frequency domain simulation.pdf

Advantages of Frequency Domain Simulation for Wireless Transceiver Design Application Note Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/032 Introduction A radio transceiver, shown in the following diagram, provides modulation and demodulation functions to enable transmitting and receiving digital information through the air link. The radio is composed of a transmitter, a receiver in conjunction with automatic gain control (AGC) and automatic frequency control (AFC), and a synthesizer. These overall transceiver blocks are made up of many modules cascaded together to process the information. The modules include modulators, demodulators, amplifiers (LNAs, Power Amps, IF Amps, etc.), mixers, oscillators, PLLs, AGCs, AFCs, and other RF components. Traditionally, RF Intermodulation distortion; and third-order intercept ? Power Added Efficiency (PAE) ? Adjacent Channel Power Ratio (ACPR) A load-pull simulation is easy to perform with RFDE. In such a simulation, a circular region on the Smith chart is specified, and load impedances within this region are presented to the output of the amplifier. Output power contour circles are plotted out and the optimum power point is then located. It is also easy to calculate DC-to-RF efficiency, and 3rd- and 5th-order intermodulation distortion versus load impedance at any bias point. The powerful and flexible data display capability enable designers to plot and apply the optimum conditions to meet their design specs. Figure 5 shows an example of simulated load impedances, swept output spectrum and intermodulation distortion versus input power. Using this output with the specified contours, you can quickly determine how changing the load impedance affects the output power, efficiency, and level of intermodulation distortion products, as well as the shape of the output waveform. Figure 5. Load impedances, swept output spectrum, and intermodulation distortion versus input power. Parameter sweep is another valuable simulation technique in RFDE. Swept parameters results in plots that help designers do fast trade-offs to optimize results. Figure 6 shows an example of this. Trade-offs may be made between output power, DC-to-RF power efficiency, and DC bias current consumption. Figure 6 also illustrates the flexibility of the data display in presenting the data, making it easier for designers to make trade-off decisions and optimize results. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/037 Figure 6. Output Power and DC- to-RF Efficiency Versus Emitter Resistor and Versus Vin_Bias allow designers to make important design trade-offs. Optimization in the Frequency Domain Because results are always in spectral frequency domain, this makes it very easy for HB to process these spectral components and effectively optimize key results like distortion and spurious signals (-1 dB compression, IP3, ACPR, etc). RFDE frequency domain linear simulator is also capable of easily optimizing wide band filter structures, matching networks, and other parameters for gain, power, noise figure, VSWR, group delay, and etc. RFDE for Modulators An I-Q, or vector, modulator is a common integrated circuit in communication systems. These modulators take two baseband data sequences (I and Q channels) and vary the amplitude and phase of a sinusoidal signal (the carrier) in response to the instantaneous I and Q channel voltages. Designers of these ICs must be concerned with a number of characteristics, some of which include: ? Modulation accuracy ? Frequency response ? Undesired leakage ? Intermodulation distortion terms ? Efficiency and output power ? Modulator amplitude and phase accuracy with realistic baseband data sources ? EVM Many of the I and Q modulator analysis and simulations would be almost impossible to complete with a purely time-domain simulator such as SPICE. The RFDE “Circuit Envelope” simulator is very well suited and very efficient for such analysis. RFDE Circuit Envelope Circuit Envelope was developed specifically to efficiently simulate the transient and complex modulated RF signals found in todays wireless circuits. The main advantage of the mixed frequency/time domain approach at the base of Circuit Envelope is that it performs the simulation only in the relatively narrow frequency band that is occupied by the modulated signal. Unlike SPICE, it does not need to analyze the complete spectrum up to the maximum frequency Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/038 set by the simulation period. This results in an enormous time savings in getting the necessary results. Figure 7 illustrates this. Figure 7. Circuit Envelope is highly efficient in analyzing circuits with digitally modulated signals, because the transient simulation takes place only around the carrier and its harmonics. Calculations are not made where the spectrum is empty. Circuit Envelope simulation output is a time-varying spectrum from which useful information, such as PLL frequency Vs time transients, ACPR (adjacent channel power ratio), EVM (error vector magnitude) and NPR (noise power ratio) can be derived. Such simulation can be carried on up to as many orders of harmonics the user specifies (5th, 7th, 9th.mixer intermodulation products analysis), and all analysis can be carried out down to the transistor level. Figure 9 shows a transistor level direct conversion IQ modulator, and the output results from the simulation that was run, with baseband signals corresponding to the IS-95 CDMA specification. The simulation took less than one hour to perform, and the resulting spectrum, output power, and ACPR are shown in the figure. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/039 Figure 8. Transistor-level direct-conversion IQ modulator, along with spectrum, trajectory, main channel power, and ACPR results from an IS-95 CDMA baseband source applied to a transistor level direct conversion IQ modulator. Simulation time was less than one hour. Figure 9 shows the output trajectory, constellation diagrams, error vector magnitude (EVM) in Volts versus time, and percent EVM. Designers can introduce arbitrary values for these imbalances and observe the EVM degradation. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0310 Figure 9. Output trajectory and constellation diagrams, as well as error vector magnitude (EVM) in Volts versus time, and percent EVM. RFDE for Oscillators For oscillators, Harmonic Balance is very efficient, both in terms of speed and memory usage, and outputs the frequency of oscillation, power, phase noise, and other simulated results. Key oscillator performance measurements include: ? Oscillation frequency and harmonics (sub) and spurious response ? Output power ? Phase noise ? Oscillation startup ? Frequency stability to temperature, load, and bias change ? Oscillator linearity RFDE Harmonic Balance provides two algorithms to calculate phase noise: ? Modulated FM Noise analysis for close in noise. ? Mixing Noise analysis for farther out noise. Interlacing the two provides the full noise spectrum and confirms the accuracy of the results. The advantages in using RFDE for noise analysis include: ? HB can handle frequency domain components with no loss of accuracy. This includes measured S2P files as well as transmission line models. ? HB with the Krylov solver uses significantly less memory, especially for multitone problems. ? HB can handle two-tone and higher analyses more efficiently. ? HB noise analysis is very accurate and it accounts for any changes in the oscillation frequency instantaneously with any small changes in bias or temperature. Figure 10 shows harmonic balance and circuit envelope simulation results for an example oscillator. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0311 Figure 10. Harmonic balance and circuit envelope simulation results for an example oscillator. Figure 11 shows an example sprial inductor, one of many distributed models that can be included in simulations with RFDE. Distributed models (multilayer lines, packaging, SMT, PCB, and so on) are easily and accurately included in harmonic balance, circuit envelope, and transient simulations through RFDEs convolution simulator. Figure 11. The spiral inductor is one example of a distributed model that can be included in simulations with RFDE. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0312 RFDE for PLLs PLL (phased-lock loop) analysis requires a variety of the simulation capabilities in RFDE. Transient analysis and transient noise analysis using the Transient simulator, Harmonic Balance, Circuit Envelope, Ptolemy system simulator (accessed through Dynamic Link for Fractional N synthesizers), and Optimization and Yield are the simulators that are used for many of the common PLL specifications. Characterization of the individual cells within a PLL (VCO, prescalars/dividers, multipliers, phase frequency detectors, charge pumps, and loop filters) can be acomplished using certain individual simulation engines within the RFDE platform. After characterization of certain parameters, these parameters can be passed to the PLL system level models contained within ADSlib, which is included with RFDE. By using behavioral models after circuit level analysis, the following specifications, listed below, can be completed: ? VCO: Harmonic balance oscillator and phase noise analysis. ? Prescalers/digital dividers: Transient Assisted Harmonic Balance (TaHB) analysis, including phase noise analysis. ? Phase Frequency Detectors (PFDs): Transient and Transient noise analysis. ? Charge Pumps: Transient and Transient noise analysis. ? Loop Filters: Transient analysis. ? System level simulation using models after characterization: Circuit Envelope Figure 12 shows a typical results plot for an example PLL. Figure 12. VCO tune voltage versus time. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0313 RFDE PLL system level models listing: ? VCO ? VCO divide by N ? Divide by N ? Phase frequency detector ? Phase frequency detector with charge pump ? Phase frequency detector tuned ? Phase noise modulator Other common items to simulate in a PLL using RFDE: ? Frequency response: The steady-state response to a sinusoidal input. It helps identify how well the loop responds to the change in phase between the dividers output and the reference, into the detector. It also outputs the loops gain and phase margins and helps to stabilize the loop. ? low pass and high response of loop ? linear analysis and nonlinear analysis ? Loop acquisition time: The time it takes to bring a loop from unlocked into locked position. ? Frequency switching time: The time it takes to switch from one locked frequency into a new desired frequency by changing the divide ratio. ? Frequency spur generation: Spurs generated through mixing and due to non-linearity. ? Carrier clock recovery applications, Costas loops: Synchronizing the phase and the timing of the signal at the receiving end. ? Reference feedthrough These are generated if the charge pump has some imbalance. This generates side bands around the VCO, at multiples of the reference signal. ? Dead zone of phase-frequency detector PLL phase noise at lock: The non-linear effects of the detector around the origin (small voltage) ? Stability; Phase margin ; Gain margin; and Loop Bandwidth Optimization ? Component noise contribution ? LO radiation and leakage ? PLL phase noise at lock ? Output power ? Frequency stability to temperature, load, and bias change Figure 13 shows a few examples of the many data plots that are available for PLL design and analysis. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0314 Figure 13. Some of the many PLL analyses that are possible using RFDE. Figure 14 shows Delta N and VCO Spectrum within Sigma-Delta PLL, including VCOs Phase Noise. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0315 Figure 14. Delta N and VCO Spectrum within Sigma-Delta PLL, including VCOs Phase Noise. Advantages of Frequency Domain Simulation for Wireless Transceiver Design 08/25/0316 Summary This application note demonstrates the value of RFDE and highlights analyses where frequency domain techniques add value to the overall design process. The radio transceiver example shows where frequency-based simulation techniques are particularly useful as a complement to time-domain simulation. Frequency-based analysis techniques provide important simulation data that give designers added insight into the performance of the transceiver and can significantly speed up the overall design process.