VCO Performance Simulations

The following tutorial assumes that you have a voltage controlled oscillator (VCO) with differential outputs already designed and you now wish to find various performance criteria such as phase noise, tuning range, etc.

Phase Noise

• Make sure that your tuning voltage is set such that your VCO oscillates at the desired frequency.
• From the Analog Design Environment (ADE) click Choose Analysis ... and choose pss
  • In the Fundamental Tones section make sure Beat Frequency is selected and enter your best guess as to what the frequency of oscillation should be (i.e., enter your desired operating frequency). Make sure that Auto Calculate is NOT selected.
  • In Output harmonics select Number of harmonics and enter 10.
  • Under Accuracy Defaults (errpreset) select conservative and enter the required stabilization time under Additional Time for Stabilization (tstab).
    • tstab is obtained from a transient simulation. It is the time that it takes for the output of your oscillator to settle to a steady state (in terms of amplitude and frequency).
  • Select Oscillator and then select one of the differential outputs for Oscillator node and the other differential output for the Reference node.
  • Click Apply

• Nose select pnoise analysis in the Choosing Analysis window
  • Set the Sweeptype to relative
  • Enter 1 for the Relative Harmonic
  • Select Start-Stop and enter 1 kHz and 100 MHz for the Start and Stop frequencies, respectively
  • Set the Sweep Type to Logarithmic and set the Points Per Decade to 201
  • Under Sidebands select Maximum sideband and enter 30
  • Set the Output to voltage and select one of the differential outputs for the Positive Output Node and the other differential output for the Negative Output Node
  • Set the Input Source to none
  • Leave the Noise Type to sources
  • Click OK

• To plot the output, in the ADE Window click Results -> Direct Plot -> Main Form ...
  • Under Analysis select pnoise
  • Under Function select Phase Noise
  • Click Plot

Tuning Range

• Set your tuning voltage sources to be controlled by a variable (e.g., vtune)
• In the ADE window click on Choose Analysis ...
  • Select tran
  • Set the Stop Time to an appropriate value that ensures that the output of your VCO has settled and that at least 3/4 of the total simulation time consists of oscillation with a stable output.
  • Set the Accuracy Defaults (errpreste) to conservative
  • Click OK

• In the ADE window click Tools -> Parametric Analysis
  • Enter the variable name for the control voltage under Variable Name
  • Set the From and To values so that the control voltage is swept from o to Vdd
  • Set the Total Steps to 10
  • Click Analysis -> Start

• To plot the results open up the calculator
  • In the calculator (under the tran tab) click vt and then select one of the differential outputs of the VCO on the schematic
  • In the special functions of the calculator click frequency
  • Click the plot button in the calculator