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TECHNICAL MEMORANDUM Page 6 of 12 For a voltage null to exist at the junction of the oscillator sections, each oscillator must generate a signal that is coherent and in phase opposition. Clearly, symmetry and circuit balance are requirements for quality push-push oscillator operation. The second harmonic of the oscillator sections is extracted at the null point while the fundamental is cancelled. The primary frequency acts as an idler to produce an output signal with significant second harmonic content; this is accomplished principally because there is no loading of the fundamental signal component and therefore, highly, non-linear operation is assured. Although the grounded collector oscillator has been selected, it should be emphasized that other oscillator configurations are applicable; in-fact, other types of devices are applicable. The principal criterion is that a negative resistance is required and a means of joining the negative resistance devices in a manner that insures synchronization and out-of-phase signal generation from each device. IV. SIMULATION TECHNIQUE The bipolar junction transistor (AT42000) is initially investigated under small and large signal conditions using the feedback capacitor to maximize the reflection coefficient over the anticipated bandwidth of the oscillator. This investigation is conducted using the schematic of Figure 6 where the device is biased at a nominal operating point and a signal is applied to the negative resistance port at variable power levels. Figure 6: Initial negative resistance investigation. The reflection coefficient is optimized with the feedback capacitor and plotted as the input power is increased. After optimization of the reflection coefficient at the base under small signal conditions using the variable feedback capacitor, the reflection coefficient is repeatedly measured under increasing input RF power conditions. As may be readily discerned from the graphic of Figure 7, the reflection coefficient decreases as the signal level is increased as one might expect from an intuitive understanding regarding the signal limiting of oscillators. Note that the negative resistance decreases as the RF input power is increased. This is expected and is the same phenomenon that occurs as the oscillator starts and settles to the final operational conditions. This exercise may also be used to predict oscillation frequency; for example, continue to increase the signal level and observe the point where the reflection coefficient magnitude is equal to unity. At this point, one may read the real and imaginary parts of the impedance. The reflected impedance from the coupled resonator is also illustrated; the point at which the negative resistance at the base of the transistor is equal to the coupled port positive resistance and the total loop reactance cancels is the oscillation frequency. Figure 7: Large signal reflection coefficient versus drive power. Note that the large signal reflection coefficient decreases as input power is increased. This observation validates oscillator theory with respect to the build-up of signal to the steady-state limiting – the net oscillator loop resistance is zero at steady- state. This is an estimate and most large signal simulation algorithms are much more sophisticated in both technique and execution. However, it is instructive to observe the behavior and validate the traditional understanding of oscillator theory. It is also instructive at this point to alter the feedback capacitance to ensure that the maximum reflection coefficient is maintained at the base of the transistor. To ensure that the oscillator starts under small signal conditions is the first requisite; to maintain a healthy oscillator, the feedback capacitor must also sustain the large signal limiting conditions. A phenomenon that 0 1.0 -0.5 -1.5 -2.0 -3.0 1.0 -1.0 2.0 -2.0 0.2 0.2 -0.2 -0.2 0.5 0.5 -0.5 -0.5 Negative Resistance Swp Max 5.5GHz Swp Min 4.5GHz 5.009 GHz r -5. 56044 Ohm x -48.5252 Ohm 5 GHz r 5.1586 Ohm x 66.5963 Ohm 5.003 GHz r -33.7796 Ohm x -31.0652 Ohm Gcomp [PORT_1,1,1] Negative Re sista nce S[1,1] Load Gcomp [PORT_1,1,2] Negative Re sista nce Gcomp [PORT_1,1,3] Negative Re sista nce Gcomp [PORT_1,1,4] Negative Re sista nce Gcomp [PORT_1,1,5] Negative Re sista nce X-Band Push-Push Oscillator Simulation and Measurement 6 www.cadence.com/go/awr

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