microwave software®
Sceptre® User Manual
Frequency Domain Circuit Analysis Utility
CIRCUIT ANALYSIS EXAMPLES
For our first example, we'll analyze the amplitude vs. frequency
response of a Chebyshev low-pass filter. It's a 50 ohm 0.5 dB ripple
design.
100 MHz Chebyshev Low-Pass Filter
Enter the Sceptre Module by choosing "(1) Sceptre" from the Main Menu. either by pressing "1" or by using the "F1" function key. The screen display will look as follows:
Sceptre Module
Filename [<Enter>=Quit] ? _
Enter "FILTER" in response to the filename question and then press <Enter>.
The circuit diagram for this filter may be familiar; its from the File Utility section. Its shown below along with its .CKT file.
 |
| CAP
|
AA
|
PA
|
54.3
|
| IND
|
BB
|
SE
|
97.88
|
| CAP
|
CC
|
PA
|
80.88
|
| IND
|
DD
|
SE
|
97.88
|
| CAP
|
EE
|
PA
|
54.3
|
| CAX
|
AA
|
EE
|
|
| PRI
|
AA
|
S1
|
50
|
| END
|
|||
| 10
|
150
|
10
|
The first five lines of code correspond to the filter elements from left to right. The sixth line, CAX AA EE, cascades all five individual two-ports into a new two-port network named AA. Next, a PRI line defines the output format and specifies a 50 ohm system reference impedance. Finally the circuit ENDs, and the analysis frequencies are given in start, stop, step format.
After entering the filename, Sceptre asks:
Analysis or Tweak (A/T) [<Enter>=Quit] ? _
Type "A," for Analysis, and press the <Enter> key. Choose not to print by pressing <Enter> once again.
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 \mwdata2 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.
Please please <Enter> to choose "N" in response to the above question.
A brief Analysis in progress message will flash on-screen, followed by data output to the screen. Your screen display should be similar to the following::
Sceptre Circuit Analysis on 07-18-2006 at 16:04:41
Filename: FILTER
| Freq
|
S11 Mag/Ang
|
S21 Mag/Ang
|
S12 Mag/Ang
|
S22 Mag/Ang
|
S21(dB)
K B1
|
| 10
|
0.165/-113.8
|
0.986/ -23.8
|
0.986/ -23.8
|
0.165/-113.8
|
-0.12
|
| 20
|
0.283/-136.5
|
0.959/ -465.
|
0.959/ -46.5
|
0.283/-136.5
|
-0.36
|
| 30
|
0.329/-158.2
|
0.944/ -68.2
|
0.959/ -68.2
|
0.329/-158.2
|
-0.50
|
| 40
|
0.295/ 179.8
|
0.956/ -90.2
|
0.956/ -90.2
|
0.295/ 179.8
|
-0.39
|
| 50
|
0.172/ 155.5
|
0.985/-114.5
|
0.985/-114.5
|
0.172/ 155.5
|
-0.13
|
| 60
|
0.027/ -51.8
|
1.000/-141.8
|
1.000/-141.8
|
0.027/ -51.8
|
-0.00
|
| 70
|
0.228/ -80.5
|
0.974/-170.5
|
0.974/-170.5
|
0.228/ -80.5
|
-0.23
|
| 80
|
0.329/-109.2
|
0.944/ 160.8
|
0.944/ 160.8
|
0.329/-109.2
|
-0.50
|
| 90
|
0.215/-142.2
|
0.977/ 127.8
|
0.977/ 127.8
|
0.215/-142.2
|
-0.21
|
| 100
|
0.331/ -12.9
|
0.944/ 77.1
|
0.944/
77.1
|
0.331/ -12.9
|
-0.50
|
| 110
|
0.852/ -65.8
|
0.524/
24.2
|
0.524/
24.2
|
0.852/ -65.8
|
-5.62
|
| 120
|
0.969/ -93.5
|
0.246/
-3.5
|
0.246
-3.5
|
0.969/ -93.5
|
-12.17
|
| 130
|
0.992/-108.3
|
0.129/ -18.3
|
0.129/ -18.3
|
0.992/-108.3
|
-17.78
|
| 140
|
0.997/-117.8
|
0.075/ -27.8
|
0.075 -27.8
|
0.997/-117.8
|
-22.53
|
| 150
|
0.999/-124.6
|
0.046/ -34.6
|
0.046/ -34.6
|
0.999/-124.6
|
-26.66
|
Task Completed
As you can see, the filter has a 0.5 dB ripple in the passband, and the cut-off frequency equals the desired 100 MHz. Also, the filter is reciprocal in that it looks the same from both ends: S11 = S22 and S12 = S21, as you would expect for a passive circuit.
Naturally, if this filter used real components, you would see some loss in the passband.
Follow the on-screen instruction to "Press any Key" to return to the opening prompt. From there, you can press <Enter> and exit Sceptre for now, or, you can go on to the next example.