliverable hardware include de-bugging, enhanced performance, smaller size, testability and reduced production costs. For example, there might be a particular issue, such as a noise, spurious or electromagnetic compatibility (EMC) problem, that requires a robust, fully verified and documented solution for a product upgrade. FCL has successfully solved many challenges such as these.

FCL's experience extends to optical transmission over monomode optical fiber (1310/1550 nm) and multi-mode optical fiber (850 nm). Monomode optical fibers are typically used in high capacity telecommunications infrastructure compliant with synchronous optical networks (SONET) or synchronous digital hierarchy (SDH), and RF over optical fiber (ROF). Support has also been provided to a project using multi-mode optical fiber systems. These have a much shorter range and smaller capacity than monomode but they are more robust with generally cheaper parts because the alignment precision of monomode is not necessary.

FCL is familiar with both the indirect and direct types of optical modulation. The former, more elaborate and expensive approach, is suited to the high capacity telecommunications and very wideband systems. The latter modulation is popular in commercial equipment used to extend cellular and terrestrial radio coverage, for example inside tunnels and large buildings. FCL has experience of the common optical components used in both applications. Examples are laser diode sources, photodiodes, WDM filters (add/drop multiplexers), optical connectors and the fiber optic cables themselves. Experience is offered with dense wavelength division multiplex (DWDM) filters, circulators, attenuators, Mach Zehnder optical intensity modulators and erbium doped fiber amplifiers (EDFAs).

Some clients have engaged FCL to develop equipment-based units and sub-units. Generally these have included bought in commercial-off-the-shelf (COTS) connectorised components, the inter-connections being effected either directly using adaptors or with short lengths of semi-rigid or similar low VSWR coaxial cable.

At system level, FCL has been a key member of a client's team responsible for the implementation of a major communications subsystem, part of a large defence ministry procurement. Other systems projects have been with point to point line-of-sight (LOS) digital communications and satellite communications (ground and space based equipment).

One client was a terrestrial broadcast service provider with whom FCL's job was to visit antenna sites and deal with both planned and unplanned maintenance issues to improve the quality of services to all customers.

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Active and Passive Components

FCL has extensive experience with both active and passive components.

The active components have encompassed amplifiers, synthesised signal generators, oscillators, VCOs and signal conversion ICs (ADCs and DACs). The active (discrete) devices have included silicon bipolar, field effect transistors (FETs), junction FETs (JFETs), metal semiconductor FETs (MESFETs), metal oxide semiconductor FETs (MOSFETs) and pseudomorphic high electron mobility transistors (PHEMTs). Several amplifiers have been developed: low noise (MESFET and PHEMT), class AB power amplifiers and general purpose examples. VHF and UHF oscillators have initially been developed as free running versions based on the Clapp and Colpitts architectures and then converted to voltage control (VCOs) by adding suitably biased varactor tuning diodes.

The passive components encountered include: antennas, couplers, splitters, filters, diplexers, duplexers, circulators, modulators and transmission lines of various types. FCL has a good grasp of transmission line theory and many of the different types used including transverse electric-magnetic (TEM), quasi-TEM and non-TEM modes (TE and TM conductor and dielectric waveguides). Examples include: coaxial cables, microstrip, co-planar waveguide, stripline, conductor waveguide (rectangular and elliptical) and dielectric waveguides. One project was dedicated to 'beam' waveguide. This achieved particularly low loss transmission just below 100 GHz using Gaussian-Laguerre quasi-TEM beams utilising dielectric lenses and ellipsoid profiled reflectors.

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Antennas and Propagation

FCL has a good understanding of antenna and propagation theory and many of the common antenna-related parameters such as gain, directivity, beamwidth, sidelobes, efficiency, polarisation, near-field and far-field properties. It has been engaged to predict analog and digital radio channel propagation performance across different paths: terrestrial (flat earth and multipath-rich), free space, microwave LOS, tropospheric, ionospheric and general non-line-of-sight (NLOS). It is also understands the general propagation mechanisms that typically exist across a range of frequencies.

FCL's early work with antennas and terrestrial propagation was across the VHF and UHF bands. It worked with directional element antennas across both spectra and more recently with multi-element passive and active phased arrays. Power levels were modest with the earlier hardware (less than 100 W transmit) moving on later to analogue TV broadcast antennas rated at up to 40 kW (peak vision carrier). Propagation predictions were based on the criteria from one of Bullington's classic papers to account for free-space path loss, ground reflections, diffraction, refraction and atmospheric absorption. This was followed by more prediction work covering the high frequency (HF) frequency range for a backup defence communications system, operating in both point to point and point to multipoint modes, so it became familiar with using HF channel prediction charts according to the frequency band, propagation path, season and time of day.

Much of FCL's work has been with microwave (passive reflector) and active antennas. The passive examples included cassegrain, Gregorian and parabolic offset feeds: designed, built and tested, operating at Ka-band. These used corrugated (scalar) horns, conical 'Potter' horns and quasi-optic (reflector and lens) beam waveguides for the feed elements. The far field radiation tests gave excellent results for gain, directivity, cross polar isolation and sidelobe performance. FCL has also specified passive microwave antennas for ground-satellite links and line-of-sight (LOS) digital terrestrial microwave links.

More recent work has been directed at the many situations where radio wave propagation is by 'non line-of-sight' (NLOS), even actually exploiting multipath propagation, such as those operating under the WiFi (IEEE 802.11), WiMAX (IEEE 802.16), low rate wireless personal area networks (IEEE 802.15.4) and DAB (Eureka 147) standards. These newer technologies are specifically designed to accommodate multipath propagation. Examples include those using the rake receiver in the 3G universal mobile telecommunications service (UMTS) and the orthogonal frequency division multiplexing (OFDM) techniques used in 4G, long term evolution (LTE/LTE-A) and digital terrestrial broadcasting (DVB-T, DVB-T2 and DAB). One study delivered to a client used algorithms developed around Rician and Rayleigh fading channel models in a heavy multipath environment, based on one of the WiMAX standards and potentially suitable for a new mobile communications system.

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Design and Development

FCL is very well equipped to support clients' requirements for RF, microwave and millimetre wave design and development. FCL also has a good grasp of electromagnetic field theory, starting with Maxwell's equations, as well as hardware hands-on experience on many projects. Several of these have operated at frequencies extending into microwaves: C, X, Ku and Ka-bands, as well as the millimetre wave bands approaching 100 GHz. The work at the millimetre wave frequencies was on passive components including horn antennas, harmonic mixers, dielectric antennas and beam waveguide feeds (reflector and lens types). One project was to design devices for performing lens surface phase and amplitude measurements at about 30 GHz, the output being fed to an HP 8510 vector network analyzer (VNA) for analysis. The VNA included the inverse fast Fourier transform capability to convert from the frequency to the time domain, thereby assisting in analysing the reflections at specific discontinuities along the transmission line under test. The work also included investigations into the transmission of fundamental (Gaussian) and higher order (Gaussian-Laguerre) beam waveguide modes for the transmission of two dimensional monopulse tracking information. FCL has a good appreciation of the challenges encountered at these frequencies both for TEM and non-TEM waves. The phenomena encountered have included: conductor and dielectric reflections, refraction, higher order waveguide modes, skin depth limitations, absorption and diffraction.

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Mixers and Switches

FCL has designed, built and tested several RF low noise amplifiers (Si bipolar, GaAs MESFET and PHEMT with input noise matching), power amplifiers (class B and AB push pull) and general purpose class A types for buffering and matching applications. The frequencies covered were mostly VHF/UHF but also extending into the microwave spectrum around Ku-band. FCL is familiar with many of the combining techniques used widely for solid state power amplifiers and low noise amplifiers to improve linearity, simplify redundancy provision and suppress harmonics. FCL has designed these products to requirements for stability, isolation, spurious, gain-frequency, gain-temperature, return loss - frequency, P1dB, IP3, NF, IMDs and AM-PM conversion.

FCL has developed double balanced mixers including characterisation and optimisation of the internal ferrite transformers for local oscillator isolation, conversion loss and noise figure. This work has spun off into the development of 'traditional' and transmission line transformers, using high frequency ferrite formers, for use in combiners and splitters as well as mixers. At microwave frequencies on microstrip FCL has developed balanced push-pull mixers using branch line couplers and packaged Schottky diodes, single ended mixers and active mixers .

Early work performed included the development of PIN diode switches on alumina microstrip operating at Ku band for a space hardware product. Later assignments included similar configurations of PIN diode switches but used in series and shunt configurations at VHF and requiring high bias voltages to achieve the necessary isolations.

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Filters, Oscillators, Modulators, Demodulators

FCL has designed, built and tested many lumped element LC filters based on filter tables (Williams and Taylor) for operation in the HF and VHF spectra. These included lowpass, bandpass (narrow and wide) and an absorptive notch. The applications have included roofing filters, IF passband, transmit rejection, image rejection and discrete spurious rejection. The absorptive notch was used to improve the useable dynamic range of a spectrum analyzer by absorbing an unwanted high level carrier nearby in frequency to those being measured. FCL has also recommended UHF filters for procurement including waveguide, ceramic resonator and surface acoustic wave (SAW) types.

Much of FCL's experience has been with oscillators of various types: fixed ovenised high stability crystals, free running Colpitts and Clapp architectures with provisioning of frequency control using hyperabrupt varactor diodes (VCOs). These have been developed from component level into complete synthesisers designed around dedicated synthesizer ICs whilst overcoming the challenges of jitter-dominated phase noise, lock time, frequency pulling, spurious, stability and control interfacing. Most of the circuit level design and development work has been at VHF, with higher frequencies tending to be implemented in the more modular forms.

Many digital receivers and transmitters today have some form of quadrature modulation or demodulation process included, referred to as in-phase/in-quadrature (IQ) mixers or modulators. A recent project was based around an IQ modulator whose baseband I and Q components were supplied by a dual output DAC. It was possible to control the digital input to the DAC to marginally deviate the analog output signals from perfect quadrature, and thereby allow some control over levels of image (unwanted) signals output from the modulator.

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LNAs and Matching

One client's requirement was to investigate the input matching of a low noise amplifier (LNA) designed around a high electron mobility transistor (HEMT) for optimum noise figure. LNAs, normally placed at the front end of a transmission system, will significantly influence the signal to noise ratio further along the system and therefore the bit error rate (BER) after the demodulator. Furthermore, the physics of the HEMT means that its optimum noise performance occurs at a particular input match which, in general, differs from the match required for maximum power. Unfortunately, the addition of input matching at the front end also incurs a small loss, dependent on the quality of the matching components, which adds to the overall noise figure of the chain. FCL was able to improve the quality factor (Q) of the matching components and revise their values to achieve a net improvement in noise figure compared to previously.

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High Frequency Matching

FCL has a good knowledge of the behavoir of components at high frequencies, well into the gigahertz (GHz)region and how their electrical parameters are affected. Factors such as skin depth, proximity effects, absorption (tan delta), Q factor, fringing, and parasitics are all taken into account in reliable simulations.

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