Experiment 12: Microwave Oscillator with Negative Conductance Diode

File: MW-OSCIL.TLM 

Objective: To simulate and visualize microwave power generation using a negative conductance diode, and to study the time and frequency characteristics of such an oscillator.

Parameters of the Structure: The file mw-oscil.tlm contains data for a waveguide cavity coupled to a waveguide section through an inductive iris. The cavity contains an active diode and a Source Point for injecting impulsive noise into the cavity. The waveguide section is matched at the right extremity by a Johns Wall. The corresponding joh file is Wg-29.joh.  A Probe allows to extract the time and frequency behavior of the output voltage. A 1Dl  wide Animation Region allows to visualize the voltage along the main axis.

The total structure has electric sidewalls and covers an area of 60Dl  x 29Dl; er = 1; s = 0 everywhere inside the structure, except for the 3Dl  x 4Dl  active diode subarea in the center of the cavity. Dl = 0.245241 mm. The structure thus is a WR(28) standard waveguide component.

The Source Point launches a Vy impulse with a magnitude of sqrt(2). It allows to inject any input signal into the cavity, either to start an oscillation, or to injection lock the oscillator.

The active diode subarea has been implemented using the Active Diode feature within the Draw menu. Its characteristics are described in Ref [2] and have the following parameters:

ereff : 1.0
Structure thickness: 0.5 mm (the height of the waveguide is irrelevant since this is a two-dimensional situation)
Series resistance: 4W
Shunt capacitance: 0.07 pF
Maximum shunt conductance  -0.004 S (the conductance must be entered with a negative sign if the diode should generate power)
Limiting voltage: 1 V

There is one Probe in the output guide on the right which is matched by a Johns Wall.

12.0 Preliminary Steps

Inspect the layout of the structure, then use the View menu to verify the diode parameters. Now change to the Graph menu and select Probe 1 > V(t). The Vy(t)  diagram appears on the screen. The program has been initialized; verify the initial values by clicking on the various items of the menu. Select an impulsive excitation of 0.2 V, and assign file WG-29 to the Johns wall 1.

12.1 Visualization of the Start of Oscillations
  1. Open the file MW-OSCIL.TLM and start the simulation. Observe the impulsive disturbance traveling forth and back in the cavity and into the waveguide.
  2. After about 700 time steps you will notice a slowly growing oscillation in the cavity. Part of the generated power is immediately transmitted into the matched load by the waveguide. Stop the simulation when you think that the amplitude of the oscillation does not grow any longer.
  3. See the time history of the start-up process. Note that the initial noise injected into the cavity slowly gives way to a coherent oscillation which grows exponentially until saturation limits the amplitude.
12.2 Visualization of the Field in the Oscillator
  1. Enter the Draw view and replace the Animation Region by a larger one which covers the entire structure.
  2. Enter the Field view, increase the number of time steps, set the update interval to 1, and start the simulation. Observe the field in the oscillator. Note that the waveform contains a third harmonic.
12.3 Visualization of the Output Spectrum of the Oscillator
  1. In the Graph menu choose Probe 1 > V(f) Magnitude
  2. View  the output spectrum of the oscillator and identify the fundamental frequency and the third harmonic.
12.4 Further Experimentation

Inject a small sinusoidal signal at a frequency close to the fundamental frequency of oscillation and study the effect of injection locking  on both the start-up process and the frequency of oscillation. Build other types of oscillators and modify the diode characteristics.