Multiple Dielectric Transmission Line Calculator

History

         August 2007 Initial Release. Tested and results found to be close to commercial calculators, but all different configurations have not been tested.

         20 Dec 2007 Updated web page to include features in version 0.2

Related Links

Project Page

Wiki Page

Wire over Plane Example

Introduction

Using the proven atlc code to provide the field solver, this transmission line calculator program provides a way to compute the transmission line characteristics of many common structures found in PCB board design. Transmission line characteristics computed include:

1.       Differential odd mode impedance

2.       Differential even mode (common mode) impedance

3.       Odd mode dielectric constant (and propagation delay)

4.       Even mode dielectric constant (and propagation delay)

5.       Even and odd mode dielectric loss

6.       Capacitance per unit length

7.       DC resistance.

Mdtlc calculations can be slow in some cases (where the electric field is widely dispersed as in surface microstrip), but is adequate for occasional or educational use.

Description

Mdtl is a GUI based application that can run the win32 platforms. Transmission line impedance calculation is a common task in printed circuit board design, and this application provides the ability to model multiple dielectrics. This ability is not common among other calculators. One other open source calculator is atlc.

Atlc can compute the transmission line characteristics for an arbitrary cross section as defined by a bitmap. Initially developed as a *nix command line program, it provides nearly the ultimate in flexibility for describing the transmission line cross section. However, this format and flexibility increases the setup time needed to describe the physical characteristics of the transmission line. Mdtl builds on the atlc base to provide a GUI based interface that allows a quick way to describe structures common in printed circuit board (PCB) design. Mdtl takes the parameters used to describe the PCB structure, and generates the bitmap required for the atlc computation. Once the computation is complete, the results are displayed in the GUI.

Multiple dielectrics are supported in this approach to field solving. As such, it is possible to observe the propagation delay differences between single-ended and differential signals caused by different dielectric constants of glass and resin.

Additionally, impedances can be checked for unusual cases such as when the differential pair is close to the edge of the board.

Usage

The main panel provides basic inputs. If details like etchback or local dielectric values are required, then press the appropriate EDIT button.

Units are currently fixed to English system. 1 mil equals 0.001 inch.

Use the bitmap EXPORT and IMPORT functions with care. The color values in the bitmap are associated with the various dielectrics. The difference in the color map between LAMINATE 1 and LAMINATE 2 is one bit. Use the bitmap editors color picker to check the 24-bit colormap value. Using unknown colors will cause a crash.

The executable is a single binary it does not install on the system. Any output files are written to the directory in which the executable is started. For example, if it is on the Desktop, output files are written to the desktop.

Output filenames have a fixed root name combined with a date and time number string.

Note that while it is possible to enter unbalanced differential structures, the output format of odd and even impedances does not fully describe the loading on a driver.

Pixels correspond to 0.0001 inches. Accuracy for copper thicknesses such as 1/8 ounce is likely affected.

Examples

Comparison to other calculators

 

Single Ended Surface Microstrip

Parameters

Name

Version

Z (Zodd/Zeven)

Propogation Delay (odd/even)

No solder mask

No etchback

W = 6

Cu = 1.8 oz

Er = 4.2

Laminate = 5mil

mdtlc

0.1.1.811

56.8

139.2 ps/in

CITS25

2.0.2.0

58.4

 

Hyperlynx

7.5

58.9

141.8 ps/in

 

Single Ended Stripline

No etchback

W = 6 mil

Cu = 0.5 oz

Er = 4.2

Laminate1=5

Laminate2=10

Mdtlc

 

47.6

173.6 ps/in

CITS25

 

47.15

 

Hyperlynx

 

47.0

174.8 ps/in

 

Differential Microstrip

No soldermask

No etchback

W1=W2 = 4 mil

Spacing = 8

Cu = 1.4mil

Er = 4.7

Laminate1=2.6

mdtlc

 

93.4/24.8

144.2/149.5 ps/in

CITS25

 

92.99

145.7 ps/in

ADS

 

88.8/25.1

147.7/159.7 ps/in

Hyperlynx

(from paper)

 

87.2/26.0

145.1/154.9 ps/in

 

Differential Embedded Microstrip

No soldermask

Etchback 0.4

W1=W2 = 3 mil

Spacing = 4

Cu = 1.4mil

Er _lam= 3.8

Er_resin=3.2

Laminate1=2.5

Laminate2=19.7

mdtlc

0.1.1.839

110.9/78.0

154.6/144.4 ps/in

Polar Si6000

 

106.53

 

 

Differential Stripline

No soldermask

No etchback

W1=W2 = 4 mil

Spacing = 8

Cu = 1.4mil

Er = 3.25

Laminate1=12

Laminate2=12

mdtlc

0.1.1.564

82.9/30.2

152.7 ps/in

LineCalc

 

84.2/30.0

 

CITS25

2.0.2.0

82.0

152.7 ps/in

Hyperlynx

7.5

82.2/30.2

 

 

Future Work

1.       The solder mask drawing algorithm is not that good. Need to coat vertical surfaces as well.

2.       Save and import configuration settings.

3.       Save and import the bitmap file.

4.       Compare results to known configurations.

5.       Modeling the glass and resin composite may require a 3D simulation or multiple 2D simulations.

6.       More trace configurations (co-planar ground).

7.       Ribbon cable and cable configurations possible.

8.       Create a properly formatted output page (in pdf) including the bitmap.

9.       Estimated breakdown strength of dielectric.

10.   Modify inner loops to speed up simulation.

11.   Incorporate multi-threading.

12.   Change output report format to support unbalanced differential impedances.

13.   Add dielectric loss.

14.   Add current filament simulation for inductance simulation and skin effect loss values.

Screenshots

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