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LTspice Circuit Elements Reference

Complete reference for all circuit element types in LTspice, including syntax, parameters, and models.

Table of Contents

  1. Element Overview
  2. R — Resistor
  3. C — Capacitor
  4. L — Inductor
  5. D — Diode
  6. Q — Bipolar Transistor
  7. M — MOSFET
  8. J — JFET
  9. Z — MESFET and IGBT
  10. V — Voltage Source
  11. I — Current Source
  12. E — Voltage-Dependent Voltage Source
  13. G — Voltage-Dependent Current Source
  14. H — Current-Dependent Voltage Source
  15. F — Current-Dependent Current Source
  16. S — Voltage-Controlled Switch
  17. W — Current-Controlled Switch
  18. K — Mutual Inductance
  19. T — Lossless Transmission Line
  20. O — Lossy Transmission Line
  21. U — Uniform RC-Line
  22. B — Behavioral Source
  23. A — Special Functions
  24. X — Subcircuit
  25. @ — Frequency Response Analyzer
  26. & — Frequency Response Analysis Probe

Element Overview

The first character of a netlist line determines the element type. Some elements use flags as parameters — a flag is active if present, or you can assign a value (>=0.5 = on, <0.5 = off).

Dynamic Nodes: Node names can be expressions using parameters, e.g., I1 {CONNECT} 0 2m.

R — Resistor

Symbols: RES, RES2

Rxxx n1 n2 <value> [tc=tc1, tc2, ...] [temp=<value>]

Temperature dependence:

R = R0 * (1 + dt*tc1 + dt^2*tc2 + dt^3*tc3 + ...)

Where R0 is resistance at nominal temperature, dt is temperature difference from nominal.

Parameters:

  • tc: Temperature coefficients (comma-separated list)
  • temp: Instance temperature

Example:

R1 in out 10K tc=0.01, 0.001

C — Capacitor

Symbols: CAP, POLCAP

Cxxx n1 n2 <capacitance> [ic=<value>]
+ [Rser=<value>] [Lser=<value>] [Rpar=<value>]
+ [Cpar=<value>] [m=<value>] [RLshunt=<value>] [temp=<value>]
Parameter Description
Rser Equivalent series resistance (ESR)
Lser Equivalent series inductance (ESL)
Rpar Equivalent parallel resistance
Cpar Equivalent parallel capacitance
RLshunt Shunt resistance across Lser
m Number of parallel units (m=0 removes from simulation)
ic Initial voltage
temp Instance temperature

Nonlinear Capacitor

Cxxx n1 n2 Q=<expression> [ic=<value>] [m=<value>]

Where x = voltage across device. Examples:

C1 n1 n2 Q=100p*x          ; constant 100pF
C2 n1 n2 Q=x*if(x<0,100p,300p)  ; voltage-dependent

L — Inductor

Symbols: IND, IND2

Lxxx n+ n- <inductance> [ic=<value>]
+ [Rser=<value>] [Rpar=<value>] [Cpar=<value>]
+ [m=<value>] [temp=<value>]
Parameter Description Default
Rser Equivalent series resistance 1m (for SMPS; set Rser=0 to disable)
Rpar Equivalent parallel resistance —
Cpar Equivalent parallel capacitance —
m Number of parallel units 1
ic Initial current —
tc1, tc2 Temperature coefficients 0

Behavioral Inductance (Flux-Based)

L1 N001 0 Flux=1m*tanh(5*x)

Where x = inductor current.

Hysteretic Core Model

L1 N001 0 Hc=16 Bs=.44 Br=.10 A=0.0000251 Lm=0.0198 Lg=0.0006858 N=1000
Parameter Description Units
Hc Coercive force A-turns/meter
Br Remnant flux density Tesla
Bs Saturation flux density Tesla
Lm Magnetic length (excluding gap) meter
Lg Gap length meter
A Cross-sectional area meter^2
N Number of turns —

D — Diode

Symbols: DIODE, ZENER, SCHOTTKY, VARACTOR, LED, TVS

Dxxx anode cathode <model> [area] [off] [m=<val>] [n=<val>] [temp=<value>]
Parameter Description
area Multiple parallel devices
m Number of parallel devices
n Number of series devices
off Initial condition hint
temp Instance temperature

Idealized Diode Model (Region-Wise Linear)

Parameter Description Default
Ron Forward conduction resistance (mOhm) 1.0
Roff Off-state resistance 1/Gmin
Vfwd Forward threshold voltage 0.0 V
Vrev Reverse breakdown voltage Infinite
Rrev Breakdown impedance Ron
Ilimit Forward current limit Infinite
Revilimit Reverse current limit Infinite
Epsilon Quadratic region width 0.0 V
Revepsilon Reverse quadratic region width 0.0 V

Berkeley SPICE Diode Model (Key Parameters)

Parameter Description Units Default
Is Saturation current A 1e-14
Rs Ohmic resistance Ohm 0
N Emission coefficient — 1.0
Tt Transit time sec 0
Cjo Zero-bias junction capacitance F 0
Vj Junction potential V 1.0
M Grading coefficient — 0.5
BV Reverse breakdown voltage V Infinite
Ibv Current at breakdown A 1e-10
Eg Activation energy (Si=1.11, Sbd=0.69) eV 1.11
Xti Saturation current temp exponent — 3.0
Kf Flicker noise coefficient — 0
Af Flicker noise exponent — 1
Tnom Parameter measurement temperature C 27
Vp Soft reverse recovery parameter — 0

Q — Bipolar Transistor

Symbols: NPN, PNP, NPN2, PNP2

Qxxx Collector Base Emitter [Substrate] <model> [area] [off] [temp=<T>]

Key Gummel-Poon Parameters

Parameter Description Units Default
Is Transport saturation current A 1e-16
Bf Ideal maximum forward beta — 100
Nf Forward emission coefficient — 1
Vaf Forward Early voltage V Infinite
Ikf Forward beta roll-off corner A Infinite
Br Ideal maximum reverse beta — 1
Var Reverse Early voltage V Infinite
Rb Zero-bias base resistance Ohm 0
Re Emitter resistance Ohm 0
Rc Collector resistance Ohm 0
Cje B-E zero-bias depletion cap F 0
Cjc B-C zero-bias depletion cap F 0
Tf Forward transit time sec 0
Tr Reverse transit time sec 0
BVcbo Collector-base breakdown V Infinite
BVbe Base-emitter breakdown V Infinite
Tnom Measurement temperature C 27

Set Level=9 for VBIC (Vertical Bipolar Inter Company) model.

M — MOSFET

Symbols: NMOS, NMOS3, PMOS, PMOS3

Monolithic MOSFET

Mxxx Nd Ng Ns Nb <model> [m=<value>] [L=<len>] [W=<width>]
+ [AD=<area>] [AS=<area>] [PD=<perim>] [PS=<perim>]
+ [NRD=<value>] [NRS=<value>] [off] [IC=<Vds,Vgs,Vbs>] [temp=<T>]

VDMOS Power MOSFET

Mxxx Nd Ng Ns <model> [L=<len>] [W=<width>] [M=<area>] [m=<value>]
+ [off] [IC=<Vds,Vgs,Vbs>] [temp=<T>]

MOSFET Levels

Level Model
1 Shichman-Hodges (default)
2 MOS2
3 MOS3 (semi-empirical)
4 BSIM
5 BSIM2
6 MOS6
7, 8, 49 BSIM3v3.3
9 BSIMSOI3.2 (SOI)
12, 44, 55 EKV 2.6
14, 54 BSIM4.6.1
73 HiSIMHV 1.2

Key Level 1/2/3 Parameters

Parameter Description Units Default
Vto Threshold voltage V 0
Kp Transconductance parameter A/V^2 2e-5
Gamma Bulk threshold parameter V^0.5 0
Phi Surface inversion potential V 0.6
Lambda Channel-length modulation 1/V 0
Rd Drain resistance Ohm 0
Rs Source resistance Ohm 0
Tox Oxide thickness m 1e-7
Uo Surface mobility cm^2/V/s 600
Tnom Measurement temperature C 27

Key VDMOS Parameters

Parameter Description Units Default
Vto Threshold voltage V 0
Kp Transconductance A/V^2 1
Lambda Channel-length modulation 1/V 0
mtriode Triode region multiplier — 1
Ksubthres Subthreshold parameter — 0
BV Breakdown voltage V Infinite
Rd Drain resistance Ohm 0
Rs Source resistance Ohm 0
Rg Gate resistance Ohm 0
Rds Drain-source shunt resistance Ohm Infinite
Cgs Gate-source capacitance F 0
Cgdmin Min gate-drain capacitance F 0
Cgdmax Max gate-drain capacitance F 0
Cjo Body diode junction cap F 0
Is Body diode saturation current A 1e-14
Bex Kp temperature exponent — -1.5
vtotc Vto temperature coefficient V/C 0

J — JFET

Symbols: NJF, PJF

Jxxx Drain Gate Source <model> [area] [off] [temp=T]

Key Parameters

Parameter Description Units Default
Vto Threshold voltage V -2.0
Beta Transconductance parameter A/V^2 1e-4
Lambda Channel-length modulation 1/V 0
Rd Drain resistance Ohm 0
Rs Source resistance Ohm 0
Cgs Gate-source capacitance F 0
Cgd Gate-drain capacitance F 0
Is Gate junction saturation current A 1e-14
Tnom Measurement temperature C 27

Z — MESFET and IGBT

Symbols: MESFET, NIGBT, PIGBT

MESFET

Zxxx D G S <model> [area] [m=<value>] [off] [temp=<value>]

Model keywords: NMF, PMF

Parameter Description Units Default
Vto Pinch-off voltage V -2.0
Beta Transconductance A/V^2 1e-4
Alpha Saturation voltage parameter 1/V 2.0
Lambda Channel-length modulation 1/V 0
Rd Drain resistance Ohm 0
Rs Source resistance Ohm 0

IGBT

Zxxx C G E <model> [area] [m=<value>] [off] [temp=<value>]

Model keyword: NIGBT (or PIGBT)

Parameter Description Units Default
Vt Threshold voltage V 4.7
KP Transconductance A/V^2 0.38
WB Base width m 9e-5
Tau Recombination lifetime sec 7.1e-6
NB Base doping 1/cm^3 2e14
MUN Electron mobility cm^2/(V*s) 1500
MUP Hole mobility cm^2/(V*s) 450

V — Voltage Source

Symbols: VOLTAGE, BATTERY

Constant

Vxxx n+ n- <voltage> [AC=<amplitude>] [Rser=<value>] [Cpar=<value>]

PULSE

Vxxx n+ n- PULSE(V1 V2 Tdelay Trise Tfall Ton Tperiod Ncycles)
Parameter Description Units
V1 (Voff) Initial value V
V2 (Von) Pulsed value V
Tdelay Delay time sec
Trise Rise time sec
Tfall Fall time sec
Ton On time sec
Tperiod Period sec
Ncycles Number of cycles —

SINE

Vxxx n+ n- SINE(Voffset Vamp Freq Td Theta Phi Ncycles)
Parameter Description Units
Voffset DC offset V
Vamp Amplitude V
Freq Frequency Hz
Td Delay sec
Theta Damping factor 1/sec
Phi Phase degrees
Ncycles Number of cycles —

Formula (for t >= Td):

V = Voffset + Vamp * exp(-(t-Td)*Theta) * sin(2*pi*Freq*(t-Td) + pi*Phi/180)

EXP

Vxxx n+ n- EXP(V1 V2 Td1 Tau1 Td2 Tau2)
Parameter Description Units
V1 Initial value V
V2 Pulsed value V
Td1 Rise delay sec
Tau1 Rise time constant sec
Td2 Fall delay sec
Tau2 Fall time constant sec

SFFM (Single Frequency FM)

Vxxx n+ n- SFFM(Voff Vamp Fcar MDI Fsig)

Formula: Voff + Vamp*sin(2*pi*Fcar*t + MDI*sin(2*pi*Fsig*t))

PWL (Piece-Wise Linear)

Vxxx n+ n- PWL(t1 v1 t2 v2 t3 v3 ...)
Vxxx n+ n- PWL REPEAT FOR <n> (t1 v1 t2 v2 ...) ENDREPEAT
Vxxx n+ n- PWL REPEAT FOREVER (t1 v1 t2 v2 ...) ENDREPEAT
Vxxx n+ n- PWL FILE=<filename>

Options: TIME_SCALE_FACTOR=<val>, VALUE_SCALE_FACTOR=<val>

WAV File

Vxxx n+ n- wavefile=<filename> [chan=<nnn>]

Full-scale range: -1V to 1V. Valid channels: 0-65535.

Trigger Parameter

All time-dependent sources support:

Vxxx n+ n- PULSE(...) Trigger=<expr> [tripdv=<val>] [tripdt=<val>]

The expression is continuously evaluated; the source restarts its time sequence when the expression transitions from false to true.

I — Current Source

Symbol: CURRENT

Same waveform types as voltage source (PULSE, SINE, EXP, SFFM, PWL, WAV), with current units instead of voltage.

Additional Current Source Features

Load flag:

Ixxx n+ n- <current> [load]

Forces dissipative behavior — current reduces to zero if voltage drops below 0.5V. Final impedance: 0.25 Ohm/A * current.

Lookup table:

Ixxx n+ n- tbl=(<voltage, current>, <voltage, current>, ...)

Step load:

Ixxx n+ n- <value> step(<val1>, <val2>, <val3>, ...)

Advances to next current value at steady state.

Resistive load:

Ixxx n+ n- R=<value>

E — Voltage-Dependent Voltage Source

Symbols: E, E2

Linear Gain

Exxx n+ n- nc+ nc- <gain>

Lookup Table

Exxx n+ n- nc+ nc- table=(<vin, vout>, <vin, vout>, ...)

Laplace Transfer Function

Exxx n+ n- nc+ nc- Laplace=<func(s)> [window=<time>] [nfft=<number>] [mtol=<number>]

Behavioral Expression

Exxx n+ n- value={<expression>}

Polynomial (legacy)

Exxx n+ n- POLY(N) (n1+,n1-) (n2+,n2-) ... c0 c1 c2 ...

G — Voltage-Dependent Current Source

Symbols: G, G2

Same forms as E source (linear gain, table, Laplace, behavioral, polynomial), but output is current. Gain units: A/V (transconductance).

Gxxx n+ n- nc+ nc- <transconductance>
Gxxx n+ n- nc+ nc- table=(<vin, iout>, ...)
Gxxx n+ n- nc+ nc- Laplace=<func(s)> [window=<time>] [nfft=<n>] [mtol=<n>]
Gxxx n+ n- value={<expression>}

H — Current-Dependent Voltage Source

Symbol: H

Hxxx n+ n- <Vnam> <transresistance>

Output voltage = transresistance * current through voltage source Vnam.

Also supports behavioral and polynomial forms.

F — Current-Dependent Current Source

Symbol: F

Fxxx n+ n- <Vnam> <gain>

Output current = gain * current through voltage source Vnam.

Also supports behavioral and polynomial forms.

S — Voltage-Controlled Switch

Symbol: SW

Sxxx n1 n2 nc+ nc- <model> [on,off]

Model Parameters

Parameter Description Units Default
Vt Threshold voltage V 0
Vh Hysteresis voltage V 0
Ron On resistance Ohm 1
Roff Off resistance Ohm 1/Gmin
Lser Series inductance H 0
Vser Series voltage V 0
Ilimit Current limit A Infinite

Hysteresis Modes

  • Vh = 0: Hard switch at Vt
  • Vh > 0: Hysteresis; trips at Vt+Vh and Vt-Vh
  • Vh < 0: Smooth transition between Vt- Vh and Vt+ Vh (recommended — avoids discontinuity)

W — Current-Controlled Switch

Symbol: CSW

Wxxx n1 n2 Vnam <model> [on,off]

Control current sensed through voltage source Vnam.

Parameter Description Units Default
It Threshold current A 0
Ih Hysteresis current A 0
Ron On resistance Ohm 1
Roff Off resistance Ohm 1/Gmin

K — Mutual Inductance

Kxxx L1 L2 [L3 ...] <coefficient>

Coupling coefficient range: -1 to 1. Listing multiple inductors creates all pairwise couplings:

K1 L1 L2 L3 L4 1.0   ; creates 6 mutual couplings for all pairs

Start with coefficient = 1.0 to eliminate leakage inductance ringing.

T — Lossless Transmission Line

Symbol: TLINE

Txxx L+ L- R+ R- Zo=<value> Td=<value>
  • Zo: Characteristic impedance
  • Td: Propagation delay

O — Lossy Transmission Line

Symbol: LTLINE

Oxxx L+ L- R+ R- <model>

Model Parameters

Parameter Description Units Default
R Resistance per unit length Ohm 0
L Inductance per unit length H 0
G Conductance per unit length 1/Ohm 0
C Capacitance per unit length F 0
Len Line length — 0

U — Uniform RC-Line

Symbol: URC

Uxxx N1 N2 Ncom <model> L=<len> [N=<lumps>]
  • N1, N2: Element nodes
  • Ncom: Common node for capacitances
  • L: Line length in meters
  • N: Number of lumped segments (auto if omitted)
Parameter Description Units Default
K Propagation constant — 2.0
Fmax Max frequency of interest Hz 1G
Rperl Resistance per unit length Ohm 1K
Cperl Capacitance per unit length F 1e-15
Isperl Saturation current per length A 0
Rsperl Diode series R per length Ohm 0

B — Behavioral Source

Symbols: BV (voltage), BI (current)

Voltage Source

Bxxx n+ n- V=<expression> [ic=<value>] [tripdv=<val>] [tripdt=<val>]
+ [laplace=<expr> [window=<time>] [nfft=<n>] [mtol=<n>]]

Current Source

Bxxx n+ n- I=<expression> [ic=<value>] [tripdv=<val>] [tripdt=<val>] [Rpar=<value>]
+ [laplace=<expr> [window=<time>] [nfft=<n>] [mtol=<n>]]

Resistor

Bxxx n+ n- R=<expression>

Acts as current = voltage/resistance.

Power Source

Bxxx n+ n- P=<expression> [vprx=<value>]

Acts as current = power/voltage.

Supported Operators (ascending precedence)

Operator Description
&, \|, ^ Boolean AND, OR, XOR
>, <, >=, <=, ==, != Comparison
+, - Addition, subtraction
*, /, % Multiplication, division, modulo
** Exponentiation
!, ~ Logical NOT

Supported Functions

Category Functions
Trig sin, cos, tan, asin, acos, atan, atan2, sinh, cosh, tanh
Exp/Log exp, ln, log10, sqrt, cbrt, pow, pwr, pwrs
Limiting limit, max, min, dnlim, uplim
Logic buf, inv, if, u (unit step), uramp
Calculus ddt (derivative), idt/sdt (integrate), absdelay, delay
Noise rand, random, white, noise
Rounding int, round, ceil, floor, sgn
Other table (lookup), hypot, mod, idtmod

Variables

  • V(node) — node voltage
  • V(n1,n2) — voltage difference
  • I(Vname) — current through voltage source
  • time — current simulation time
  • pi — 3.14159265358979323846

A — Special Functions

Symbols: INV, BUF, AND, OR, XOR, SCHMITT, SCHMTBUF, SCHMTINV, DFLOP, VARISTOR, MODULATE

Axxx in1 in2 in3 in4 in5 out+ out- com <model> [params]

Terminals 1-5: inputs. Terminals 6-7: complementary outputs. Terminal 8: device common. Unused terminals connected to terminal 8 are optimized out.

Gate Parameters

Parameter Default Description
Vhigh 1 Logic high level (V)
Vlow 0 Logic low level (V)
Trise 0 Rise time (sec) — highly recommended to set non-zero (e.g., 10n)
Tfall Trise Fall time (sec)
Td 0 Propagation delay (sec)
Rout 1 Output impedance (Ohm)
Cout 0 Output capacitance (F)
Ref (Vhigh+Vlow)/2 Logic threshold (V)

Schmitt Trigger Parameters

  • Vt: Trip point center voltage
  • Vh: Half-hysteresis; low trip = Vt-Vh, high trip = Vt+Vh (must be positive — negative hysteresis not supported)

OTA (Transconductance Amplifier)

Symbols: OTA, OTA2, MOTA, MOTA2, MOTA3, MOTA8

Parameter Default Description
G 1 Transconductance (S)
Vhigh 2V High compliance limit
Vlow 0V Low compliance limit
Iout 10u Maximum source current
Isink -Iout Maximum sink current
Rout Infinite Output resistance
Cout 0 Output capacitance

MODULATE (VCO)

  • mark: Frequency at FM input = 1V
  • space: Frequency at FM input = 0V

X — Subcircuit

Xxxx n1 n2 n3 ... <subckt_name> [param1=val1] [param2=val2]

Example:

X1 in out 0 divider top=9K bot=1K

.subckt divider A B C
R1 A B {top}
R2 B C {bot}
.ends divider

Parametric instantiation with string expression:

X1 in out 0 {name} top=9K bot=1K

Pin Name Annotation (;§pnba)

Subcircuit instances generated by the LTspice netlister may end with a special comment that records the pin names of the symbol. The comment begins with ;§pnba followed by a list of pin names delimited by a right parenthesis ).

X§U1 0 N001 NC_01 N004 NC_02 N003 N002 NC_03 IN IN LT8609S ;§pnba GND)SW)INTVcc)RT)SYNC)FB)TR/SS)PG)VIN)EN/UV

When present:

  • The pin names appear in the same order as the node (net) assignments on the instance line.
  • Each pin name maps positionally to a node: the first pin name corresponds to the first node, the second to the second, and so on.
  • The names are the labels on the symbol’s pins, letting you recover the symbolic meaning of each net connection.

In the example above, the mapping is:

Node Pin Name
0 GND
N001 SW
NC_01 INTVcc
N004 RT
NC_02 SYNC
N003 FB
N002 TR/SS
NC_03 PG
IN VIN
IN EN/UV

Because ;§pnba is introduced by ;, it is an ordinary comment and is ignored by the simulator — it exists purely to preserve the symbol pin names alongside the net assignments.

@ — Frequency Response Analyzer

Symbols: FRA

@xxx in out [zm] fstart=<val> fend=<val> [delay=<val>] [oct=<val>] [fcoarse=<val>]
+ [nmax=<val>] [[pp0=<val>] [[pp1=<val>] [f0=<val> f1=<val>]]] [tavgmin=<val>]
+ [tsettle=<val>] [rpar=<val>] [flist=<values>] [acmag=<val>] [acphase=<val>]
+ [refnode=<netname>] [intnode=<netname>]

During a .fra simulation, the analyzer applies a range of sinusoidal stimuli and measures the circuit response. Used to analyze loop gain versus frequency or impedance versus frequency. For a step-by-step procedure using this device, see SMPS Bode Plots (FRA).

Analysis mode:

  • Default — gain analysis: applies voltage stimuli and analyzes the absolute voltage at each terminal.
  • With the zm keyword — impedance analysis: applies current stimuli and analyzes the voltage across the terminals.

Example:

@1 A B delay=1m fstart=1k fend=500k oct=1 fcoarse=10k nmax=1 pp0=2m pp1=1m f0=1k f1=2k tavgmin=100u tsettle=200u

Analysis Frequencies

Frequency is stepped from fstart to fend at a resolution set by oct, the number of points per octave — oct=1 doubles the stimulus frequency at every step.

fcoarse forces coarse stepping (maximum one point per octave, oct<=1) below the given frequency; the oct setting then applies only above it. Setting fcoarse to 2–10× fstart can greatly reduce simulation time.

flist applies a list of specific frequencies individually instead of sweeping.

Stimulus Amplitude

Amplitude may be specified by one of three methods:

Method Behavior
pp0 alone Sets one stimulus amplitude across all frequencies.
pp0, pp1, f0, f1 Amplitude is pp0 up to f0, pp1 above f1, and logarithmically interpolated between (f0, pp0) and (f1, pp1). Recommended for most circuits.
pp Piecewise logarithmic amplitude vs. frequency, as space-delimited frequency-voltage pairs. pp=1k 10m 10k 1m is equivalent to f0=1k pp0=10m f1=10k pp1=1m.

Typically pp0 > pp1 — a larger stimulus at low frequencies, where high loop gain suppresses the injected perturbation, and a smaller one at high frequencies, where the loop no longer attenuates it and too much drive would disturb the operating point.

Simultaneous Harmonic Injection

To reduce simulation time, the FRA device may inject a sinusoid and several of its harmonics simultaneously. nmax sets the maximum number of overlaid simultaneous sinusoids per stimulus, given either as an integer or as piecewise-logarithmic frequency-value pairs.

nmax defaults to 1 (no harmonic injection), and generally should be left there. Injecting harmonics alongside the fundamental means any harmonic distortion the circuit itself produces lands on exactly the frequencies being measured, where it cannot be distinguished from the real response. Raise nmax above 1 only when you are specifically trying to speed up a simulation whose setup is already known to be good — never while still validating a measurement.

When you do raise it:

  • Higher nmax reduces simulation time but may reduce accuracy; nmax=2 is a reasonable first step, and beyond nmax=4 there is generally little further benefit.
  • LTspice automatically scales the total amplitude to the specified pp* value.
  • Compare the result against an nmax=1 run to confirm the speed-up has not changed the answer.

Timing

  • tavgmin — minimum time each sinusoid is analyzed. For each applied frequency, if 1/f < tavgmin, the analysis time is increased in integer period increments until it exceeds tavgmin. For an SMPS, a good starting point is 100/fsw, where fsw is the switching frequency.
  • tsettle — time between a stimulus first being applied and analysis beginning. A good starting point is 2/fcross, where fcross is the approximate expected 0 dB crossover frequency. Defaults to 10/fend.
  • delay — time at which the analyzer applies its first stimulus. Set it long enough for the circuit to reach steady state before stimulus begins: measuring during start-up or while the output is still settling perturbs an operating point that is itself still moving, giving a meaningless loop gain. Determine the settling time from the plain .tran run of Step 1 in the SMPS Bode plot procedure and set delay beyond it.

Alternate Measurement Nodes

  • refnode — measures the in and out node voltages relative to refnode instead of ground. This allows FRA to be used on, for example, circuits that regulate current via the voltage across a sense resistor. Not applicable to impedance analysis.

    The & probe device is generally preferred over refnode. Both address the same differential-feedback cases, but the probe takes an explicit differential pair at each end (o+/o- and i+/i-) instead of redefining the reference for the analyzer’s own terminals, and it adds measurement points without altering how the loop is broken.

  • intnode — specifies an intermediate node for an additional gain analysis, useful for analyzing the external compensation point of a regulator. Gain is calculated from the FRA device to intnode, and the complex value is included in the raw output file so it can be plotted in the waveform viewer. To measure gain between arbitrary differential point pairs rather than to a single node, use the & probe device.

Results and Bode Plot

When simulated in the GUI, LTspice opens a Bode plot and may populate it automatically — but only in two configurations, which depend both on the FRA device’s terminals and on how many & probes are in the circuit:

FRA device terminals & probes in circuit Auto-plotted trace
Neither grounded none one trace per @ FRA device
One grounded exactly one that probe’s trace
Neither grounded one or more none — blank Bode plot
One grounded none, or more than one none — blank Bode plot

A blank Bode plot does not mean the analysis failed: the results are in the raw output file either way, and the traces can be added manually. Check the FRA device’s terminals and the probe count against the table above before looking for a problem in the circuit.

Parameters

Parameter Description Units Default
delay Stimulus start time sec 0
fstart Frequency sweep start value Hz —
fend Frequency sweep end value Hz —
oct Points per octave for sweep resolution. Supported values: 0.25, 0.5, 1, 2, 3, 4 — 4
fcoarse Upper frequency for coarse stepping in the sweep Hz —
flist List of specific frequencies (applied individually) Hz —
nmax Maximum number of simultaneously injected harmonic frequencies — 1
pp0 Stimulus amplitude for frequencies below f0 (if specified) V (gain) / A (impedance) 1mV (gain) / 10mA (impedance)
pp1 Stimulus amplitude for frequencies above f1 V (gain) / A (impedance) —
f0 Maximum frequency for pp0 Hz —
f1 Minimum frequency for pp1 Hz —
pp Piecewise logarithmic amplitude vs. frequency Hz,V pairs —
tavgmin Minimum analysis time for each stimulus frequency sec 0
tsettle Settling time at each frequency before analysis begins sec 10/fend
rpar Parallel resistance Ohm 1m (gain) / 1T (impedance)
acmag AC current magnitude (.ac simulations only) A 0
acphase AC current phase (.ac simulations only) degrees 0
ac AC current magnitude, phase pair (.ac simulations only) A,degrees 0,0
enabled Analyzer enable (0 or 1) — 1
refnode Reference node for voltage gain analysis — 0
intnode Intermediate node for additional gain analysis — —

& — Frequency Response Analysis Probe

Symbols: FRAPROBE

&xxx o+ o- i+ i-

Used in conjunction with an @ Frequency Response Analyzer during a .fra simulation, the probe analyzes gain between any two differential points. It accepts no control parameters — the stimulus is controlled entirely by the FRA device.

Result: the complex quantity V(o+,o-) / V(i+,i-) versus frequency, written to the FRA complex raw output file <circuit>.fra_<fra_instance_name>.raw as a signal named probe_<fraprobe_instance_name>. Whether that signal is plotted automatically depends on the FRA device’s terminals and how many probes are present — see Results and Bode Plot.

The differential input and output pairs suit applications such as the following, each with an example schematic in *File > Open Examples > Educational\FRA*:

  • SMPS micromodules with integrated top feedback resistors — fra_eg8_ltm8074_probe.asc
  • Analyzing the gain of an intermediate portion of a control loop — for example from the compensation point of an SMPS to the output, known as the modulator gain — &mod in fra_eg6_LT3763_probe_current.asc, which measures from the compensation point VC to the output current sense
  • Differential feedback, such as current-feedback circuits — &1 in the same example, taken differentially across the output current sense resistor
  • Inverting (negative output) SMPS circuits — fra_eg10_LT8609_inverting_probe.asc

For differential feedback, prefer this device over the FRA device’s refnode parameter.

Multiple Loops and Probes

A circuit with more than one feedback loop can have its loops analyzed simultaneously by configuring a separate FRA device for each independent loop.

Probes may also be used in simulations with multiple FRA devices, but each probe must be associated with a specific FRA device, and LTspice makes that association by instance name — &1 pairs with @1, &2 with @2, and so on.

Examples: in that same directory, any schematic with “probe” in the filename demonstrates the probe device.

See also: SPICE-SYNTAX-REFERENCE.md for netlist conventions, SIMULATION-COMMANDS-REFERENCE.md for dot commands

Documentation source: github.com/analogdevicesinc/ltspice-reference