Let's turn our attention to a different kind of circuit, a negative resistance common-base oscillator. Here, we'll use the SER command to insert an inductor into the base lead of a bipolar transistor. Then, we'll tweak it to maximize the input reflection coefficient.

The base lead inductance, when properly chosen, will raise the magnitude of either the S11 or S22 scattering parameter, (or both), to a value greater than one. Then, when that happens, a negative resistance will be created. Typically, you would tune the port with the highest reflection coefficient, and output match the other port.

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. You'll see the following  familiar screen display:

            Sceptre Module

            Filename [<Enter>=Quit]  ? _

Enter "CBOSC" to load the common-base oscillator circuit file stored in the \mwdata2\ sub-directory, and then press <Enter>.

The basic model used to evaluate the negative resistance of this particular active device is shown below. Its followed by the .CKT file listing.

TWO	AA	S1	50
IND	BB	PA	1.0
SER	AA	BB
TWO	DD	S1	50
PRI	AA	S1	50
END
2750
0.9	150	1.7	-80	0.07	120	1.08	-56

              
Study the .CKT file listing shown above. The first two lines describe the circuit elements, and the SER command, (please see Appendix B),  is used to series connect them in line 3. Note that the new two-port always takes on the name of the first element, AA in this case.

After you enter the filename above, Sceptre asks:

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

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

Sceptre will analyze the circuit and give the following on-screen output. It will then wait for you to tell it what to do:

Sceptre Circuit Analysis on 07-18-2006 at 08:11:45

Filename:   CBOSC

Freq	S11 Mag/Ang	S21 Mag/Ang	S12 Mag/Ang	S22 Mag/Ang	S21(dB)  K  B1
2750	1.502/  119.0	2.071/ -114.7	0.492/  111.2	1.217/ -84.3	6.32  -0.63  0.32

Task Completed

This first analysis shows that the emitter port, as represented by S11, is probably the best place to place a resonant tank since it offers the highest negative resistance.

The analysis concludes with the following question:

            Tweak Variable (Y/N) [<Enter>=No]  ? _

If you respond with "Y," (as you should here), you'll see the following:

            Line,Column,New Value (L,C,NV)  ? _

What does this mean? If you look above at the "CBOSC" .CKT file, you'll see that in line 2 we specified a 1.0 nH parallel connected inductor to start with. The size of this inductor is specified in column 4, so, if you wanted to tweak this element, and change its value from 1.0 to 1.5 nH, you would respond to the question as follows:

            2,4,1.5

Is this clear? You've now entered new values for the inductor by entering L,C, and NV. The new values are: L=2, C=4, and NV=1.5.

After pressing <Enter>, you should see the following:

2750

1.737/  98.0

2.073/ -138.9

0.735/  92.9

1.149/ -104.6

6.33 -0.57 1.09

Note that S11, the input port reflection coefficient magnitude has increased from 1.502 to 1.737. This means that the negative resistive is increasing, which is what you want for an oscillator.

O.K., so far so good. Now, let's tweak the inductor value upwards in increments of 0.1 nH, starting with 1.6 nH, until we reach, or pass, a peak in the magnitude of S11M.

We'll show you below what happened when we did this. If you like, repeat our entries for practice.

            Tweak Variable (Y/N) [<Enter>=No]  ? Y
            Line,Column,New Value (L,C,NV)  ? 2,4,1.6

2750

1.761/  93.6

2.044/ -143.9

0.776/  89.0

1.118/ -108.8

6.21  -0.56  1.26

            Tweak Variable (Y/N) [<Enter>=No]  ? Y
            Line,Column,New Value (L,C,NV)  ? 2,4,1.7

2750

1.777/  89.3

2.006/ -149.0

0.813/  85.1

1.081/ -113.1

6.05  -0.54  1.43

            Tweak Variable (Y/N) [<Enter>=No]  ? Y
            Line,Column,New Value (L,C,NV)  ? 2,4,1.8

2750

1.783/  85.1

1.959/ -154.0

0.845/  81.3

1.040/ -117.2

5.84  -0.53  1.58

Enter the fColumn,New Value (L,C,NV)  ? 2,4,1.9

2750

1.781/  81.0

1.906/ -158.9

0.872/  77.5

0.995/ -121.3

5.60  -0.51  1.72

            Tweak Variable (Y/N) [<Enter>=No]  ? N

As you can see, the magnitude of S11 peaked at about 1.8 nH. Choose "N" to stop tweaking, and return to the SmithMatch Module opening screen.

Now, if you would like to see the impedance equivalent of S11, you may edit the "CBOSC" .CKT file using the File Utility. To do this, first press <Enter> to return to the Main Menu, and then enter the File Utility. If you would rather not, skip the rest of this section and go on to design Example 4.

You edit the "CBOSC" file by using the "Replace"  and "Save" commands. You want to change line 2 and line 4 so that they read:

            IND BB PA 1.8
            PRI AA IR 50

Make sure you make the changes ONE line at a time. First edit "CBOSC" and "Replace" line 2, "Save" it using the new name "CBOSCIR," then edit the new "CBOSCIR," replace line 4, and save again as "CBOSCIR."

Keep in mind, that in Sceptre you don't enter any "three-letter extensions" or "path" commands. Just type the name you want for a file and press <Enter>.

Return to the Sceptre Module and run the file "CBOSCIR."

            Sceptre Module

            Filename [<Enter>=Quit]  ? _

Enter the filename as "CBOSCIR" and then press <Enter>. Choose the analysis option, and choose not to print. Here is what the IR output format looks like:


Sceptre Circuit Analysis 07-18-2006 at 08:20:11