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TECHNICAL MEMORANDUM Page 12 of 12 APPENDIX B Oscillator Simulation using the Test Element 5 The oscillator test element is used to determine loop gain in oscillator design under large-signal conditions. Referring to Figure B-1, large-signal excitation is applied to port 3, and the large signal S parameter S 21 , at the fundamental frequency is monitored. The cutoff frequency fc must be set to any value between the fundamental frequency and the second harmonic. When the excitation is weak, |S 21 | must be greater than unity to ensure that the start conditions are satisfied. As the excitation level is increased, |S 21 | decreases as the circuit enters saturation, and at some frequency and some excitation level, |S 21 | = 1 and its phase is zero. This point corresponds to the oscillation frequency, and the output power under these conditions is the oscillator's output power. This operational procedure is and has been experimentally verified. Figure B-1: Feedback Oscillator with Test Element Before the oscillator test element can be used under large-signal conditions, an initial small-signal design of the oscillator should be made. The circuit should be adjusted so that |S 21 | > 1 and the phase is zero, under small-signal conditions, at the desired frequency of oscillation. As the excitation is increased, the frequency of zero phase shift changes, and it may be necessary to "tweak" the design to keep it on the right frequency. One would expect this phenomenon simply 5 The oscillator test element is used to determine loop gain in oscillator design and to break the feedback loop of an oscillator in the forward direction, at the fundamental frequency. When this is done, a source at port 3 is used to replace the feedback signal and the feedback itself is measured at port 4. This allows you to because the active device parameters change with signal level. The oscillator test element permits the total loop gain to be measured under small and large signal conditions and evaluated without disturbing the circuit operation; it is especially useful in analysis of oscillators with discrete feedback loops. An additional feature of the oscillator test element is the determination of frequency control using the phase shift element. This is accomplished via measurement of large signal magnitude of S 21 while the phase shift is varied. The oscillator tuning range is determined by observing the frequency points at which large signal S 21 is less than unity. The S-parameters of the oscillator test element (Figure B-2) are similar to those of a three-port isolator and permit circuit excitation without altering circuit function for the measurement of closed-loop gain and phase. c c f f S f f S = = @ 0 0 0 1 0 0 0 0 0 0 0 1 0 0 1 0 and @ 0 0 0 1 0 0 0 0 0 1 0 0 0 0 1 0 Figure B-2: Oscillator Test Element S-Parameters Valid large signal models of active devices are essential for accurate oscillator simulation. determine the open-loop gain of the circuit under large- signal conditions. Source: Help resource, Cadence AWR software. X-Band Push-Push Oscillator Simulation and Measurement 12 www.cadence.com/go/awr

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