Transmission Line Delay & Length Matching
From the effective dielectric constant, get propagation delay per millimetre, the λ/4 length, the critical trace length that needs termination and how much length a skew budget allows.
Key points
Transmission Line Delay & Length Matching: From the effective dielectric constant, get propagation delay per millimetre, the λ/4 length, the critical trace length that needs termination and how much length a skew budget allows. Formula: t_pd = 3.336·√εeff ps/mm ｜ l_crit = t_r/2 ÷ t_pd. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. FR-4 rule of thumb: microstrip ~6–7 ps/mm, stripline ~7–8 ps/mm; a 1ns edge starts needing termination around 15cm. Length matching only means something within the same layer and dielectric stack — changing layers changes the propagation velocity.
Inputs
t_pd = 3.336·√εeff ps/mm ｜ l_crit = t_r/2 ÷ t_pdCalculation model (updates with parameters)
| Propagation velocity v_p | 160.25 mm/ns | c/√3.50 |
| Delay per unit length t_pd | 6.24 ps/mm | |
| Delay of this trace | 312.02 ps | 50.0mm |
| Signal wavelength λ | 0.0 mm | 100.0MHz 下的wavelength in the dielectric |
| λ/4 length | 0.0 mm | A λ/4 trace becomes an antenna / impedance transformer |
| Critical length needing termination | 80.1 mm | Rule of thumb: delay > t_r/2 |
| Skew converted to length | 8.01 mm | 50ps 对应的Trace length差 |
Engineering notes
- FR-4 rule of thumb: microstrip ~6–7 ps/mm, stripline ~7–8 ps/mm; a 1ns edge starts needing termination around 15cm.
- Length matching only means something within the same layer and dielectric stack — changing layers changes the propagation velocity.
FAQ
- What is Transmission Line Delay & Length Matching for?
- From the effective dielectric constant, get propagation delay per millimetre, the λ/4 length, the critical trace length that needs termination and how much length a skew budget allows.
- What is the formula for Transmission Line Delay & Length Matching?
- Core formula: t_pd = 3.336·√εeff ps/mm ｜ l_crit = t_r/2 ÷ t_pd. The tool returns results instantly using this formula and the selected parameters.
- What engineering notes should I keep in mind when using Transmission Line Delay & Length Matching?
- FR-4 rule of thumb: microstrip ~6–7 ps/mm, stripline ~7–8 ps/mm; a 1ns edge starts needing termination around 15cm. Length matching only means something within the same layer and dielectric stack — changing layers changes the propagation velocity.