INTRODUCTIONNew generation radios operate in wider bandwidth. For example, the bandwidth of the Very High Frequency(VHF)or Ultra High Frequency(UHF)ranges from 30-512 MHz. If these radios operate at 30 MHz then there could be a harmonic signal at 60 MHz. That is, here 30 MHz is the fundamental frequency and 60 MHz is an integral multiple of the fundamental frequency, known as harmonic. For the operation of 30-512 MHz it is possible to have 6-7 octaves.
An octave means anintervalwithalfordoublethefrequency.Soto?lterouttheseoctavesbetweenfrequency range 30-512 MHz, 6-7 low pass ?lters arranged in banks is required. These radios operate between the power levels 10 W to 50 W or more. So it is dif?cult to get tunable ?lters in these power levels. These radios has built in Communication Security (COMSEC) and Transmission Security(TRANSEC).Communication Security(COMSEC) is de?ned as the measures taken to prevent unauthorized interceptors from accessing the telecommunications while still deriving the content of the intended recipients. This is implemented by encryption and decryption. Transmission Security (TRANSEC) is the component of communication security that results from the application measures designed to protect transmission from interception and exploitation by means other than cryptanalysis.
This is implemented by frequency hopping algorithms to prevent jamming. A higher number of hops per second ensures effective jamming proof and this requires a fast switching of ?lter banks which demands a hopping interval of 500-1000 hops per second. Ideally, a low loss, fast switching, high power handling capacity Harmonic Rejection Filter (HRF) is required for new generation radios. The cost of COTS (Commercially or Component off the shelf) ?lter is very low but their Non Recurring Engineering Charges (NREC)is very high. Commercially or Component off the shelf(COTS)describes software orhardwareproductsthatarereadymadeandavailableforsaletothegeneralpublic. COTS products are designed to be implemented easily into existing systems without the need for customization. For example, MS Of?ce is a COTS product that is packaged software solution for business.
So it is necessary to design and develop an indigenous ?lter bank.LITERATURE REVIEWZabdiel Brito-Britoetal presented 1 a switchable band stop ?lter which is able to switch between two different central frequency states while precisely maintaining a ?xed bandwidth. Switchable ?lters can reduce the complexity of a system by allowing ?ler re-con?gurability instead of having switched ?lter banks. The ?lter topology allows precise control over the design parameters frequency and bandwidth, achieved by choosing adequate resonator sections which are switched by PIN diodes to obtain two discrete states. The central frequency control was obtained by modifying resonator length. Bandwidth control was achieved by choosing a resonator width and controlling the normalized reactance slope parameter of a decoupling resonator by means of a switchable resonator extension.
The ?lter was designed to have center frequencies of 2 and 1.5 GHz both having an 8 percent fractional bandwidth. Liew Hui Fangetal presented2 the design of a compact Butterworth low pass ?lter operating at frequency range of 300-400 MHz with input and output impedence of 50 ohm. The butterworth ?lter was developed into UHF range as harmonic ?lter for portable 2 ways radio application by allowing the desired frequency signal to pass through the antenna and attenuate the higher signal frequencies and to reduce the minimal losses. Minimallosseson signal transmission reduces energy consumption during communication thus making this product potentially invaluable for signal transmission on regios whereby a power source is dif?cult to locate.
To achieve better performance, the design of butterworth ?lters are concentrated on lumped elements than distributed elements. The 2 ways-radio experience harmonic produced by the transmitter and enters the receiver and thus damages the receiver circuit. In order to solve this, a low pass ?lter can be employed. It will attenuate the harmonic signal and allow the wanted signal to pass through the antenna with minimum loss.
The butterworth ?lter are devices of combination of two-port network which allows the transmissionofwantedfrequencyinthepassbandanddiscriminatetheunwantedfrequency in the stop band. By this method, the harmonic ?lter is also used to attenuate the excessive harmonic that is generated by the transmitter chain. Tatiana Pavlenkoetal presented 3 the design of bandpass ?lters tunable at 400-800 MHz. Microwave ?lters are vital components which provide frequency selectivity in wide variety of electronic systems operating at high frequencies.
Due to the occurrence of multi frequency band communication and diverse applications of wireless devices, requirements of tunable ?lter exists. The one of the potential implementation of frequency agile ?lters is front-ends and sensors in Cognitive Radio(CR).The principle of CR is to detect and operate at a particular available spectrum without interfering with the primary user’s signals. The focus of this work is development of suf?ciently compact, low cost tunable ?lters with quite narrow bandwidth using currently available lumped element components and PCB technology. Filter design, different topologies and methods to tune band pass ?lters were explored to choose the best suitable variant to comply with the required purpose. I.
M. Alexander et al established 4 a junction circulator that has characteristics of a shunt resonator. The bandwidth over which these characteristics are maintained depends on the relationships between frequency, ferrite material properties and applied magnetic ?eld.
By appropriate material design, broadband low loss devices can be realized in the VHF/UHF region using lumped element ?lter matching techniques. An extension of these techniques allows the tailoring of the out-of-band response to provide a predictable degree of rejection at harmonic frequencies. Fabrizio Gentili et al presented 5 the design of an SFB in the S-and C-band. The aim was to realize a low-cost four-channel SFB with arbitrary driving voltages, which increases the exibility of its application. The conventional driving of an SP4T with bipolar voltages was presented and compared with the proposed approach (with arbitrary driving voltages) avoiding the use of dcdc converters so as to save the current needed when additional devices are introduced. The architecture consists of separating the dc ground below the SP4T and the rest of the RF circuit by a gap. In this way,the reference voltage of the SP4T is changed to a value different from zero and the switch can be driven with a single voltage command i.e.
, 0 or 5 V (TTL compatible). In order to preserve the integrity of the RF signal, capacitors of suitably designed value are placed uniformly over the gap. Furthermore, as a consequence of the adopted architecture, the active channel of the switch results powered with a portion of the current owing on the other channels,thus determining afurthercurrentsaving(40 percent in the case of a SP4T).Settling time performances were also evaluated and the results showed fast switching time(less than 250 ns for both rise and fall times)SOFTWARE DEFINED RADIO(SDR)Software de?ned radio (SDR)(also known as “software radio”) is a radio communication system where components that have been traditionally implemented in hardware(e.g. mixers, ?lters, ampli?ers, modulator/demodulators, detectorsetc.
) are instead implemented by means of software on a personal computer or embedded system.3.1 SDRConceptPriortotheproliferationofdigitalsignalprocessingtechnologyinradiosystemsmost transceiver functions were implemented in analog circuitry. This con?ned the capabilities of the transceiver to the limitations of the analog technology. Some complex communications algorithms weresimply impossible toimplementwithanalogcomponentsfor a given project budget. Since analog circuitry is speci?ed for a certain function it is dif?cult to multitask, thus analog systems tend to be physically large and power hungry. Although it is true that in some applications such as ?ltering, it can outperform its digital counterpart, an analog systems performance is jeopardized by environmental variations. Digital signal processors (DSPs), ?eld programmable gate arrays (FPGAs), and microprocessorsallowanalogcircuitssuchas?lters,equalizers,andphase-lockedloops(PLL) to be packed into one chip, consuming a fraction of the power, area, and cost.
This has led to the implementation of sophisticated signal processing algorithms such as convolutional encoding, interleaving, and dynamic power control in small hand-held devices such as cel6Harmonic Rejection Filter Design for Software De?ned Radio Applicationlular phones. Todays transceivers consist of a radio-frequency (RF) front end, and a baseband processing section. The RF front-end is a loose term referring to the analog circuitry between theantennaandthedataconverters. ThemainfunctionsoftheRFfrontendaretomodulate and demodulate the carrier with and from the data, respectively. Base band signal processing, voice processing, user interface, power management, and networking functions are done by a combination of analog and digital chips. Mixed signal (analog and digital) chip design, which would allow the integration of many of the current analog and digital functionalitiesintoonechip,isapopularconceptintodayswirelessindustry. TexasInstruments for example, has announced that by 2004 it will introduce a one-chip GSM phone. Software radio strives to pack as much of the transceivers functionality into a programmable signal processor as possible.
A block diagram of an ideal software radio is shown in Figure 3.1 where the data converters are placed very close to the antenna.Fig. 3.1: In an ideal software radio the RF front end is eliminated.In this system, the RF front end is eliminated and the DSP is tasked with the modulation and demodulation, in addition to the baseband signal processing. Thus, if the DSP is programmable, the characteristics of the radio can be signi?cantly de?ned by the software thatitrunson.
Adesignercanaltertheperformanceoftheradiosimplybyreprogramming the DSP.This concept has far-reaching implications in the wireless communications industry. Basestationtransceiverequipmentatcellsiteswillnolongerbecomeobsoletewithchanges in wireless standards. Thus, migration to newer and more powerful systems will be inexpensive. Wireless switches, access points, and routers will no longer have to be replaced with system upgrades, but reprogrammed. Satellites and other spacecraft can be reprogrammed from Earth to alter their transmission and reception characteristics, thus making them more powerful and ?exible.
Interoperabilityisanotherpotentialbene?tofsoftwareradiosystems. Differentwireless communication systems operating on different standards can communicate with each other. This has been amajor focus of the USmilitary becausedifferent battle?eld units use different communication systems.
Softwareradioscandrasticallyreducetimetomarketbecausesoftwaremodi?cations can be done at a fraction of the time of hardware modi?cations. Complex 3G handsets take many months to design and implement. Any errors in this process can lead to a delay of many months for the product to reach market. Software radio systems cut down this correction time signi?cantly. New software features and upgrades can be downloaded to the handset automatically orondemand,thusgreatlyenhancingthedegreeandqualityofservicesavailabletocellular customers. Handsets will not become obsolete as often as they do now with changing standards, thus saving customers money.
Signal processing algorithms such as ?ltering, encoding/decoding, equalization, and modulation/demodulation can be adaptively altered remotely. For example, in current CDMA systems the base station controls the power emissions of the handsets to minimize the near-far effects, as well as multiuser interference. This can be applied to all parameters of the handset, and as a result, transmission quality and capacity can increase. The concept of cognitive radio, which seeks to make radio systems intelligent and adaptive to their environments, is a future goal for software radios.3.2 SDRAchitectureA traditional or typical receiver, besides the classic demodulation, performs three other operations: (1) carrier frequency tuning to select the desired signal, (2) ?lter to separate it from others received, and (3) ampli?cation to compensate transmission losses. Most traditional receivers have used conventional heterodyne schemes for almost a century.
The superheterodyne internals blocks are shown in Figure 3.2.Fig.
3.2: Superheterodyne Receiver’s Interbnal Block.In the previous scheme, after the signal enters through the antenna, it is typically ampli?ed by an RF stage that operates only in the frequencies of interest region. Then, the signal is passed to the mixer which receives the local oscillator contribution by its other input. The local oscillators frequency is set by the radios tuning control 11. The mixer is in charge of translating the signal to the Intermediate Frequency (IF).
Typically, the oscillators frequency is set to a value that ensures that its difference fromthedesiredsignalsfrequencyisequaltotheIF.Thenextstageisabandpass?lterthat attenuates every signal except a speci?c portion of the spectrum. The bandwidth of this stage limits the band width of the signal thats being received. At the end, the demodulator recovers the original modulating signal from the IF ampli?ers output employing one of several alternatives. Further processing of the signal depends on the purpose for which the receiver is intended.3.2.
1 SDRTransmitterSDRtransmittersreceiveabasebandsignalasaninput,typicallygeneratedbyaDSP step as it is shown in Figure 3.3.Fig. 3.3: Block Diagram of a SDR transmitter.The ?rst block is a Digital Up Converter (DUC) which transfers the baseband signal to IF. The DAC that follows transform the samples to the analog domain. Next, the RF converter shifts the signal towards higher frequencies.
Finally, the signal is ampli?ed and directed to the antenna. Within the DUC, the Interpolation Filter is responsible for raising the baseband signalssampleratetomatchtheoperatingfrequencyofthecomponentsthatfollow. Then, the digital mixer and the local oscillator shift the samples to IF, the shift being controlled by the local oscillator.3.
2.2 SDRReceiverFigure3.4showstheblockdiagramofaSDRreceiver. At?rst,theRFtunerconverts the analog signal to IF, performing the same operation that the ?rst three blocks of the superheterodyne receiver. Up to this point the two schemes converge 13. Next, the IF signal is passed to the ADC converter in charge of changing the signals domain, offering digital samples at its output. The samples are feed to the following stagesFig.
3.4: Block Diagram of the SDR Receiver.inputwhichisaDigitalDownConverter(DDC).TheDDCiscommonlyamonolithicchip anditstandsasthekeypartoftheSDRsystem.
Itconsistsofthreemaincomponents: (1)a digitalmixer,(2)adigitallocaloscillator,and(3)aFiniteImpulseResponse(FIR)low-pass ?lter. The components operation is similar to their analog counterparts. The digital mixer and the local oscillator shift the IF digital samples to baseband, while the FIR low-pass ?lter limits the bandwidth of the ?nal signal 13. For the implementation of each of its parts, the DDC includes a high number of multipliers, adders and shift registers. Observe that the signals are transferred to their baseband equivalent at the digital mixers output by the disintegration into the I and Q counter phase components 12. If the tuning of the digital local oscillator is modi?ed, the desired signal can be shifted away or towards the point where it reaches 0Hz. This variation, together with the bandwidth adjustment of the low pass ?lter, de?nes which part of the reception is treated as a useful signal.
Another procedure, known as decimation, is commonly performed for reducing the sampling frequency or sample rate. Thus, the new sampling frequency in baseband results from the division of the original sampling frequency by an N factor, called decimation factor. The ?nal sample rate can be as little as twice the highest frequency component of the useful signal, as proposed by the well-known Nyquist theorem 14. This can beDept.of ECE, SJCET, Palai Page 11Harmonic Rejection Filter Design for Software De?ned Radio Applicationexpressed numerically asfb2 = 0.8fb =fs N/(3.
1)Where fb isthefrequencyatbaseband, fs isthesamplingfrequency,Nisthedecimator factor and fb2 is the new calculated baseband frequency after the decimation is applied. Finally, the baseband samples are passed to the Digital Signal Processing (DSP) block, where task such as demodulating and decoding are performed, among others.Chapter4WORKPLANThe?lterbanksusing6,7or8channelsistobedesignedusingMicrowavesimulation tool.
A high power switched ?lter bank is also to be designed using CAD tool. The results obtained from Microwave simulation tool and CAD tool is to be validated and the best result has to be chosen. Then a high power harmonic rejection ?lter has to be fabricated and tested.
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