Hey everyone! Today, we're diving deep into the world of Cadence PSS (Periodic Steady State) simulations. If you're an RFIC designer or someone working with oscillators, mixers, or frequency dividers, you've probably heard of PSS. It's an indispensable tool for analyzing circuits with periodic or nearly periodic signals. This tutorial will provide you with a practical, step-by-step guide to setting up and running PSS simulations in Cadence. Let's get started, guys!

Understanding PSS Simulations

First, let's understand what PSS simulation is all about. Traditional transient simulations can be computationally expensive when you're dealing with circuits that take a long time to reach a steady state. Imagine simulating an oscillator for thousands of cycles just to see its stable oscillation frequency and amplitude! PSS simulation offers a shortcut. It directly calculates the steady-state periodic response of the circuit, bypassing the initial transient behavior.

Why is this so powerful? Because it saves you a ton of simulation time and allows you to analyze critical performance metrics like phase noise, conversion gain, and stability much more efficiently. Think of it as finding the equilibrium of a system directly, instead of watching it evolve over time. The PSS analysis is a frequency-domain analysis technique, but it does its magic in the time domain first to find the periodic steady state.

To really grasp this, picture a swing. A transient simulation would be like pushing the swing from a standstill and watching it oscillate until it reaches a stable height and rhythm. A PSS simulation, on the other hand, would be like instantly knowing the stable height and rhythm of the swing without needing to go through all those initial pushes and oscillations.

So, when should you use PSS? Anytime you're dealing with circuits that exhibit periodic behavior and you're interested in their steady-state characteristics. Oscillators are the prime example, but it's also extremely useful for mixers (analyzing conversion gain and image rejection), frequency dividers (checking division ratio and output waveform), and even clock recovery circuits.

Key Parameters: Before we jump into the Cadence setup, let's familiarize ourselves with some key parameters you'll encounter in PSS simulations.

• Beat Frequency (Fundamental Frequency): This is the frequency of the periodic signal you expect in your circuit. It's the most crucial parameter, as it tells the simulator what periodicity to look for. Get this wrong, and your simulation will be meaningless! For an oscillator, it's the oscillation frequency. For a mixer, it's typically the LO (Local Oscillator) frequency.
• Number of Harmonics: PSS doesn't just analyze the fundamental frequency; it also considers harmonics (multiples of the fundamental frequency). The number of harmonics determines how many multiples are included in the analysis. More harmonics can improve accuracy, especially if your signal is not a perfect sinusoid, but they also increase simulation time. A good starting point is usually 5-10 harmonics.
• Accuracy Settings: Like any simulation, PSS has accuracy settings that control the trade-off between simulation speed and result precision. You can adjust these settings to fine-tune the simulation for your specific needs. Relaxed accuracy settings will run faster but might sacrifice some accuracy, while tighter settings will be slower but more accurate.
• Transient Analysis Options: PSS often starts with a short transient analysis to help the simulator find an initial guess for the steady-state solution. You can control the duration and accuracy of this initial transient analysis.

By carefully choosing these parameters, you can optimize your PSS simulations for both speed and accuracy.

Setting Up a PSS Simulation in Cadence

Okay, now for the fun part! Let's walk through setting up a PSS simulation in Cadence. I'll assume you have a circuit schematic ready. For this example, let's consider a simple LC oscillator.

1. Open the Analog Design Environment (ADE): Launch ADE from your schematic window. This is where you'll configure and run your simulations.
2. Choose the Analysis Type: In ADE, go to Analysis -> Choose. Select Periodic Steady State (PSS) from the list of available analyses.
3. Specify the Beat Frequency: This is the most important step. Enter the expected oscillation frequency of your LC oscillator in the Beat Frequency field. You might need to do a quick transient simulation or some hand calculations to get a reasonable estimate. Getting the beat frequency right is crucial for the PSS simulation to converge correctly. If you're not sure, start with a rough estimate and refine it later if needed.
4. Set the Number of Harmonics: In the Harmonics field, enter the number of harmonics you want to include in the analysis. As I mentioned earlier, 5-10 is a good starting point. You can always increase this later if you need more accuracy. Remember that increasing the number of harmonics will increase the simulation time.
5. Configure Accuracy Options: Click on the Options tab. Here, you can adjust various accuracy settings. For initial simulations, you can leave these at their default values. However, if you're having convergence problems or need more accurate results, you can experiment with these settings. Some key settings to consider are relref, reltol, and vabstol. These control the relative and absolute tolerances of the simulation.
6. Set the Initial Transient Analysis: PSS often uses a short transient analysis to find an initial guess for the steady-state solution. In the Transient tab, you can specify the duration of this initial transient analysis. A good rule of thumb is to set the duration to a few periods of the beat frequency. You can also adjust the accuracy settings of the initial transient analysis.
7. Choose the Solver: The Solver tab allows you to select the solver algorithm used for the PSS simulation. The default solver is usually a good choice, but you can experiment with other solvers if you're having convergence problems. Some common solvers are shooting, harmonic balance, and time-domain. Each solver has its own strengths and weaknesses, so it's worth trying different solvers if you're facing issues.
8. Save the Simulation Setup: Once you've configured all the settings, save the simulation setup for future use. This will save you time and effort when you need to rerun the simulation with different parameters.
9. Run the Simulation: Click the Run button to start the PSS simulation. The simulator will first perform the initial transient analysis and then solve for the steady-state periodic response.

Analyzing PSS Simulation Results

After the simulation is complete, it's time to analyze the results. PSS provides a wealth of information about your circuit's periodic behavior.

1. Plotting Waveforms: You can plot waveforms of voltages and currents at various nodes in your circuit. This allows you to visualize the periodic signals and verify that they are behaving as expected. To plot a waveform, go to Results -> Direct Plot -> Transient. Select the node you want to plot and click OK. You should see the steady-state periodic waveform.
2. Calculating Key Metrics: PSS allows you to calculate key performance metrics such as oscillation frequency, amplitude, phase noise, and conversion gain. These metrics are essential for evaluating the performance of your circuit. To calculate a metric, go to Results -> Calculator. Use the calculator functions to define the metric you want to calculate. For example, to calculate the oscillation frequency, you can use the frequency() function. To calculate the amplitude, you can use the peakToPeak() function.
3. Phase Noise Analysis (using Pnoise): One of the most important applications of PSS is phase noise analysis. Phase noise is a measure of the spectral purity of an oscillator signal. PSS, in conjunction with Pnoise (Periodic Noise) analysis, allows you to accurately simulate the phase noise of your oscillator. To perform a Pnoise analysis, you first need to run a PSS simulation. Then, go to Analysis -> Choose and select Periodic Noise (Pnoise). In the Pnoise setup, specify the frequency range over which you want to analyze the phase noise. You also need to specify the output node where you want to measure the phase noise. After running the Pnoise simulation, you can plot the phase noise using Results -> Direct Plot -> Pnoise. The phase noise is typically plotted in dBc/Hz as a function of frequency offset from the carrier frequency.
4. Analyzing Mixer Performance: For mixers, PSS can be used to analyze conversion gain, image rejection, and other important performance metrics. To analyze a mixer, you need to set up a PSS simulation with the LO frequency as the beat frequency. You also need to specify the input signal frequency. After running the PSS simulation, you can use the calculator to calculate the conversion gain and image rejection. The conversion gain is the ratio of the output signal power to the input signal power. The image rejection is the ratio of the desired signal power to the unwanted image signal power.
5. Troubleshooting Convergence Issues: Sometimes, PSS simulations can fail to converge. This can be due to various reasons, such as incorrect beat frequency, insufficient number of harmonics, or poor initial guess. If you're having convergence problems, try the following:
   • Double-check the beat frequency: Make sure the beat frequency is accurate. If you're not sure, try running a quick transient simulation to estimate the oscillation frequency.
   • Increase the number of harmonics: Adding more harmonics can improve accuracy and help the simulation converge.
   • Adjust the accuracy settings: Try tightening the accuracy settings to improve the convergence. However, be aware that this will increase the simulation time.
   • Try a different solver: Experiment with different solvers to see if one of them converges better.
   • Simplify the circuit: If the circuit is very complex, try simplifying it to see if that helps with convergence.

Advanced PSS Techniques

Once you've mastered the basics of PSS, you can explore some advanced techniques to further enhance your simulations.

• Shooting Method: The shooting method is an alternative solver algorithm for PSS. It's particularly useful for circuits with strong nonlinearities. The shooting method works by guessing an initial state and then simulating the circuit over one period. The final state is then compared to the initial state, and the difference is used to adjust the initial state. This process is repeated until the initial and final states converge.
• Harmonic Balance: Harmonic balance is another solver algorithm for PSS. It's particularly well-suited for circuits with a small number of dominant harmonics. Harmonic balance works by representing the circuit's voltages and currents as a sum of harmonics. The circuit equations are then solved in the frequency domain, taking into account the interactions between the harmonics.
• PSS with Envelope Tracking (PXF): PXF is an extension of PSS that allows you to simulate circuits with slowly varying envelopes. This is useful for analyzing circuits such as mixers and modulators. PXF works by combining PSS with envelope tracking. PSS is used to analyze the periodic behavior of the circuit, while envelope tracking is used to analyze the slow variations in the envelope.

Conclusion

So there you have it – a comprehensive tutorial on Cadence PSS simulations! We've covered the basics of PSS, how to set up a simulation in Cadence, how to analyze the results, and some advanced techniques. With this knowledge, you should be well-equipped to tackle a wide range of periodic circuit simulations. PSS is a powerful tool, and mastering it will significantly enhance your RFIC design capabilities. Keep practicing, and don't be afraid to experiment with different settings and techniques. Happy simulating, folks! I hope this was helpful, let me know if you have any further questions! Peace out.