{"id":1868,"date":"2017-11-02T08:00:16","date_gmt":"2017-11-02T15:00:16","guid":{"rendered":"http:\/\/www.autodesk.com\/products\/eagle\/blog\/?p=1868"},"modified":"2023-07-17T23:30:06","modified_gmt":"2023-07-18T06:30:06","slug":"spice-simulation-part-3-dc-ac-sweep-analysis","status":"publish","type":"post","link":"https:\/\/www.autodesk.com\/products\/fusion-360\/blog\/spice-simulation-part-3-dc-ac-sweep-analysis\/","title":{"rendered":"SPICE Simulation Part 3: DC and AC Sweep Analysis"},"content":{"rendered":"\n

Welcome back to our SPICE Simulation Series, Part 3! In <\/span>Part 2<\/span><\/a> we showed you how to run a fundamental operating point analysis to simulate current and voltage at a single reference point. Now it\u2019s time to ramp up the difficulty with DC sweep and AC sweep analysis. These two SPICE simulation methods sweep a circuit over a defined voltage or frequency range. This can give you an accurate representation of how your circuit will behave. Running these two simulations will also give us our first look at plot data, probes, multipliers, and more. Let\u2019s get started!<\/span><\/p>\n\n\n

DC Sweep Simulation<\/h2>\n\n\n

We\u2019ll be using DC sweep to calculate our circuit\u2019s bias point over a range of voltage values. By defining the start and stop voltage values we can get an accurate representation of how voltage fluctuates in specific scenarios.<\/span><\/p>\n\n\n

Example Project<\/h3>\n\n\n

The example project we\u2019ll be working with is a FET characterization schematic. Let\u2019s open this now. Look for the <\/span>Fet_Characterization<\/b> project folder<\/b> within the <\/span>Projects \u00bb ngspice<\/b> directory in your <\/span>Autodesk EAGLE Control Panel<\/b>. Then open <\/span>FETCHAR.sch<\/b>.<\/span><\/p>\n\n\n\n

This schematic is a basic circuit that many beginner engineers use to learn what a FET is and what voltage curves look like in a SPICE simulation. With this circuit, we\u2019ll be sweeping voltage from the FET gate to ground with a defined range of 0 to 10 volts.<\/span><\/p>\n\n\n\n

\"fetchar<\/figure>\n\n\n

Simulation Configuration<\/h3>\n\n\n

We\u2019ll begin by selecting the <\/span>Simulation \"spice-simulation-icon\" <\/b>icon<\/b> to open the <\/span>Simulation dialog<\/b>. Here we can see that DC sweep is the chosen simulation type with preconfigured <\/span>Source<\/b>, <\/span>Start Value<\/b>, and <\/span>End Value<\/b> parameters.<\/span><\/p>\n\n\n\n

\"dc<\/figure>\n\n\n\n

One important thing to note is that all of these values come up as presets because there\u2019s a SPICE simulation file saved in this project\u2019s folder. Every time you open the Configuration tab in a SPICE simulation and adjust settings, EAGLE saves a .simcfg file that sits next to your .sch file. <\/span><\/p>\n\n\n\n

\"simcfg<\/figure>\n\n\n\n

Before running our simulation, let\u2019s talk about two quick points – range accuracy and multipliers. EAGLE always checks for proper parameters in your selected simulation type before the simulation completes. This might seem obvious, but it\u2019s good to point out that the simulation simply won\u2019t run if you enter an invalid start or end value. For example, if we were to enter a letter for the start value we\u2019d get an error after selecting the Simulate button as shown below:<\/span><\/p>\n\n\n\n

\"error<\/figure>\n\n\n\n

Next, take a look at the bottom portion of the Simulation dialog, and you\u2019ll see a text description about <\/span>Multipliers<\/b>. This is explaining that you can use standard units in combination with scientific notification and multipliers to define specific frequency ranges or start values. <\/span><\/p>\n\n\n\n

\"multipliers\"<\/figure>\n\n\n\n

For example, if you were doing a transient analysis you could type in 1ms for the start time. For a stop time, you could use scientific notation and type in 5e-3s. Both methods of notation end up being translated in the same way, so it comes down to personal preference. <\/span><\/p>\n\n\n

Simulation Analysis<\/h3>\n\n\n

Now that we have all the basics covered let\u2019s run our DC sweep simulation. We\u2019re going to sweep our gate voltage and should see the current increase as the transistor turns on. Press the <\/span>Simulate button<\/b> to see what happens.<\/span><\/p>\n\n\n\n

Notice that because we ran a DC sweep the results go directly to the Plot tab instead of the Simulator Output. Here we can see voltages on the left-side Y axis, current on the right-side Y-axis, and our voltage sweep range on the X-axis. vgs is what was swept.<\/span><\/p>\n\n\n\n

If you move your cursor from left to right on the plot graph, it will follow all of the lines and update the values on the right-hand side. In this example, we want to determine the exact point where vds and vcur_1 intersect.<\/span><\/p>\n\n\n\n

\"plot<\/figure>\n\n\n\n

You can zoom into a specific point on the graph by left-clicking and drag over a specific area. Double-clicking will restore the default view. Here we can see that the FET turns on as vgs increases to about 1.6 volts. This is also the point where our current starts flowing from drain to source.<\/span><\/p>\n\n\n\n

\"plot<\/figure>\n\n\n\n

Great! Let\u2019s move on to a more advanced simulation with AC sweep. <\/span><\/p>\n\n\n

AC Sweep Simulation<\/h2>\n\n\n

AC sweep simulation is used when you want to calculate the small-signal response of a circuit. This method will first use operating point analysis to gather linear, small-signal models for all nonlinear components. The snapshot of this circuit is then analyzed based on a defined frequency range.<\/span><\/p>\n\n\n

Example Project<\/h3>\n\n\n

The example project we\u2019ll be working with is an RC filter schematic. Let\u2019s open this now. Look for the <\/span>rcfilter project folder<\/b> within the <\/span>Projects \u00bb ngspice<\/b> directory in your <\/span>Autodesk EAGLE Control Panel<\/b>. Then open <\/span>rcfilter.sch<\/b>.<\/span><\/p>\n\n\n\n

In this circuit we see the input signal, V1, flowing through three RC filters with different cutoff points. We\u2019re expecting cutoffs to occur at 1.59KHz, 159KHz, and 15.9MHz. All of the resistor values are the same, so our focus will be on tweaking the capacitor values.<\/span><\/p>\n\n\n\n

\"rcfilter<\/figure>\n\n\n

Simulation Configuration<\/h3>\n\n\n

We\u2019ll begin by selecting the <\/span>Simulation<\/b> <\/b>\"spice-simulation-icon\" icon<\/b> at the top of our interface to open the Simulation dialog<\/b>. Notice that AC sweep is already set up with some default values. Starting Freq<\/b> is set to 1Hz, End Freq <\/b>is set to 500 MHz. Also make a note of the Type<\/b> set to Dec<\/b>, or Decade. If you choose Decade, you can\u2019t use zero as a starting frequency due to the logarithmic nature of how AC sweep analysis works. Because of this, we start our sweep at 1Hz.<\/p>\n\n\n\n

\"ac<\/figure>\n\n\n

Simulation Analysis<\/h3>\n\n\n

There\u2019s nothing else to configure on this tab, so let\u2019s press the <\/span>Simulate button<\/b>. Again we\u2019re taken to the Plot tab to see our results. Let\u2019s take a look at the buttons on the bottom which provide some quick filtering options for our data:<\/span><\/p>\n\n\n\n

\"plot<\/figure>\n\n\n\n