Tips for better chances of success when using the OSCPORT

automated nonlinear closed loop oscillator analysis:

Oscillator circuits can be tricky to analyze in closed loop form.

Sometimes, the oscillation frequency in closed loop form can shift

due to impedance interaction of the two ports that were previously

used to analyze the circuit in open loop form. Also, the higher the

loaded Q of the oscillator, the more difficult it generally is to analyze

steady state large signal conditions. (The easier it is to miss the

frequency with steady state oscillation conditions).

The OscPort module provides an automated means for finding

closed loop detection of oscillator circuit operation. To best apply

the OscPort, here are some tips:

1) Choice of node to locate OscPort: It's important to locate the

OscPort at a node which exhibits certain impedance conditions

at the oscillation frequency. (The Real portion of the port's Zin

is negative and the Imag portion of the port's Zin is zero). There

can exist situations where the closed loop conditions change

sufficiently to invalidate predictions that may have been made in

the open loop analysis - this is part of the challenge of oscillator

circuit simulation. The actual location of the OscPort may end

up being somewhere on the input of the circuit or somewhere

on the output, in order to satisfy the criteria above.

2) Add a nonlinear (Harbec) analysis to the "Analysis" folder.

In the "Harbec Options" window, "Oscillator" tab, set the minimum

and maximum search frequency, and number of points for small

signal analysis. Note that searching too narrow a band, or searching

an insufficient number of data points... can result in not finding

oscillation conditions. You may need to try different frequency

intervals, or increase the number of data points in a given interval.

3) Check the checkbox at bottom for "Display spectrum and waveform graphs".

This will cause the Harbec module to create the oscillator analysis equations

and output measurements.

4) Click the button "Find initial oscillator port frequency". The Harbec module should

find a frequency and display it in the dialogue window. If a frequency is not found,

close the Harbec options dialogue box and re-run the Harbec analysis by right-clicking

the Harbec analysis in the file tree and choosing "recalculate".

At first running, a set of output equations and graphs will be created.

Open the graph "Oscillation criteria for HB"

(note if the filename used for the Harbec analysis is too long, the graph may

come out with a short name such as Graph1).

The graph shows a sweep of the OscPort impedance. In order to detect

oscillation, the OscPort is looking for the presence of three conditions:

a) The phase angle of the impedance at the OscPort must be sufficiently near zero

b) The impedance signature contains a relatively sharp change

c) The Real portion of impedance magnitude is <0 (negative)

5) If the circuit does not demonstrate oscillation conditions in the predefined frequency

band, move the OscPort to a different node. The OscPort may be placed anywhere

in the feedback path from output to input, or within nodes associated with the resonator.

For high Q oscillators, make sure to use enough frequency data points to avoid missing

the sharp resonance conditions.

6) When the frequency of oscillation is found, proceed to the next step: nonlinear analysis.

At this point, click the radio button "use oscillator solver" on the "oscillator" tab of the nonlinear

analysis. This activates the Newton algorithm of the oscillator analysis, which seeks to solve

for Real and Imaginary portions of the first Harmonic content to be zero.

Automated analysis of an oscillator circuit involves conflicting requirements.

On one hand, we desire high sensitivity to very small changes (in Z, I, V, etc.) On the other

hand, the numeric processing associated with the higher sensitivity promotes

stronger likelihood of the existence of natural numerical errors. This tends to

increase with circuit complexity.

automated nonlinear closed loop oscillator analysis:

Oscillator circuits can be tricky to analyze in closed loop form.

Sometimes, the oscillation frequency in closed loop form can shift

due to impedance interaction of the two ports that were previously

used to analyze the circuit in open loop form. Also, the higher the

loaded Q of the oscillator, the more difficult it generally is to analyze

steady state large signal conditions. (The easier it is to miss the

frequency with steady state oscillation conditions).

The OscPort module provides an automated means for finding

closed loop detection of oscillator circuit operation. To best apply

the OscPort, here are some tips:

1) Choice of node to locate OscPort: It's important to locate the

OscPort at a node which exhibits certain impedance conditions

at the oscillation frequency. (The Real portion of the port's Zin

is negative and the Imag portion of the port's Zin is zero). There

can exist situations where the closed loop conditions change

sufficiently to invalidate predictions that may have been made in

the open loop analysis - this is part of the challenge of oscillator

circuit simulation. The actual location of the OscPort may end

up being somewhere on the input of the circuit or somewhere

on the output, in order to satisfy the criteria above.

2) Add a nonlinear (Harbec) analysis to the "Analysis" folder.

In the "Harbec Options" window, "Oscillator" tab, set the minimum

and maximum search frequency, and number of points for small

signal analysis. Note that searching too narrow a band, or searching

an insufficient number of data points... can result in not finding

oscillation conditions. You may need to try different frequency

intervals, or increase the number of data points in a given interval.

3) Check the checkbox at bottom for "Display spectrum and waveform graphs".

This will cause the Harbec module to create the oscillator analysis equations

and output measurements.

4) Click the button "Find initial oscillator port frequency". The Harbec module should

find a frequency and display it in the dialogue window. If a frequency is not found,

close the Harbec options dialogue box and re-run the Harbec analysis by right-clicking

the Harbec analysis in the file tree and choosing "recalculate".

At first running, a set of output equations and graphs will be created.

Open the graph "Oscillation criteria for HB"

(note if the filename used for the Harbec analysis is too long, the graph may

come out with a short name such as Graph1).

The graph shows a sweep of the OscPort impedance. In order to detect

oscillation, the OscPort is looking for the presence of three conditions:

a) The phase angle of the impedance at the OscPort must be sufficiently near zero

b) The impedance signature contains a relatively sharp change

c) The Real portion of impedance magnitude is <0 (negative)

5) If the circuit does not demonstrate oscillation conditions in the predefined frequency

band, move the OscPort to a different node. The OscPort may be placed anywhere

in the feedback path from output to input, or within nodes associated with the resonator.

For high Q oscillators, make sure to use enough frequency data points to avoid missing

the sharp resonance conditions.

6) When the frequency of oscillation is found, proceed to the next step: nonlinear analysis.

At this point, click the radio button "use oscillator solver" on the "oscillator" tab of the nonlinear

analysis. This activates the Newton algorithm of the oscillator analysis, which seeks to solve

for Real and Imaginary portions of the first Harmonic content to be zero.

Automated analysis of an oscillator circuit involves conflicting requirements.

On one hand, we desire high sensitivity to very small changes (in Z, I, V, etc.) On the other

hand, the numeric processing associated with the higher sensitivity promotes

stronger likelihood of the existence of natural numerical errors. This tends to

increase with circuit complexity.