Transcript
MICROWAVE OSCILLATOR DESIGN
Presented by:
Imane Hafnaoui
M’Hamed Bouguara University – Boumerdes – IGEE
2011 -2012
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OUTLINES
Introduction Control theory approach Two-port Oscillator Design Optimum Oscillator Design Summary
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INTRODUCTION
In the most general sense, an oscillator is a nonlinear circuit that converts DC power to an AC waveform without requiring a input signal.
They are used to: Stabilize time-frequency generators, which in
turn provide carrier and pilot signals for electronic communication and navigation systems.
Provide the clock signals used by data processing equipment.
As a reference signal for other special-purpose systems.
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CONTROL THEORY APPROACH
Block Diagram of a feedback model Oscillator
This approach is a good starting point for it enables us to better understand and design two-component oscillators. It helps in deriving conditions for oscillation.
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CONTROL THEORY APPROACH
The closed-loop gain:
For oscillation to occur,
Þ Barkhausen Criteria (startup condition)Þ When the criteria is met, the poles are
located on the imaginary axis. This is a necessary but not sufficient condition.
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TWO-PORT OSCILLATOR DESIGN
oOne port is made to resonate so that K < 1oThe other port is designed to match the output
impedence with the negative resistance Rin
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TWO-PORT OSCILLATOR DESIGN
Oscillation Conditions: Stability factor K < 1
For a non-zero output,
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TWO-PORT OSCILLATOR DESIGN
Design Steps1. Check if the transitor is potentially
unstable (K < 1)Remark: i. if the device is not potentially unstable:
Use feedback element like an inductor to make the device unstable.
Change the configuration to common-gate or common-base.
ii. Shunt/Series feedback will increase , increasing instability.
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TWO-PORT OSCILLATOR DESIGN
2. Design the terminating networkMake . Can be attained by selecting far away in the instability region of the input stability circle.
Remark:We can confirm that by computing,
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TWO-PORT OSCILLATOR DESIGN
3. Design the load network to resonate Zin
Remark: Rin must be chosen wisely so oscillations won’t cease before it reaches steasy conditions.
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TWO-PORT OSCILLATOR DESIGN
Example
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TWO-PORT OSCILLATOR DESIGN
1. Checking for unstability at 8-GHz:(Potentially unstable)
2. Draw input stability circle
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TWO-PORT OSCILLATOR DESIGN
The associated impedence:
This reactance can be implemented by an open-circuited 50-Ohm line of length
This gives:
The load matching network:
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TWO-PORT OSCILLATOR DESIGN
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OPTIMUM OSCILLATOR DESIGN
Since oscillators tend to work at maximum power, small-signal parameters will no longer be accurate for a precise design. Therfore,the use of large-signal parameters is essential.
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OPTIMUM OSCILLATOR DESIGN
Oscillator Circuit Configurations
There are 06 configurations, where the choice of the embedding elements will make the circuit oscillate.
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OPTIMUM OSCILLATOR DESIGN
Series Oscillator-circuit Configurations
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OPTIMUM OSCILLATOR DESIGN
Shunt Oscillator-circuit Configurations
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OPTIMUM OSCILLATOR DESIGN
Design Steps1. Measure the large-signal S-parameters of the
transistor.2. Check for potential unstability (K < 1).3. Convert S-parameters to Y- and Z- parameters.4. Compute embedding elements of a desired
oscillator circuit configuration (one of the six (06) previous configuration).
5. Realize the oscillator circuit.
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OPTIMUM OSCILLATOR DESIGN
ExampleA common-source packaged GaAs MESFET has the following S-parameters measured at 10-GHz
Design a high power oscillator for 10-GHz by using series oscillatorcircuit-1.
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OPTIMUM OSCILLATOR DESIGN Converting S-parameters to Z- and Y- parameters
Computing the values of embedding elements
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OPTIMUM OSCILLATOR DESIGN
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SUMMARY
In this talk, we were exposed to microwave oscillators, their use, a general idea about their inner workings.
Also, we discussed two-port network design. The conditions for the circuit to oscillate and the steps to design such a circuit.
We finished with optimum design of oscillators where large-signal parameters are employed rather than small-signal parameters to insure maximum power and accuracy.
END OF TOPIC
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