In this example, we'll analyze a four-element lumped / distributed line network. It is used to match a FET whose input impedance was modelled, over the range of 8000 to 18000 MHz, as a series RC network. The FET input model consists of 15 ohms in series with 0.5 pF.

The step-by-step creation of the first cut network was detailed by George Vendelin in his book "Design of Amplifiers and Oscillators by the S-Parameter Method." See the References in Appendix G for further information.

We took the basic network designed by George and optimized it across the band, to improve its VSWR, using our OptiMatch program.

To begin, choose "(1) Sceptre" from the Main Menu. either by pressing "1" or by using the "F1" function key, you'll enter the Sceptre Module. This should be routine by now! You'll see the following  familiar screen display:

            Sceptre Module

            Filename [<Enter>=Quit]  ? _

Enter "FETMATCH" to load the FET input match circuit file, and then press <Enter>.

The FET match circuit, (with the optimized values in place), followed by the .CKT file is shown below:
.

TRL	AA	SE	31.424	90	13000
OST	BB	PA	26	45	13000
SST	CC	PA	27.6	45	13000
IND	DD	SE	0.319
CAP	EE	SE	0.5
TRF	FF	TR	0.5477
CAX	AA	FF
PRI	AA	IR	50
END
8000	10000	12000	13000	14000	16000	18000

              
Study the .CKT file listing shown above. The first four lines describe the match network circuit elements. The next two lines are something you've not seen before. These two represent the load impedance presented by the FET model. Can you see where they come from? The 0.5 pF series capacitor is obvious: it was mentioned earlier on as being the reactive part of the load. The TRF command is new. It is an ideal transformer and is shown in Appendix A - take a look at it. If you are "up" on basic "transformer theory," you'll remember that Zin =  N-squared times ZL is what you see looking into the primary, given that N is the turns ratio, TR, and ZL is the load on the secondary. So, to create a 15 ohm series resistor out of a 50 ohm load, calculate N = square root (Zin/ZL) = 0.5477. This is the value listed in line 6, column 4, above.

You can use this same scheme to analyze a wide variety of circuits that have source and/or load terminations that don't equal 50 ohms.

One new convention in Sceptre, is that the electrical length of transmission line elements are always specified in degrees at a specific frequency. So, in the .CKT file listing above, the input TRL, on the far left of the circuit, is 90 degrees at 13000 Mhz, which is band center.

After you enter the filename above, Sceptre asks:

            Analysis or Tweak (A/T) [<Enter>=Quit]  ? _

In response, type "A," and then press the <Enter> key to use the analysis mode.

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.

Sceptre will analyze the FET match circuit and display the following results on-screen:


Sceptre Circuit Analysis on 07-18-2006 at 08:56:30

Filename:   FETMATCH

Input Reflection Coefficient and VSWR in a 50 ohm system.

Freq	Rho Mag/Ang	VSWR	R.L. (db)	Z(R+jX) ohm
8000	0.288/  -67.6	1.811	-10.800	53.086-j30.881
10000	0.233/ 142.3	1.607	-12.656	33.231+j10.004
12000	0.158/   49.0	1.375	-16.039	59.604+j14.563
13000	0.137/  -14.7	1.318	-17.250	65.122-j 4.619
14000	0.166/  -71.7	1.397	-15.615	52.670-j17.035
16000	0.186/-150.0	1.457	-14.616	35.590-j 6.860
18000	0.159/   37.4	1.377	-15.991	63.046+j12.464

Create SmithMatch .IMP File (Y/N) [<Enter>=No] ? _

Task Completed

At this point, if you wanted to, you could create a .IMP impedance file, and view the match shown above on a detailed Smith Chart using our SmithMatch program. Caution: Don't respond "Y" to the "Create SmithMatch .IMP File" question above, unless you have our SmithMatch software installed. An error will result.

Press <Enter> to by-pass creating such a file, and then "Press any Key" to return to the Sceptre Module entry point.