Oscillator Design Microwave and Radio Frequency from M Pozar 2nd edition

1. RF & Microwave Engineering
Oscillator Design
Oscillator Design
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2. Oscillator Design
Contents.
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1. Introduction
2. One port negative resistance
 Example 11.8
3. Transistor oscillator
 Amplifier
 Oscillator
4. Dielectric resonator oscillator
 Comparison with other oscillators
 Parallel & series Configuration
 Example 11.10
5. Applications
6. Advantages & Disadvantages
7. References
Oscillator Design

3. Introduction
Oscillators
An oscillator provides a source of repetitive A.C. signal across
its output terminals without needing any input (except a D.C.
supply).
Microwave oscillators
A microwave oscillators converts DC power to RF power.
Range
 RF (radio frequency) oscillators working at frequencies
above about 30 to 50kHz .
 These may also be classified as HF, VHF, and UHF
oscillators, depending on their frequency.
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4. Design
A solid-state oscillator uses an active device, such as a diode or transistor,
in conjunction with a passive circuit to produce a sinusoidal steady-stare
RF signal.
Requirements
 At startup, however oscillation is triggered by transients or noise
 After which a properly designed oscillator will reach a stable oscillator.
 The active device be non linear.
 RF power must have negative resistance.
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5. One Port Negative Resistance Oscillators
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Figure 1. Circuit for one-port negative-resistance oscillator

6. One Port Negative Resistance Oscillators
 Principle
This material will also apply to two-port (transistor) oscillators.
Figure 1 shows the canonical RF circuit for a one-port negative-resistance oscillator,
Zin = Ri + jXi (input impedance of the active device)
input impedance is current (or voltage) dependent as well m frequency dependent
So,
Zin (I,jw) = Rin (I,jw) + jZin (I,jw)
The device is terminated with a passive load impedance.
ZL = RL + XL.
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7. One Port Negative Resistance Oscillators
(Zin+Zl) I = 0 (11.79)
Oscillations is occurring RF current will be non-zero. Then following
conditions must be satisfied
Rl+Rin = 0 (11.80-a)
Zl+Zin = 0 (11.80-b)
Since the load is passive. Rl > 0 and (11.80-a) indicates that Rin < 0. Thus,
while a positive resistance implies energy dissipation, a negative resistance
implies an energy source. This condition of (11.80-b) controls the frequency
of oscillation.
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8. One Port Negative Resistance Oscillators
The equation (11.79) will be
ZL = -Zin
The reflection coefficient will be
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9. Oscillator Design
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10. Example 11.8 Negative-resistance Oscillator Design
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Data
f=6GHz
Solution
From either the smith chart or by direct
calculation, we find the input impedance as
𝑍𝑖𝑛 = −44 + 123𝑗Ω.

11.  Then by 𝑅 𝐿 + 𝑅𝑖𝑛 = 0 ,
the load impedance must be
 𝑍 𝐿 = 44 − 123𝑗Ω.
 A shunt stub and series section of line can be used
to convert 50 Ω to 𝑍 𝐿 , as shown in the circuit of
figure 2
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Figure 2 Load matching circuit for the one port oscillator

12. Oscillator Design
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Distance
Length

13. Transistor Oscillator
 In a transistor oscillator, a negative resistance one-port is effectively created by
terminating a potentially unstable transistor with an impedance design to drive the
device in an unstable region. The circuit model is shown in Figure 3.
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Figure 3. Circuit for a two-port transistor oscillator

14. Amplifier
 preferred a device with a high degree of stability
 Ideally, an unconditionally stable device
Oscillator
 preferred a device with a high degree of instability
 common source or common gate FET configuration are used
 Positive feed back to enhance the instability of the device
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Figure 4

15. Transistor
 Transistor configuration is selected
 Output stability circle can be drawn in the plane
 selected to produce a large value of negative resistance at the input to the transistor
 Load impedance 𝑍 𝐿 can be chosen to match 𝑍𝑖𝑛
 Such design uses small signal parameters
 Rin become will become less negative as the oscillator power builds up
 It is necessary to choose 𝑅 𝐿 so tha
𝑅 𝐿 + 𝑅𝑖𝑛 < 0
 Otherwise oscillation will cease when increase Rin to the point where
𝑅 𝐿 + 𝑅𝑖𝑛 > 0
 In practice a value of
 𝑅 𝐿 =
−𝑅 𝑖𝑛
3
 Is typically used. The reactive part 𝑍 𝐿 is chosen to resonate the circuit
 𝑋 𝐿 = −𝑋𝑖𝑛
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16. Oscillations
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17. Oscillator Design
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 Then form (11.6b) we have that
 Hence oscillation condition satisfied

18. Dielectric resonator oscillator:
 Frequency determining element
 Produce signals with high stability
 High Q tuning network
 Can be made from ceramic materials
(have excellent temperature stability).
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Figure 5

19. Dielectric resonator oscillator:
Drawbacks of waveguide cavity resonators:
 Have unloaded Qs of 10^4 or more, not suited for microwave integrated
circuitry.
 Significant frequency drift.
 Temperature variations.
Comparison with other oscillators:
 Better power efficiency.
 High frequency pulling factor.
 Dielectric resonator oscillator exhibits less variation over temperature than
other oscillators as it can be made from ceramic materials that have excellent
temperature stability.
 Due to these reasons, Transistor Dielectric resonator oscillator are in common
use over entire microwave.
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20. Dielectric resonator oscillator:
Working:
 A dielectric resonator is usually coupled to an oscillator circuit by positioning it proximity
to a microstrip line.
 Operates in the TE10 mode.
 Couples to fringing magnetic field of microstrip
 The strength of coupling is determined by the spacing, d, between the resonator and
microstrip line.
 Because coupling is via the magnetic field. the resonator appears as a series load on the
microstrip line.
 The resonator is modeled as a parallel RLC circuit
 Coupling to the feedline is modeled by turns ratio of transformer.
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21. Geometry of dielectric oscillator coupled to micro
strip line:
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Figure 6 (a) Geometry of a dielectric resonator coupled to a Microstripline;
(b) Equivalent circuit

22. Oscillator Design
22 Equivalent series impedance seen by microstrip line:
Coupling factor:
Reflection coefficient:
Incorporation of dielectric resonator into circuit:
 Many oscillator configurations using FET or bipolar transistor.
 Incorporated into circuit using parallel or series feedback technique.

23.  Resonator coupled two microstrip lines.
 functioning as a high-Q bandpass filter that couples a portion of transistor output back to its
input.
 Amount of coupling is controlled by spacing between resonator and lines.
 Phase is controlled by length of lines
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Parallel configuration:
Figure 7 Parallel configuration

24. Series configuration:
 Using single microstrip feedline.
 Does not have a wide tuning range.
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Figure 8 Series configuration

25. Example11.10
 A wireless local area network application requires a local oscillator operating at 2.4 GHz.
Design a dielectric resonator oscillator using the series feedback circuit of Figure 1 1.27b
with a bipolar transistor having the following S parameters (Zₒ = 50Ω) : S11 = 1.8<130", S12
= 0.4<45. S21 = 3.8<36", S22 = 0.7<-63'. Determine the required coupling coefficient for
dielectric resonator and a micro-strip matching network for the termination network. The
termination network should include the output load impedance. Plot the magnitude for
small variations in frequency about the design value, assuming M unloaded resonator Q of
1000.
 DATA:
 Frequency = f= 2.4 GHz
 Zₒ = 50Ω
 TO FIND:
 Micro-strip matching network for the termination network=?
 Coupling coefficient for dielectric resonator=?
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26. SOLUTION
 The DRO circuit is shown in Figure 1 1 -28a. The dielectric resonator is placed X/4 from
the open end of the micro-strip line: the line length t, can be adjusted to match the phase of
the required value. The stability circles for the load and termination sides of the transistor
can be plotted if desired, but are not necessary to the design. Since, we will begin by
choosing TL to provide a large value of Form (1 1.89) we have
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27. Oscillator Design
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The magnitude of the reflection coefficient is unchanged so we have the relation
ZT = RT + j XT
= -Rout/3 – j Xout
RT = - Rin/3
XT = - Xin

28.  A short computer program or a microwave CAD package can be used to generate data for -0.01 <∆ f/fₒ
< 0.01, which is shown in the graph of Figure 11.28b. Observe that decreases rapidly with a change in
frequency as small as few hundredths of a percent, demonstrating the sharp selectivity that can be
obtained with a dielectric resonator.
 GRAPH:
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29. Oscillator Applications
Oscillators are a cheap and easy way to generate specific Frequency of a signal. For example,
 RC oscillator is used to generate a Low Frequency signal,
 LC oscillator is used to generate a High Frequency signal, and an Op-Amp based oscillator is used
to generate a stable frequency
Some common applications of oscillators include:
 Quartz watches (which uses a crystal oscillator)
 Used in various audio systems and video systems
 Used in various radio, TV, and other communication devices
 Used in computers, metal detectors, stun guns, inverters, ultrasonic and radio frequency
applications.
 Used to generate clock pulses for microprocessors and micro-controllers
 Used in alarms and buzzes
 Used in metal detectors, stun guns, inverters, and ultrasonic
 Used to operate decorative lights (e.g. dancing lights)
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30. Advantages
 Portable and cheap in cost.
 An oscillator is a non-rotating device. ...
 Frequency of oscillation may be conveniently varied.
 Voltage or currents of any frequency (20 Hz to 100 MHz) adjustable over a wide range can be
generated.
 Frequency once set remains constant for a considerable period of time
Disadvantages
 Because of inductor L circuit becomes bulky and cost of circuit is more.
 Poor frequency stability.
 Difficult to adjust feedback as capacitor values has to be changed.
 Poor Isolation (Load impedance v/s frequency).
 Hard to design.
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31. References
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[1] Microwave Engineering 2nd edition David M. Pozar University of Massachusetts at Amherst
[2] J. B. Beyer. S. N Parasad. R. C. Becker. J.E Nordman. And G. K. Hohenwarter, “MESFET Distributed Amplifier
Design Guidelines,” IEEE Trans. Microwave Theory and techniques. Vol.MTT-32, pp.268-275, March 1984.
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