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   Running Multitone Oscillator Analysis from the Schematic Editor       


Running Multitone Oscillator Analysis from the Schematic Editor

Multi-tone oscillator analysis is typically used on a circuit that includes one or more oscillation sources:

•  One unknown resonant frequency (F1) plus one or more known driven oscillations (RF1 through RFM) such as sinusoidal voltage sources.

•  Two or more unknown resonant frequencies (F1 through FN) with no driven oscillations in the circuit.

•  Two or more unknown resonant frequencies (F1 through FN) plus one or more known driven oscillations (RF1 through RFM).

An example would be a mixer circuit connecting one or more voltage-controlled oscillators (VCOs) of unknown frequencies with one or more RF sources of known frequencies to produce a single resultant IF output.

From the Schematic Editor, perform the following steps to set up and run a multi-tone oscillator analysis using Nexxim, and view the results.

1. Set up the schematic circuit for multi-tone oscillator analysis by inserting oscillator probes between pairs of nodes in your circuit. Each probe represents one unknown oscillation. Edit the probe properties and specify an initial guess for the frequency (FREQ parameter) and an initial voltage (A parameter) for each probe. See Using the Oscillator Probe for more information.

2. Right-click on the Analysis menu in the Project window to open a menu.

3. Select Add Nexxim Solution Setup from the menu, then slide the cursor to select Oscillator Analysis (N-Tone) from the subordinate menu.

4. The Oscillator N-Tone Analysis dialog box appears.

5. Type an Analysis Name (or accept the default name, for example “OscillatorNtone”).

6. For most simulations, leave the Disable box unselected (the default setting). Selecting this box lets you store multiple solution setups for later use. (Note that if a solution setup is disabled before the analysis is run, any changes made to the design will invalidate the simulation results.)

7. Click the upper Edit button to open the Edit Oscillator Tones/Max Harmonic Number dialog:

•  Specify the number of unknown frequencies to be analyzed in the No. of Tones field. The display changes to show that number of tones, labeled F1 through Fn. The names cannot be changed. The illustration above shows two frequencies (however, most analyses have only one unknown frequency). The number of unknown frequencies must be the same as the number of oscillator probes in the circuit.

•  Click on the Value fields for the tones to specify the initial guess frequencies. The initial guess frequencies should be the same as the ones entered in the FREQ parameters of the oscillator probes in the circuit. The Oscillator Tones values override the probe FREQ parameters if they are not the same. The default frequency is 1 GHz.

•  Click on the MaxK field for a tone to specify its maximum harmonic number. The default is 7 harmonics.

•  Click OK to return to the Oscillator N-Tone Analysis dialog.

8. Click the lower Edit button to open the Edit Driven Tones/Max Harmonic Number dialog:

•  Specify the number of driven frequencies in the No. of Tones field. The display changes to show that number of tones, labeled RF1 through RFn. The names cannot be changed. The illustration above shows two driven frequencies. These frequencies represent the known, fixed inputs to the analysis.

•  Click on the Value fields for to specify the frequencies of the driven oscillations. The default frequency is 1 GHz.

•  Click on the MaxK field for a tone to specify its maximum harmonic number. The default is 7 harmonics.

•  Click OK to return to the Oscillator N-Tone Analysis dialog.

9. Optional: To run Phase Noise analysis, check the Enable Noise Calculation box. See Running Phase Noise Analysis from the Schematic Editor for details on the other steps in setting up the phase noise analysis.

10. To add a sweep, use the Sweep Variable area in the Oscillator N-Tone Analysis dialog box.

•  In the Sweep Variable area, click Add, and the Add/Edit Sweep dialog box appears.

•  In the Variable list, select Temp or the name of a variable (when a variable has been defined for the design), and then select one of the following: Single value, Linear step, Linear count, Decade count, or Octave count.

•  Type the sweep values into the Value text box (for Single value), or into the Start, Stop, and Step text boxes (for Linear, Decade, or Octave count), and make sure that the appropriate units are selected for each.

•  For details on sweeps, see Variable Sweep.

•  Click Add, and then click OK to close the Add/Edit Sweep dialog box.

•  The Oscillator N-Tone Analysis dialog box reappears.

11. Click on the Edit Quantities button in the Output Quantities field to open the Defined Output Quantities dialog box.

Expand the icons for the circuit components (Nets and individual devices), and use the checkboxes to select quantities for output.

Expand the Harmonic Combinations icon to display a list of harmonics including combinations. The number of available harmonic combinations is set by the No. of Frequencies and the MaxK for each frequency set in the Edit Oscillator Tones/Max Harmonic Number dialog in Step 8. Select the harmonics you want to analyze. If you do not select any harmonics, harmonics DC, F1, 2F1, 3F, and 4F1 are automatically selected. In the Report dialog, only selected harmonics are available for plotting.

Click OK to return to the Oscillator Solution Setup dialog box.

12. Optionally, use the fields in the Solution Option panel to select or add Oscillator analysis options and other Nexxim options to the design.

•  Click the Select button on the Name field to open the Select Solution Options dialog.

If any options sets have been defined, their names appear in the Select Solution Options list. To select a named option set that you have previously defined, click the name of the option set, then click OK to return to the Oscillator Analysis dialog. The named option set appears in the Name field in the Solution Options panel. See Analysis-Specific Options for details on creating option sets outside of the Solution Setup dialog.

To create a new option set, click New. The Solution Options dialog box appears.

•  Select the Oscillator Options tab:

•  Enter a name for the options, or accept the default name.

•  To select an Initial Guess option, click in the pulldown field. The options are Frequency Sweep, Transient, and DC.

For an Initial Guess of Frequency Sweep, the dialog changes to:

Enter the Frequency Start and Frequency End values. Use the pulldown at the right of the value to select the unit (Hz or kHz).

For an Initial Guess of Transient, the dialog changes to:

Enter a Number of Cycles if the default is not as desired.

•  Click the Result as Initial Guess box to use the result from the previous oscillator sweep step as the initial guess for the current step. Use of this setting can avoid repeated analysis runs.

•  To select a Convergence Option, click in the pulldown field. The options are Newton, Descent, and Damped Newton.

•  Make the appropriate changes to option values, then click OK to return to the Select Solutions dialog box. On the Select Solutions dialog, click the name of the new option settings, then click OK to return to the DC Analysis dialog. The name of the new option settings appears in the Name field in the Solution Options panel.

•  Click the Edit button on the Additional field to open a text-entry dialog, Edit additional options. Use the text box to enter any Nexxim options exactly as they are to appear in the netlist. Click OK to return to the Oscillator Analysis dialog.

•  For more information on the option settings, see Nexxim Oscillator Analysis Options.

13. When all setup steps are as desired, click Finish on the Oscillator Analysis dialog. The solution setup is added to the Project tree under the Analysis icon.

14. Run the simulation:

•  Click on the solution setup in the Project tree and click Analyze from the menu. If the circuit is set up correctly, the analysis begins immediately and a red progress bar appears.

•  If the analysis is not successful, an error message appears in the Message Window. Check the Message Window for an explanation, and then take corrective action.

15. View results:

•  After a successful oscillator analysis, Nexxim displays a message like the following in the Message Window, near the end of the messages:

[info]analysis:osc(status):
Lowest oscillation frequency is 2.28514e_006 Hz (Time, Date)

The frequency value will be the one calculated for your circuit, and the time and date will show the time of the analysis on your machine.

Note 

As analysis progresses, oscillator analysis may display messages like the following:

analysis:osc(status): Probe 0: selecting (node_name1,node_name2) as alternative location 1.

analysis:osc(status): Probe 0: selecting (node_name3, node_name4) as alternative location 2.

The node_names are circuit-specific. If oscillator analysis fails, Nexxim automatically reruns the analysis after moving the probe to alternative location 1. If that analysis fails as well, Nexxim retries using alternative location 2.

 



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