Transmission Line

Transmission Line #

The transmission line component models distributed RLC parameters for accurate simulation of signal propagation, impedance matching, and high-frequency effects. Breadpad uses the LTRA (Lossy Transmission Line) model for realistic simulations.

Basic Properties #

  • Terminals: 4 (Differential input and output)
  • SPICE Designation: O (LTRA model)
  • Placement: Fixed 2x3 grid area on breadboard

Key Parameters #

Per-Unit-Length Parameters #

  • Resistance (R): 10 Ω/m - Series resistance per meter
  • Inductance (L): 1e-15 H/m - Series inductance per meter
  • Capacitance (C): 1e-15 F/m - Shunt capacitance per meter
  • Length: 1.0 m - Total transmission line length

Calculated Total Parameters #

The component automatically calculates:

  • Total Resistance = R × Length
  • Total Inductance = L × Length
  • Total Capacitance = C × Length

SPICE Netlist Format #

.model LTRAMODEL1 LTRA R=10 L=1e-15 C=1e-15 LEN=1
O1 in+ in- out+ out- LTRAMODEL1

Common Transmission Line Types #

Coaxial Cable (RG-58) #

  • R = 54 Ω/m
  • L = 250 nH/m
  • C = 100 pF/m
  • Characteristic Impedance: Z₀ = 50Ω

PCB Microstrip (50Ω) #

  • R = 2 Ω/m
  • L = 330 nH/m
  • C = 132 pF/m
  • Typical for FR-4 substrate

Twisted Pair (CAT5) #

  • R = 188 Ω/m
  • L = 490 nH/m
  • C = 49 pF/m
  • Characteristic Impedance: Z₀ = 100Ω

PCB Stripline #

  • R = 1.5 Ω/m
  • L = 475 nH/m
  • C = 95 pF/m

Characteristic Impedance #

The characteristic impedance is calculated as:

Z₀ = √(L/C)

For lossless lines at high frequency. At DC or low frequencies, resistance dominates.

Applications #

  1. Signal Integrity: Model PCB traces for high-speed digital signals
  2. RF Circuits: Impedance matching networks and filters
  3. Power Distribution: Model power delivery networks
  4. Cable Modeling: Simulate long cable runs with distributed effects
  5. Time Domain Reflectometry: Analyze impedance discontinuities

Design Tips #

  1. Impedance Matching: Set L and C to achieve desired Z₀
  2. Loss Modeling: Adjust R for realistic attenuation
  3. Propagation Delay: td = Length × √(LC)
  4. Frequency Response: The model is accurate up to frequencies where the wavelength » segment length

Placement Notes #

Unlike other components, transmission lines have a fixed placement pattern:

  • Occupies 2 columns horizontally
  • Spans 3 rows vertically (rows 7-9)
  • Input terminals on the left, output on the right

See Also #