Eldo RF Frequently Asked Questions
On this page:
- General Questions
- Technical Questions
- Performance Considerations
- Flow and Integration Considerations
Q - Does Mentor have a solution for RF IC simulation?
A - Yes, Eldo RF extends Eldo into the RF domain. Eldo RF is a high-performance and high-capacity simulator built on top of Eldo, for large RF ICs which contain thousands of devices and/or nodes.
Q - What is the target market for Eldo RF?
A - Eldo RF has been designed to handle complex RF ICs that are common in the 3G Telecom Wireless market. Eldo RF is also suitable for the broader wireless market for RF IC designs. The typical circuits Eldo RF is addressing are the RF transceivers found in virtually all mobile phones for example. These circuits include low noise amplifiers, mixers, filters, oscillators etc. which operate in the RF frequency range (typ. from a few 100MHz to 5 or 10 GHz)
Q - Is Eldo RF suitable for board/system designs?
A - No, Eldo RF does not support the models required by system and board designers. The focus for Eldo RF is really on the IC design market. For board level applications, the Mentor product RF Design can be used in conjunction with the ADS (Agilent) environment, providing a solution for board level RF design and simulation.
Q - How is Eldo RF functionality integrated into Eldo?
A - Eldo RF includes additional analysis types for steady-state analysis and modulated steady-state analysis. These analyses are specified through Eldo-style commands (.SST, .SSTAC, .SSTXF, .SSTNOISE, .MODSST). Additional sources are also available to directly generate RF signals, either continuous wave (e.g. sinusoids) or digitally modulated (e.g. GMSK, OFDM etc.).
Common RF measurements (e.g. IP3, compression point) are handled with dedicated functions. The plotting commands have also been enhanced with new formats to support the type of output generated by the new steady-state analyses. The new functionality represents additional commands to the users, which can be combined with any other standard Eldo analysis.
Q - What are the prerequisites for using Eldo RF?
A - Eldo RF is an option ‘on top’ of standard Eldo. Running Eldo RF requires Eldo and the AMS Waveform Processor (Ezwave) licenses.
Q - Does Eldo RF require special device models?
A - No, Eldo RF, uses the same device models as Eldo, including BSIM3v3, BSIM4, PSP, HiSIM, BSIM3SOI, VBIC, HICUM, VBIC etc. In case proprietary models must be used, the GUDM option of Eldo may be used to integrate them. Eldo RF supports charge-based MOS models only.
Q - Does Eldo RF support microstrip models?
A - Yes. Eldo RF comes with a library of microstrip models, implemented as primitives in the simulator (no need to purchase or even install any additional library). The main motivation for support of a few microstrip models with Eldo RF is to help designers working with Power Amplifier applications. These devices indeed heavily rely on microstrip components for input/output matching particularly, and it is difficult to get useful information out of a simulation if these components are not included.
Q - Which frequencies can Eldo RF handle?
A - There is no built-in limitation to the frequencies that Eldo RF can handle. The algorithms can be used at any frequency. The limitations come from the accuracy and availability of the device models for high frequency usage. In an IC design context, which is the usage context for Eldo RF, foundries usually provide the device models, after thorough characterization of their process. The device models are ‘valid’ (i.e. they accurately represent the device behavior) up to a certain frequency, which becomes the practical usage limit. The typical maximum frequencies for state-of-the-art RF processes are in the range of a few GHz.
For example a foundry may provide models for its 0.25um RF process, which are valid up to 10GHz. If the user wants to simulate beyond this frequency, and ultimately fabricate an IC operating beyond this frequency, he can do it. Eldo RF will simulate without complaining, but the results of the simulation may not match the silicon (they probably won’t). However, the foundry will not be responsible for any type of mis-functioning in said IC. Thus practically the limit on the frequency is set by the foundries, who ‘stamp’ their processes up to certain maximum operating frequencies. The simulator itself does not impose any limit.
Q - What are the analyses provided by Eldo RF compared to Eldo?
A - The fundamentally different (compared to ‘regular’ Eldo) analyses supported by Eldo RF are the steady-state analysis and the modulated steady-state analysis.
The “steady” state is defined as the state when all initial transients have vanished, and the circuit operates with periodic or quasi-periodic large signals only. Eldo RF computes this steady state in the frequency domain. Thus the results are typically presented as a spectrum in the frequency domain. Eldo RF also automatically converts the results in the time domain, if requested by the user.
Modulated steady-state analysis is a mixed time-frequency algorithm, which efficiently handles the modulation information carried by RF signals. The output of the modulated steady-state analysis is a time-varying spectrum. The modulated steady-state algorithm is the key to Questa ADMS RF.
Other analyses are also possible, such as the steady-state small signal analysis, steady-state noise analysis and local stability analysis.
Q - Does Eldo RF support non-linear noise analysis?
A - Yes, Eldo RF supports steady-state non-linear noise analysis. The noise analysis is performed in either single-tone or multi-tone conditions whereas most other tools only support single-tone. The noise models that are used are those from the Eldo device models. This analysis is typically used to predict the noise spectrum in low noise amplifiers, filters and mixers. Eldo RF also supports direct extraction of the noise factor and minimum noise factor of a circuit. Eldo RF also supports noise circles.
Q - Does Eldo RF support phase noise analysis?
A - Yes, Eldo RF supports phase noise analysis. The prediction of phase noise from voltage controlled oscillators is a critical aspect in the design of frequency synthesizers. Eldo RF includes robust and reliable algorithms, which can predict the phase noise density both close to and far from the carrier. Phase noise close to the carrier is sometimes referred as ‘close-in’ phase noise. Eldo RF can also predict phase noise for circuits containing mixers and/or dividers.
Q - Does Eldo RF support Phase Locked Loops?
A - Yes, Eldo RF supports the analysis of phase locked loops described at the transistor level. A dedicated algorithm handles specifically PLLs. The algorithm computes the steady sate solution (i.e. the locked sate waveforms), and the closed loop non linear phase noise. The algorithm supports integer-N PLLs with full SPICE netlists, and possibly fractional PLLs if the divider is modeled with the frequency divider macro-model. The algorithm also reports the individual contributions of the building blocks to the global output phase noise.
Q - Does Eldo RF predict ‘jitter’ ?
A - Yes. Jitter is a time-domain view of the statistical distribution of the variation of a clock period. Under certain conditions, this time-domain quantity can be related to the phase noise spectrum (which is the frequency-domain view of this same ‘jitter’). Eldo RF provides functions that return average jitter and long-term jitter, computed from the phase noise spectrum.
Q - Does Eldo RF support extraction of 1dB compression point?
A - Yes, Eldo RF supports extraction of 1dB compression point through dedicated functions called ‘compress()’ and ‘xcompress()’, which can be used in the standard .EXTRACT commands of Eldo.
Q - Does Eldo RF support extraction of IM3/IP3?
A - Yes, Eldo RF supports extraction of 3rd order intermodulation and intercept points. Users must enter the appropriate .EXTRACT commands to obtain the values, or use built-in functions (OIPx and IIPx). The commands are simple but they have to be entered manually - advantage is the flexibility (no hard-coded formula). However some users don’t like typing, thus with the Cadence integration of Eldo RF (Artist Link), the user only has to fill predefined forms, and no typing is required
Q - Does Eldo RF support load-pull contour analysis?
A - Yes.
Q - Is Eldo RF able to handle S parameters?
A - Yes. Eldo RF can extract S parameters from a steady state analysis (i.e. large signal). These S parameters can be displayed in a Smith chart within EZwave. Eldo and Eldo RF can also simulate blocks defined by their small-signal S parameters. For Eldo, it allows AC and transient simulations. For Eldo RF, it allows SST simulations. The S parameters are given using the TouchStone file format which is a ‘de facto’ standard. Any RF equipment may have measured these parameters. This can be useful to incorporate a representation of an antenna or any block with a linear behavior, without having to provide a transistor level representation.
Q - Can Eldo RF use other Eldo commands during analysis?
A - Yes. Since Eldo RF uses Eldo, all the common Eldo commands are available. Eldo RF can take advantage of Eldo parametric sweeping, temperature sweeping, .EXTRACT, .DEFMAC, etc.
Q - Can Eldo RF use the .alter and MonteCarlo features of Eldo?
A - Yes. Since Eldo RF uses Eldo, all the common Eldo commands are available, including .alter and .mc
Q - Can Eldo RF perform parametric analyses (design parameter sweeping)?
A - Yes. Again, since Eldo RF leverages Eldo capabilities, all the common Eldo commands are available. Eldo RF can take advantage of Eldo parametric sweeping or temperature sweeping. For RF analysis, the input power and the frequency are the commonly ‘swept’ parameters. Furthermore, the Harmonic Balance algorithm used in Eldo RF is very efficient at parametric sweeps. Indeed, information from the previous point can be reused at each point of a sweep, leading to considerable CPU time savings when doing a 1dB compression point analysis for example (this analysis requires sweeping the input power).
Q - Can Eldo RF handle oscillators and VCOs?
A - Yes. Eldo RF provides efficient analysis of VCOs (LC-tuned, ring oscillators, etc.). It is not necessary to ‘start’ the oscillator in the time domain, everything is directly computed in the frequency domain. This is one of the main advantages of frequency-domain analysis.
The circuit can also contain mixers and or dividers.
Eldo RF can also analyze circuits that contain several independent oscillators.
A flexible VCO macro-model is also available.
Q - Can Eldo RF do optimization?
A - Yes. Eldo RF includes the built-in optimizer of Eldo. The RF analyses are supported by the optimizer, so users can optimize input/output matching networks, compression points, phase noise, or anything that Eldo RF can simulate
Important note: the optimizer is a built-in feature of Eldo/Eldo RF. It does not require any additional license.
Q - Does Eldo RF support Verilog-A?
A - Yes. Eldo RF analyses (SST and MODSST) are compatible with Verilog-A, which means you can simulate circuits described with a mix of plain transistor-level blocks and Verilog-A ‘modules’ (the ‘module’ is the basic Verilog-A design unit).
This option provides a solution for those customers looking for a way to speed-up simulations by using behavioral models instead of detailed transistor-level models for all or part of their circuits. This also allows ‘system-level’ simulations, where all circuit blocks are defined using Verilog-A models.
Not all Verilog-A statements are compatible with the Eldo RF analyses (basically the statements which create (or test for) ‘events’ - Verilog-A includes the notion of event, even though this is an analog language - are not supported.
The ddt() and idt() operators (time-derivatives) are fully supported, and also the Laplace blocks for easy implementation of filters. The noise functions are also supported.
Using Verilog-A with Eldo RF requires a Verilog-A license
A library of behavioral models is available, named CommLib RF. It is free of charge, and delivered in source code format, both in Verilog-A and in VHDL-AMS. It contains models for LNA, MIXER, VCO, filters and math operators components. This library is regularly enhanced with new models. Note that using the VHDL-AMS flavor requires ADMS and the ADMS RF option (Eldo RF does not support VHDL-AMS). Only the Verilog-A flavor is supported by Eldo RF.
Q - Does Eldo RF support VHDL-AMS ?
A - No, you need ADMS RF to use VHDL-AMS. VHDL-AMS is a mixed signal language with all the digital features of VHDL. Eldo RF does not handle anything ‘digital’.
To use VHDL-AMS models for RF analyses, ADMS and its ADMS RF option are required (even if the VHDL-AMS models are 100% analog).
A library of behavioral models is available, named CommLib RF. It is free of charge, and delivered in source code format, both in VerilogA and in VHDL-AMS. It contains models for LNA, MIXER, VCO, filters and math operators components. This library is regularly enhanced with new models. Note that using the VHDL-AMS flavor requires ADMS and the ADMS RF option (Eldo RF does not support VHDL-AMS). Only the VerilogA flavor is supported by Eldo RF.
Q - Can Eldo RF handle modulated signals?
A - Yes. Eldo RF supports digitally modulated sources and an analysis called ‘modulated steady-state’.
The types of modulation supported are GMSK, GFSK, OQPSK, /4QPSK, MPSK, MFSK, EDGE, MQAM, OFDM and HPSK.
The sources incorporate a low-pass filter (Gaussian, Square Root Raised Cosine or Raised Cosine), and they are driven with either an explicit pattern (user-defined bit stream), or a pseudo-random sequence (using a built-in pseudo-random pattern generator). Eldo RF also supports the standard PRBS used in test instruments. If the user is using an unsupported modulation format, arbitrary (generic) IQ modulators can also be implemented using the IQMOD source.
Q - Can Eldo RF do ‘envelope’ analysis?
A - Yes. Eldo RF provides support for digitally modulated signals thanks an analysis called ‘modulated steady-state’. The signals are high frequency carriers, modulated by a baseband signal, according to selected digital modulation schemes. The outputs can be analyzed both in the time and the frequency domain. Some of the typical applications are the prediction of standard figures such as ACPR or NPR for Power Amplifiers.
Q - Does Eldo RF support ‘envelope’ analysis for oscillators?
A - Yes. Eldo RF supports oscillators with the MODSST analysis. This allows simulating the transient ramp-up phase of an oscillator with the modulated steady-state algorithm. This can be useful if the oscillator or the VCO has a long initial transient, which is long to simulate with transient analysis (quartz oscillators for ex.)
Q - What is the capacity limitation of Eldo RF?
A - Eldo RF has been designed for high capacity. It routinely simulates designs with tenths of thousands elements, with 2 or 3 fundamental tones.
Q - I heard that Harmonic Balance has memory problems. What about Eldo RF ?
A - Indeed, ‘brute force’ implementation of the harmonic balance algorithm does not allow handling large circuits with many harmonics. Memory usage has been thoroughly optimized in Eldo RF to overcome this limitation.
Q - Is Eldo RF able to handle the parasitics from a RC or RLC extraction?
A - Yes. Again, Eldo RF has been designed for high capacity. The RC or RLC parasitic extractions from layout tend to produce large netlists with thousands of elements, which in very challenging for simulators. Eldo also incorporates a RC reduction algorithm that can help reducing the size of the netlist to simulate.
Q - What is the speed advantage of Eldo RF ?
A - As for ‘regular’ analog simulation, this question has unfortunately no straightforward answer. The speed ratio can vary widely depending on the circuit under analysis. What is consistently observed with Eldo RF vs. alternate solutions, is a much faster simulation (typically 10x to 100x) for really large circuits – those in the range of several thousands elements – under multi-tone conditions. Analysis of oscillators is also much faster, and much easier. For small circuits, the speed advantage may be less impressive, unless the stimuli are close multi-tones, in which case Eldo RF has substantial advantage.
Flow and Integration Considerations
Q - Can Eldo RF and Questa ADMS work together?
A - Yes. The solution is called Questa ADMS RF. The product is often abbreviated as “ADMS RF”.
ADMS RF basically offers support for the SST and MODSST algorithms inside ADMS. This targets designs having an analog/RF content plus a purely digital (VHDL-D or Verilog-D) content. The user interface is the regular ADMS interface. See the ADMS RF datasheet for further details. Analog behavioral models in VHDL-AMS or VerilogA (for example those from CommLib RF) can also be used, with some restrictions.
ADMS RF allows partitioning of the circuit into TRAN and MODSST partitions. The TRAN partitions are simulated with the regular transient algorithm of Eldo, and the MODSST are simulated using the Modulated Steady-Sate algorithm. The TRAN partitions are assigned to baseband blocks, whereas the MODSST partitions contain the high speed RF blocks.
Note that ADMS RF is an option ‘on top’ of ADMS. It has ADMS and Eldo RF as prerequisites.
Q - Is Eldo RF integrated in the Mentor IC flow?
A - Yes. The Mentor IC Flow integrates Eldo, Eldo RF and Questa ADMS into Design Artchitect IC, using its netlister and EZWave as the waveform viewer.
Q - Is Eldo RF integrated in the Cadence flow (ADE)?
A - Yes. Artist Link (the Mentor layer needed to run the Mentor simulators from ‘Artist’) supports Eldo RF. Cadence supported versions are 5.1.41 (CDBA) and 6.1 (OpenAccess).
Artist Link has many forms dedicated to Eldo RF, to support RF source parameterization (FOUR sources and digitally modulated sources as well), analysis control (steady-state analysis etc.), output selection etc.
Both the Cadence waveform viewer and Mentor’s viewer EZWave can be used to post-process the results. Cross-probing is supported with both viewers.
An interface for parameter sweeping is available. Also dedicated interfaces for Corner Case, Monte Carlo and Optimization simulations are available.
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