microwave software®
MStrip+® User Manual
Transmission Line Analysis and Synthesis Utility
MICROSTRIP COUPLED LINE MODULE
A microstrip coupled transmission line is formed by etching two lines,
separated by a small gap, on the top surface of a sheet of dielectric
material; the backside plating forms a ground plane.
Introduction
Propagation of a signal down these lines is, like microstrip, in a "quasi-TEM" mode. However, while a single microstrip line is described in terms of its "Z0," coupled lines are described in terms of their even and odd mode impedance, Zoe and Zoo. There are corresponding values of ERe and ERo. MSC computes a value midway between the two, which works quite well, for use in microwave filter and coupler design.
The even and odd mode impedances, Zoe and Zoo, and the effective relative dielectric constant, ER', (the midway value), are a function of the following physical and electrical parameters.
- Line width
- Gap width
- Dielectric thickness
- Dielectric constant
- Metallization thickness
- Frequency
- Cover height
A sketch of a typical coupled microstrip line is shown below. The terms used to describe it are standard in the industry.
As mentioned earlier, propagation in
coupled microstrip is dispersive
and in a "quasi-TEM" mode. Also, like microstrip, these lines are
affected by cover height and frequency, and, to a small extent, by
metallization thickness.
The current version of MSC does not take these latter two effects into account, nor does the original Bryant-Weiss MSTRIP program. However, the effects of cover height and frequency may be approximated by evaluating one of the coupled lines using the MS program.
In regard metallization thickness, all design equations found in the literature treat the ideal "T=0" case. The metallization error is small, and can be minimized by using 1/4 oz. or 1/2 oz. plating.
Be aware that the metallization supplied by a dielectric materials vendor, can be specified to be either electrodeposited or rolled copper. In the case of filter design, where insertion loss is usually critical, you should choose rolled copper.
Synthesis Mode
Enter Zoe,Zoo,Er [<Enter>=Quit] ? _
Enter each of the three values requested. The units are ohms.
Let's design a 10 dB directional coupler for use at 10 GHz. Standard design equations give Zoe = 69.5 and Zoo = 36 ohms.
We'll use alumina with a dielectric constant of ER=9.0 and 250 micro-inch CrCuAu metallization.
Enter the data as follows and then press <Enter>:
69.5,36,9.0
Note: At this point, you'll be asked "Print Logfile (Y/N) [<Enter>=No] ? _" Please press <Enter>.
If you choose the option to 'Print Logfile,' the data will be directed to 'Logfile.txt' within the \mwdata3 sub-directory, and to the screen. Right-click on 'Logfile.txt' to print it, and then DELETE the file; it will re-create when next needed.
After the question on 'Print Logfile,' if you press <Enter>, MStrip+ will display the input data on-screen and then enter an iterative loop. Your screen should look as follows:
If you use standard 25 mil thick alumina, the coupled-line
width and gap would be, respectively, 0.0226 and 0.0067 in. At 10 GHz,
the length of the coupled region would be 0.1228 in., a quarter-wave,
calculated using ER' = 5.7737.
Follow the on-screen instructions to "- Press any Key -" and return to the Coupled Microstrip Menu.
Analysis Mode
Enter W/H,S/H,Er [<Enter>=Quit] ? _
The format is the same as before; the only difference is that now we enter dimensional ratios in place of impedances. Try this mode working backwards with the data from the last example.
You should get the following output on-screen:
Follow the on-screen instructions to " - Press any Key - " to return to the Coupled Microstrip Menu.
As is the case elsewhere in MStrip+, the impedances are tabulated as ratios.
Values for the independent variables, W/H and S/H, must be >=0.01
The following is an example of Zoe/Zoo vs W/H on alumina (ER=9.6) with S/H held constant at 0.5
Choose "(1) Zoe/Zoo vs W/H" from the Tabular Data menu. When asked, enter S/H=.5 and ER=9.6 You'll see the following analysis:
Follow the on-screen instruction to " - Press any Key - " to return to the Tabular Data menu.
The current version of MSC does not take these latter two effects into account, nor does the original Bryant-Weiss MSTRIP program. However, the effects of cover height and frequency may be approximated by evaluating one of the coupled lines using the MS program.
In regard metallization thickness, all design equations found in the literature treat the ideal "T=0" case. The metallization error is small, and can be minimized by using 1/4 oz. or 1/2 oz. plating.
Be aware that the metallization supplied by a dielectric materials vendor, can be specified to be either electrodeposited or rolled copper. In the case of filter design, where insertion loss is usually critical, you should choose rolled copper.
The Microstrip Coupled Line Menu
When you choose "(2) MSC" from the Main
Menu, either by pressing "2" and then <Enter>,
or by using the "F2" function key, you'll enter the Microstrip
Coupled Line Module. The screen display will look as follows:
| Coupled Microstrip Menu
|
| (1) Synthesize
|
| (2) Analyze
|
| (3) Tabular Data
|
| (4) Main Menu
|
| Choose (1-4) ? _
|
Coupled-Line Synthesis Mode
When use choose "(1) Synthesize" from the Coupled
Microstrip Menu, in the same manner as before, the screen
display will look as follows:
Synthesis Mode
Enter Zoe,Zoo,Er [<Enter>=Quit] ? _
Enter each of the three values requested. The units are ohms.
Let's design a 10 dB directional coupler for use at 10 GHz. Standard design equations give Zoe = 69.5 and Zoo = 36 ohms.
We'll use alumina with a dielectric constant of ER=9.0 and 250 micro-inch CrCuAu metallization.
Enter the data as follows and then press <Enter>:
69.5,36,9.0
Note: At this point, you'll be asked "Print Logfile (Y/N) [<Enter>=No] ? _" Please press <Enter>.
If you choose the option to 'Print Logfile,' the data will be directed to 'Logfile.txt' within the \mwdata3 sub-directory, and to the screen. Right-click on 'Logfile.txt' to print it, and then DELETE the file; it will re-create when next needed.
After the question on 'Print Logfile,' if you press <Enter>, MStrip+ will display the input data on-screen and then enter an iterative loop. Your screen should look as follows:
| MStrip+ MSC Synthesis
|
| Input Data
|
| Zoe
= 69.5 ohms
|
| Zoo
= 36 ohms
|
| ER = 9.0 |
| Iterating |
| Zoe = 69.85958099365234
|
| Zoe = 67.19611358642578 |
| Zoe = 69.39781188964844 |
| Zoe = 69.49992370605469 |
| Output Data |
| W/H = 0.9034 |
| S/H = 0.2680 |
| ER' = 5.7737 |
| - Press any Key - |
Follow the on-screen instructions to "- Press any Key -" and return to the Coupled Microstrip Menu.
Coupled-Line Analysis Mode
When use choose "(2) Analyze" from the Coupled
Microstrip Menu, as before, the screen display will look as
follows:
Analysis Mode
Enter W/H,S/H,Er [<Enter>=Quit] ? _
The format is the same as before; the only difference is that now we enter dimensional ratios in place of impedances. Try this mode working backwards with the data from the last example.
You should get the following output on-screen:
| MStrip+ MSC Analysis
|
| Input Data |
| W/H
= .9034
|
| S/H = .268 |
| ER
= 9.00
|
| Output Data |
| Zoe = 69.5011 ohms |
| Zoo = 35.9990 ohms |
| ER' = 5.7737 |
| - Press any Key - |
Follow the on-screen instructions to " - Press any Key - " to return to the Coupled Microstrip Menu.
Tabular Data Mode
When use choose "(3) Tabular Data" from the Coupled
Microstrip Menu, as before, the screen display will look as
follows:
| Tabular Data
|
| (1) Zoe/Zoo vs W/H
|
| (2) Zoe/Zoo vs S/H |
| (3) MSC Menu |
| Choose (1-3) ? _ |
As is the case elsewhere in MStrip+, the impedances are tabulated as ratios.
Values for the independent variables, W/H and S/H, must be >=0.01
The following is an example of Zoe/Zoo vs W/H on alumina (ER=9.6) with S/H held constant at 0.5
Choose "(1) Zoe/Zoo vs W/H" from the Tabular Data menu. When asked, enter S/H=.5 and ER=9.6 You'll see the following analysis:
| MStrip+ MSC
Zoe/Zoo vs W/H Analysis
|
| Input Data
|
| S/H = .5
|
| ER =
9.6
|
| W/H
Zoe
Zoo
ER'
|
| 0.10
136.9029 81.6681
5.7049
|
| 0.20 115.3242 66.9212 5.7822 |
| 0.30 101.8661 58.9932 5.8472 |
| 0.40 92.0363 53.6727 5.9068 |
| 0.50 84.3214 49.7116 5.9631 |
| 0.60 78.0057 46.5770 6.0173 |
| 0.70 72.6888 43.9942 6.0698 |
| 0.80 68.1222 41.8040 6.1209 |
| 0.90 64.1406 39.9063 6.1709 |
| 1.00 60.6281 38.2343 6.2197 |
| - Press any Key - |
Follow the on-screen instruction to " - Press any Key - " to return to the Tabular Data menu.