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TECHNICAL MEMORANDUM Page 11 of 12 APPENDIX A Oscillator Simulation using the Probe Element 4 The oscillator probe element is a popular technique used in large signal simulation of oscillators. To understand the operation and simulation algorithm, consider Figure A-1 where a typical feedback oscillator is shown with the probe element connected to a resonator located within the feedback loop. Figure A-1: Feedback Oscillator with Probe Element The probe element has the following properties: • Presents a short circuit at the source frequency and an open circuit elsewhere. • Probe voltage and frequency are adjusted to exactly equal the steady state operating voltage. • No current flows through the probe at the probe frequency or any other harmonic. The probe frequency and amplitude are scanned in a search algorithm that identifies the signal frequency and amplitude at the connection point of the probe. When the signal frequency and amplitude are identified at the connection node, the probe no longer disturbs the circuit and its frequency equals the oscillation frequency while its amplitude equals the amplitude at the node to which the oscillator probe is connected. Oscillator analysis is reduced to standard harmonic balance analysis running in the inner loop of a routine that attempts to locate probe parameters 4 The oscillator probe element initiates a large-signal oscillator simulation. The oscillator probe element is an ideal source in series with an ideal impedance element. The impedance presents an open circuit at all frequencies other than the fundamental frequency of (amplitude and frequency) that result in zero current flow through the terminals of the probe. In negative resistance oscillators, the probe should be connected between the resonator and the negative resistance-generating active device. In feedback oscillators, the probe should be connected to one of the nodes belonging to the feedback loop. Other connections of the oscillator probe generally result in excessive time to achieve convergence of the harmonic simulation algorithm and also decrease accuracy; sometimes, an alternate connection will fail to predict oscillation in an otherwise properly operating circuit. The author has found rapid convergence and frequency prediction when the probe element is connected to the frequency determining element of the oscillator circuit; while this has not been analytically verified, it is suspected that the slope of the reactive impedance at the node of the probe is believed to be a component of the search algorithm. oscillation. Source: Help resource, Cadence AWR software. X-Band Push-Push Oscillator Simulation and Measurement 11 www.cadence.com/go/awr

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