agilent - cdma2000 rf system design softwares & spreadsheets

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odays complex designs, particularly

Twith the advent of digital communica-

tions standards such as code-division mul-tiple access (CDMA2000), require a reliableperformance predictive methodology thatensures a high level of confidence that adesign will work right in the first iteration,before committing significant time and re-sources to the project. The ultimate predictorof a practical, complex system design is sys-tem design software. System design softwaredetermines the RF system performance bymonitoring the trade-offs of various systemspecifications.

Although spreadsheet programs are eco-

nomical, they often require enormous amountsof time and effort before they contribute tosuccessful system design. Critical measure-ments such as dynamic circuit response, con-trol loop characteristics and so on, cannot bedetermined by spreadsheets. These measure-ments are important for the accurate selec-tion of hardware components.

Many real-world impairments affect sys-tem design. And simple tools like spread-sheets do not fully consider these ef-fects. To characterize these effects,several measurements are required.Advanced Design System (ADS) from

Agilent EEsof EDA is the tool that isused throughout this article for mak-ing measurements and displaying re-sults for analysis.

Analyzing AGC loopsAutomatic Gain Control (AGC)

loops are important in any communi-cations system where wide amplitudevariations in the output signal lead toa loss of information. These signalsneed a good control to maintain aconstant signal level at the output.Figure 1 shows a block diagram of anAGC loop for an amplifier with aCDMA2000 input signal.

Fig. 1. A block diagram of an AGC for an amplifier with a CDMA2000 signal source.

Fig. 2. Input Power, Amplifier Output Power and IF Output Powerversus Time.

Fig.3. A high-level amplifier block pushed into the circuit level to design input and output match. Thegraphs show the power gain before and after matching the amplifier.

CDMA2000 RF system design softwareand spreadsheets

For deca des , engineers have used sprea ds heets forRF system design. During the initial phase of thedes ign cyc le, s prea ds heets provide res ults for s impleRF sys tems q uickly. How ever, s imple s prea ds heettoo ls fa il to provide ins ight into ma ny c ritica l is sueswhen selecting the proper hardware.

By Anitha Swaminathan

The complexity of an AGC system is de-

termined by the requirements of the commu-nications system. An integrated software tool

that provides top-downRF design can be usedto analyze the steady-state and transient re-

sponse of the controlloop.Figure 2 shows a plot

of input and outputpower of the AGC am-plifier. The amplifier andintermediate frequency(IF) output powerchanges in response tovariations in input

power due to CDMA2000 modulation,

the loop forces them back to their tarlevels. Here, the dynamic circuit characte


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tics are measured, whereas spreadsheets areonly capable of performing steady-state mea-surements and cannot address time-varying

signals.

Spurious responses, TOI,and matching

Spurious responses are another exampleof a real-world impairment often seen in RF

systems. Spurious responses usually are gen-erated by mixers, antennas (if no pre-ampli-fier stage is present), nonlinear amplifiers and

spectrally impure oscillators. To analyze spu-rious signals, the designer needs adequateselectivity ahead of the mixer stage and goodlinearity in the RF stage to obtain a wide,spurious-free dynamic range. Spreadsheetscannot predict spurs at all levels accurately.

Traces of spectral coponents, spurious fquencies, and powlevels for various noof the design can be termined using RF stem design softwa

The software also ctains mixer modewhich carry informtion about intermodulation pructs due to mixer snal input and local cillator mixiCircuit-level mixers cachieve accurate pdiction of spurs at levels. The softwalso allows changthe intercept point a

spur in mixers analysis.When designing

systems, the linearitya broadband receiplays an important rin the overall perfmance. It affects stem level parametsuch as sensitivity alink budget. Third-

der intercept point (TOI) is a good measof linearity of the receiver. Designers can equations on a spreadsheet to predict

worst-case, third-order intercept, but the tual TOI is beyond the scope of the toActual TOI measurement is vital to impring the design performance because it cindicate a need to include selectivity betwestages. System design software predicts

Figure 4. Single-step .P2D file extraction for an amplifier circuit using a .P2D extractor.

Figure 5. Distorted trajectory of CDMA2000 signal input to an amplifier .P2D model and circuit.


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Table 1. RF system design measurements - Spreadsheets vs. RF system design software.

actual TOI for multi-tone frequencies withharmonics in seconds if the software containsa harmonic balance simulator.

During the initial design phase of a high-level system, certain assumptions are madefor each block in the system. One such as-sumption, very common in spreadsheet analy-sis, is that the amplifier is matched to 50Ohms. But in reality, the amplifier usually hasmismatches in the input and the output of theactual hardware implementation. Any suchmismatch will degrade the overall system gain.RF design experts can perform narrowbandmatching using spreadsheets. However, RFsystem design software usually contains anarrowband and broadband matching synthe-

sis utility that designers can use to designmatching elements that work well in the firstrun. This type of linear analysis is available inalmost all RF design software and quicklyidentifies a poor match.

Perfect matching of amplifier designs isimportant for aerospace defense/satellite com-munications applications. Consider a high-level system such as the one in Figure 3,which shows a series of blocks that containcircuit representations inside them in the hi-erarchy. The complex terminations in everyblock can be identified with linear analysis. Ifthe amplifier is poorly matched, the designercan open up (push into) the circuit level ofthe amplifier block and perform narrowband/

broadband matching using the matching uity, as shown in Figure 3. After the matchare designed, the circuit can be pushed bato block level and simulated. The gain of overall system is improved. Using hierarccal designs reduces the complexity of design and saves simulation time. Each blocan be accurately designed by pushing ithe circuit level of each block and optimizits performance.

Now consider a system built on a sstrate. Spreadsheets fail to design such stems accurately because narrowband effecannot be predicted unless the designer knothe component data such as voltage stand

Measurements/Features Spreadsheet RF System CommentsRequired for System Design SoftwareAnalysis

Signal-to-Noise Ratio

Power/Voltage FrequencySpectrum !

Input/Output third-orderIntercept Point ! Worse-case TOI predicted using

spreadsheets

Spurious Signal Analysis ! Spreadsheets can perform spuriousanalysis but does not predict alllevels of spurs accurately

Accuracy of Results ! Less accurate in spreadsheets whencompared to EDA software

CAD operating Cost ! License Required

Multi-Platform ! EDA software work on mostoperating systems

Steady-state Analysis

Quickness of obtaining results ! Spreadsheets can update resultsvery quickly

Interaction between functional ! For example, yield optimizationmodels. For e.g. match gives interactions betweendependencies component including mismatch

effects on gain and noise figure (NF).

General non-linear analysis ! Includes compression curves, phasedistortion, non linear noise, etc

Detailed linear analysis ! Includes s-parameter analysis

Statistical sweep to studyworst-case performance !

Analysis with complex ! For example, CDMA2000, 3-GPP,modulated sources wireless local area network (WLAN)

standard signals

Predicting control loop effects ! For e.g. AGC loop time in amplifier

In-band and out-of-band ! For example, a spur can travelintermodulation for multiple from a phase locked loop (PLL)harmonics to other functional modules even

if filters filter most of it

Dynamic circuit response using ! Spice, convolution, circuit envelope Time-Based Analysis

Analyze physical design ! Electromagnetic (EM) analysis, layoutsteady-state behavioral and dynamiccircuit

Use of instrumentation to ! Connecting the software to a networkform connected solutions analyser and pulling the componentdesign verification details.

Continued on page 30


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Figure 6. A snapshot of a high-level CDMA2000 signal source in the background and EVMresults using a circuit and P2D model of an amplifier in the foreground.

wave ratio (VSWR). One way to do this is toread the component datasheet and manuallytype the data into the spreadsheet to performnarrowband analysis. Sophisticated RF sys-tem design software can be connected to avector automated network analyzer to auto-

matically read the component data to thesoftware via a hardware-software connec-tion. This method requires minimum timeand effort and allows much data to be im-ported for various components, resulting inmore accurate performance predictions.

Behavioral modelsBehavioral models are commonly used

high-level system design in most system sign software. The advantage is that behioral models reduce system simulation timThe disadvantage is often a compromiseresults accuracy. On the other hand, behioral models that are extracted from circlevel components reduce simulation time aprovide more accurate results. Figure 4 shothe extraction of the .p2d model of the ampfier. These models are extracted in a singstep process using the model extractor. Aother benefit of model extraction is intellectproperty (IP) protection because dataset cbe passed around without the circuit detaThese models contain RF circuit impairmeffects, S-parameters, power, and narrowbaeffects.

Due to the complex modulation CDMA2000 signal source, performancethe system cannot be determined without csidering the the modulation. With electrodesign automation (EDA) software that hadesign and verification environment for a comunications standard, a designer can genate a viable design easily that will work crectly in the first iteration. Wirelstandard-specific component libraries amenu-accessed pre-built templates and t

Circle 19 or visit freeproductinfo.net/rfd


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ing the amplifier circuit with an extracted.P2D model. The results shown in Figure 6are close when compared to the actual circuitresults. Designers have the advantage of se-lecting the components and system specifica-tions under actual operating conditions dur-ing the initial phase of the design. This willsave significant design and simulation timefor large complex systems.

Statistical sweepAnother major advantage of using RF sys-

tem design software is statistical sweep analy-

sis. Performing statistical sweep of variablesis complex in spreadsheets. In most cases,the analysis is done at a single point in eachrun. Design software can analyze results for arange of values in the first run to study theworst-case performance. For example, mea-surements like yield optimization that con-tains interactions between each component

benches are available to verify if the system iscompliant with a particular wireless stan-dard. The complex modulated sources can beused in the system/circuit simulations with acircuit envelope simulator. This is the onlysimulation technology that allows simulationof circuits with time-varying complex modu-lation. Complex modulated sources also canbe used for circuit simulation without co-simulation using circuit envelope simulator.

The CDMA2000 signal is input to thecircuit amplifier in the first setup and then tothe extracted .P2D model of the amplifier in

the second setup. The distorted trajectory ofthe signal is compared in these two setups.They have excellent matching, as shown inFigure 5.

The CDMA2000 complex modulatedsource is input to an amplifier circuit, and theerror vector magnitude (EVM) is measured.The same result is again measured by replac-

including mismatch effects on gain and nofigure (NF) can be easily done using systdesign software.

Comparison summaryTable 1 gives a comparison of spre

sheets and RF system design software various measurements that are key to scessful RF system design.

This article demonstrates many of the reworld impairments that can affect system sign. Simple tools like spreadsheets do consider these impairments and provide incurate results during the initial phase of design. Ever-increasing system complexcontinues to absorb more effort and destime. By carefully implementing these effevia design simulation software, accurate sults can be achieved in the initial phasethe design to minimize manufacturing riRFD

Circle 21 or visit freeproductinfo.net/rfd

Many real-world impairments affect system design.

And simple tools like spreadsheets do not fully

consider these effects. To characterize these

effects, several measurements are required.

ABOUT THE AUTHORAnitha Swaminathan is Product Ma

ager for RF System Simulation at AgileEEsof EDA. She has a BSEE from PSTech, India, and MSEE from VirginTech. Anitha can be reached at:

[email protected]


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agilent - cdma2000 rf system design softwares & spreadsheets

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