2. OUTLINES
Introduction
Control theory approach
Two-port Oscillator Design
Optimum Oscillator Design
Summary
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3. 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. 3
4. CONTROL THEORY APPROACH
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.
Block Diagram of a feedback model Oscillator 4
5. CONTROL THEORY APPROACH
The closed-loop gain:
For oscillation to occur,
Barkhausen Criteria (startup condition)
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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6. TWO-PORT OSCILLATOR DESIGN
o One port is made to resonate so that K < 1
o The other port is designed to match the output impedence
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with the negative resistance Rin
8. TWO-PORT OSCILLATOR DESIGN
Design Steps
1. 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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9. TWO-PORT OSCILLATOR DESIGN
2. Design the terminating network
Make . 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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10. 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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12. TWO-PORT OSCILLATOR DESIGN
1. Checking for unstability at 8-GHz:
(Potentially unstable)
2. Draw input stability circle
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13. 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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15. 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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16. 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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19. OPTIMUM OSCILLATOR DESIGN
Design Steps
1. 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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20. OPTIMUM OSCILLATOR DESIGN
Example
A 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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21. OPTIMUM OSCILLATOR DESIGN
Converting S-parameters to Z- and Y- parameters
Computing the values of embedding elements
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23. 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.
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