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Oscillator Open-Loop Test Element (Closed Form): OSCTEST

Symbol

Summary

OSCTEST 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 determine the open-loop gain of the circuit under large-signal conditions.

The following figure illustrates the use of OSCTEST. A large-signal excitation is applied to port 1, and the large signal S-parameter S21, 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, |S21| must be greater than unity, or the circuit will not oscillate. As the excitation level is increased, |S21| decreases as the circuit enters saturation, and at some frequency and some excitation level, |S21| = 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.

Before OSCTEST 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 |S21| > 1 and its 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.

Parameters

Name Description Units Type Default
ID Element ID Text O1
*FC f1 < fc < 2*f1 Frequency 1.0 GHz
*Z0 Port 1 and 2 impedance Resistance 50 ohm

* indicates a secondary parameter

Equations

At frequencies below FC, OSCTEST has the following S-parameters:

At frequencies above FC, the S-parameters are

All other S-parameters are zero.

Layout

This element uses a special layout cell for a short circuit. The layout cell allows the elements connected on either side of the element to look through this element. For example, when a MLIN is hooked to a TFCM (capacitor) model and the project has bridge code configured, the line draws the proper interconnect between the line and the cap. If this element is placed between the line and the cap, the layout still draws the same.

You typically do not assign artwork cells to these items.

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