Spectre Tech Tips: Measuring Noise in Digital Circuits

As a designer, verification engineer, or CAD expert, you use Spectre® APS for analyzing your designs. Sometimes, you use Spectre to measure noise in digital circuits. Are you confused which method to use for noise measurement since Spectre provides different methods for measuring noise? This blog discusses the different noise measuring methods and explains why they may provide different results.

PNOISE Overview

Pnoise has two basic modes, timeaverage and sampled.  Both use PSS solution as the basis of noise measurement and both include noise folding defined by maximum sidebands for default pnoise, or the pss option maxacfreq for fullspectrum pnoise. Timeaverage calculates the noise power averaged over all the PSS timepoints and is usually used for noise figure measurements, and for measuring oscillator phase noise. Sampled follows the default pnoise result with an ideal sampler that measures the noise at one specific time in PSS analysis. This is usually used to measure noise for an output that crosses a threshold of a succeeding stage.

Pnoise sampled has three modes: Edge Crossing, Edge Delay, and Sampled Phase.

• Edge Crossing mode measures the noise on a net where the time is defined by an edge. You specify the threshold voltage and whether the voltage is rising or falling. Typically, this is half of VDD. There are natural variations in the digital threshold created by process variations. Multiple measurements can be made at different thresholds of interest. This mode has direct plot functions to calculate the jitter on the measured net.
• Edge Delay mode measures the noise defined by two edges. Both edges are defined by a threshold voltage and rising or falling edges. This mode can be used to measure the noise of a pulse where the rising edge is the start and the falling edge is the end of the measurement. This measures the noise of the pulse itself. This mode also has direct plot functions to calculate the variation of the pulse width. In addition, this mode can be used to measure the jitter between any two edges in the circuit.
• Sampled Phase mode is seldom used. It was originally created so that noise could be measured at different points in an edge caused by natural process variations. First, you run PSS analysis and plot the waveform of the edge you want to measure.  Next, you get the times that correspond to the voltages in the edge. These times are then manually entered in the Additional Points field of the analysis. Unlike the other two modes, there is no direct plot function to calculate the jitter from Sampled Phase mode.

 PNOISE Setup

 The following figure displays a multiple inverter chain circuit driven at 2.5GHz. 

Because this is a digital circuit with square wave output, shooting is used for this example.  The stop frequency in pnoise is set to half the input frequency, which is 1.25GHz. The start frequency is at least 3 decades below that to capture at least 99.9% of all the noise. You can use either default pnoise or fullspectrum pnoise for this circuit since the input frequency is high. If the input frequency is less than about one fiftieth of the maximum noise frequency, fullspectrum should be used because it will run faster.  Both timeaverage and sampled Edge Crossing mode are run, as shown below.

 In timeaverage, the USB result is plotted in dBc/Hz and Edge Phase Noise is plotted for sampled, as show below.

 The waveforms are shown below.

Result Explanation

To determine why the sampled result is larger than the timeaverage result, pnoise sampled phase was run with noise analyses taken at 100 points in the PSS waveform. Then the direct plot function integrated noise was used. The output waveform and the integrated noise are plotted below.

Note that the noise is largest near the VDD/2 point. When the output is low or high, the transistors are just resistors without the ability to amplify. During the edge, both devices are active where they can amplify the noise. The specific shape will depend on the devices, however, most will have similar shapes. In the case of a digital circuit, noise in low and high states does not matter because the noise will never reach the switching threshold. Noise near the switching threshold is what matters, and this is what is calculated using pnoise sampled.

Conclusion

For digital gates, pnoise sampled Edge Crossing mode with the direct plot function Edge Phase Noise is the correct way to measure phase noise.

Related Resources

Spectre Circuit Simulator and Accelerated Parallel Simulator RF Analysis in ADE Explorer User Guide

 You may also contact your Cadence support AE for guidance.

 For more information on Cadence products and services, visit www.cadence.com.

About Spectre Tech Tips

Spectre Tech Tips is a blog series aimed at exploring the capabilities and potential of Spectre®. In addition to providing insight into the useful features and enhancements in Spectre, this series broadcasts the voice of different bloggers and experts, who share their knowledge and experience on all things related to Spectre. Enter your email address in the Subscriptions box and click SUBSCRIBE NOW to receive notifications about our latest Spectre Tech Tips posts.