SPICE Simulation

Overview

KiCad includes integrated SPICE simulation capabilities through ngspice, allowing you to simulate analog and mixed-signal circuits directly from your schematic. The simulation system supports AC, DC, transient, and noise analysis.

Architecture

The simulation system consists of several key components:

NGSPICE Interface

Defined in /eeschema/sim/ngspice.h:

class NGSPICE : public SPICE_SIMULATOR
{
public:
    NGSPICE();
    virtual ~NGSPICE();
    
    // Initialize simulator
    void Init( const SPICE_SETTINGS* aSettings = nullptr ) override;
    
    // Attach model and prepare simulation
    bool Attach( const std::shared_ptr<SIMULATION_MODEL>& aModel,
                 const wxString& aSimCommand,
                 unsigned aSimOptions,
                 const wxString& aInputPath,
                 REPORTER& aReporter ) override;
    
    // Load and run netlist
    bool LoadNetlist( const std::string& aNetlist ) override;
    bool Run() override;
    bool Stop() override;
    
    // Query simulation results
    std::vector<std::string> AllPlots() const override;
    std::vector<std::string> GetPlotVectors( const std::string& aPlot ) override;
    std::vector<double> GetRealPlotVector( const std::string& aName ) override;
    std::vector<double> GetImagPlotVector( const std::string& aName ) override;
};

Simulation Library Manager

Defined in /eeschema/sim/sim_lib_mgr.h:

class SIM_LIB_MGR
{
public:
    SIM_LIB_MGR( const PROJECT* aPrj );
    
    void SetLibrary( const wxString& aLibraryPath, REPORTER& aReporter );
    
    // Create models from symbols
    SIM_LIBRARY::MODEL CreateModel(
        const SCH_SHEET_PATH* aSheetPath,
        SCH_SYMBOL& aSymbol,
        bool aResolve,
        int aDepth,
        const wxString& aVariantName,
        REPORTER& aReporter,
        const wxString& aMergedSimPins = wxEmptyString
    );
    
    // Get all libraries and models
    std::map<wxString, std::reference_wrapper<const SIM_LIBRARY>>
        GetLibraries() const;
    std::vector<std::reference_wrapper<SIM_MODEL>> GetModels() const;
};

SPICE Model Types

KiCad supports various SPICE model types:

Built-in Primitive Models

R: Resistor
C: Capacitor
L: Inductor
V: Voltage source (DC, AC, transient)
I: Current source
D: Diode
Q: BJT transistor
M: MOSFET
J: JFET
X: Subcircuit

Assigning Models to Symbols

In the schematic, symbols are assigned simulation models through fields:

Sim.Device: R
Sim.Type: RESISTOR
Sim.Params: r=10k

Or reference external SPICE models:

Sim.Library: /path/to/model.lib
Sim.Name: 2N2222
Sim.Pins: 1=C 2=B 3=E

Running Simulations

DC Operating Point

Analyze DC voltages and currents:

.op

DC Sweep

Sweep a voltage or current source:

.dc V1 0 10 0.1

AC Analysis

Frequency domain analysis:

.ac dec 10 1 100k

Parameters:

dec: Decade sweep (10 points per decade)
1: Start frequency (1 Hz)
100k: End frequency (100 kHz)

Transient Analysis

Time-domain simulation:

.tran 1u 100u

Parameters:

1u: Time step (1 microsecond)
100u: Total time (100 microseconds)

Noise Analysis

.noise v(out) V1 dec 10 1 100k

SPICE Netlist Export

KiCad generates SPICE netlists from schematics using /eeschema/netlist_exporters/netlist_exporter_spice.h:

Netlist Structure

* KiCad SPICE Netlist
* Circuit: My Amplifier
* Date: 2024-01-15

.title My Amplifier

* Components
R1 1 2 10k
R2 2 0 1k
C1 2 3 100n
Q1 3 2 0 2N2222

* Sources
V1 1 0 DC 12

* Analysis
.tran 1u 1m
.ac dec 10 1 1MEG

.control
run
plot v(3)
.endc

.end

Probe Points and Measurements

Voltage Probes

.print tran v(out) v(in) v(ref)

Current Probes

Insert voltage source with 0V to measure current:

Vmeas 5 6 DC 0
.print tran i(Vmeas)

Power Calculations

.print tran {v(out)*i(Vmeas)}

Python Scripting Integration

While KiCad’s simulation is primarily GUI-based, you can export and run netlists programmatically:

import subprocess
import os

def run_spice_simulation(netlist_path, output_path):
    """
    Run ngspice simulation from Python
    """
    # Create control file
    control_script = f"""
    source {netlist_path}
    run
    print all > {output_path}
    quit
    """
    
    control_file = netlist_path.replace('.cir', '.control')
    with open(control_file, 'w') as f:
        f.write(control_script)
    
    # Run ngspice
    result = subprocess.run(
        ['ngspice', '-b', control_file],
        capture_output=True,
        text=True
    )
    
    return result.returncode == 0

def parse_results(output_path):
    """
    Parse simulation results
    """
    with open(output_path, 'r') as f:
        lines = f.readlines()
    
    # Parse data (simplified)
    data = {}
    for line in lines:
        if line.strip() and not line.startswith('#'):
            parts = line.split()
            if len(parts) >= 2:
                data[parts[0]] = float(parts[1])
    
    return data

# Usage
if run_spice_simulation('circuit.cir', 'results.txt'):
    results = parse_results('results.txt')
    print(f"Output voltage: {results.get('v(out)', 'N/A')}")

Advanced SPICE Features

Behavioral Sources

* Arbitrary voltage source
BV1 out 0 V={sin(2*pi*1k*time)}

* Arbitrary current source
BI1 out 0 I={V(in)*0.001}

Subcircuits

.subckt opamp in+ in- vcc vee out
Rin in+ in- 1MEG
Cin in+ in- 1pF
Gm 0 int {(V(in+)-V(in-))*0.001}
Rout int out 75
Cout int 0 10pF
.ends opamp

* Usage
X1 inp inn vcc vee output opamp

Parameter Sweeps

.param R1val=1k
R1 1 2 {R1val}

.step param R1val list 1k 2k 5k 10k

Temperature Analysis

.temp 0 25 50 75 100
.tran 1u 1m

Model Libraries

Loading External Models

.lib /usr/share/ngspice/models/standard.lib
.include /path/to/custom_models.mod

IBIS Models

For signal integrity analysis, KiCad supports IBIS models through /eeschema/sim/kibis/:

class KIBIS
{
public:
    bool Init( const std::string& aFileName );
    KIBIS_COMPONENT* GetComponent( const std::string& aName );
    std::vector<KIBIS_PIN> GetPins( const std::string& aComponent );
};

Performance Optimization

Convergence Issues

* Improve convergence
.options RELTOL=1e-4
.options ABSTOL=1e-10
.options VNTOL=1e-4

* Reduce timestep
.tran 0.1u 1m 0 0.01u

Memory Management

* Limit memory usage
.options MAXORD=2
.options ITL1=300
.options ITL4=100

Simulation Results Analysis

Fourier Analysis

.fourier 1kHz v(out)

Distortion Analysis

.distortion dec 10 1 100k -0.1

Pole-Zero Analysis

.pz v(out) V1

Common Simulation Patterns

RC Low-Pass Filter

* Input
V1 in 0 AC 1

* Filter
R1 in out 1k
C1 out 0 159n

* Analysis
.ac dec 10 1 100k
.control
run
plot vdb(out)
.endc

BJT Amplifier

* Bias
VCC vcc 0 DC 12
R1 vcc base 47k
R2 base 0 10k
Rc vcc coll 2k
Re emit 0 500

* Coupling
Cin in base 10u
Cout coll out 10u

* Transistor
Q1 coll base emit 2N2222

* Signal
Vin in 0 AC 0.01

.ac dec 10 10 1MEG

Troubleshooting

Common Errors

“Singular matrix”

Add small resistor to floating nodes
Check for loops of voltage sources or capacitors

“Timestep too small”

Reduce RELTOL
Add small resistance to capacitors
Check for unrealistic component values

“No convergence”

Set better initial conditions
Use .nodeset to hint at solutions
Reduce circuit complexity

Best Practices

Start simple: Verify basic operation before adding complexity
Use realistic values: Avoid extreme component values
Set appropriate tolerances: Balance speed vs. accuracy
Include parasitics: Add ESR, ESL for real-world behavior
Validate with measurements: Compare simulation to hardware
Document models: Comment model sources and parameters

​Overview

​Architecture

​NGSPICE Interface

​Simulation Library Manager

​SPICE Model Types

​Built-in Primitive Models

​Assigning Models to Symbols

​Running Simulations

​DC Operating Point

​DC Sweep

​AC Analysis

​Transient Analysis

​Noise Analysis

​SPICE Netlist Export

​Netlist Structure

​Probe Points and Measurements

​Voltage Probes

​Current Probes

​Power Calculations

​Python Scripting Integration

​Advanced SPICE Features

​Behavioral Sources

​Subcircuits

​Parameter Sweeps

​Temperature Analysis

​Model Libraries

​Loading External Models

​IBIS Models

​Performance Optimization

​Convergence Issues

​Memory Management

​Simulation Results Analysis

​Fourier Analysis

​Distortion Analysis

​Pole-Zero Analysis

​Common Simulation Patterns

​RC Low-Pass Filter

​BJT Amplifier

​Troubleshooting

​Common Errors

​Best Practices

​See Also