Download Microwave Oscillators

Solid State Microwave Oscillators
• Convert dc energy to microwave signals
• Can be used as generators in all communication systems, radars,
electronic counter measures (ECM), etc
• Replacing more and more low power tubes such as small klystrons
The main advantages of solid state oscillators are:
• Low voltage supply
• Reliability
• Small weight
• Simple in fabrication and low cost
Solid state oscillators are generally used as a Local Oscillator for
mixing operation.
The Local Oscillator is a pure sinusoidal signal source used to provide
the mixing function in a mixer. It is fed into the mixer LO port, and is
used for down conversions and up conversions.
Oscillators are characterized by the following parameters:
1)
2)
Frequency
Frequency stability:
- Frequency pushing (frequency change with poor supply
voltage or current)
- Frequency pulling (frequency change with load mismatch)
- Temperature stability
- Shock and vibration effect
3)
4)
5)
6)
Phase noise
Spurious Performance
Power and efficiency
Tuning:
Mechanical
Electronically
Tuning sensitivity (Hz/V or Hz/ mA)
Settling time (the time it takes to respond to frequency control signal)
Post-tuning drift (drift after the oscillator reaches its desired frequency )
Oscillator parameters are mainly determined by the type
of the resonance circuit or the cavity used
Oscillator is Composed of three parts:
1.
A resonant structure (is mainly characterized by its quality factor
(Q) ). Quality factor is determine by the type of resonator at a
specified frequency of operation.
2.
A negative resistance system controlled by the power
3.
A coupling system able to deliver power to the load.
Microwave Resonators generally used in microwave Oscillators:
A typical microwave resonators are shown in the following Figure
Resonator
Lumped
l/2
Microstrip
Dielectric
Q=fo/ Df
Tran. power
Df
fo
SAW
Coaxial
Active Devices used for microwave oscillators
Microwave semiconductors used as the active oscillator
devices are:
•
•
•
•
•
•
BJT at the low end of the microwave band
MESFETFET
HEMT
IMPATT
Gunn devices at the high end of the microwave band
Varactor multiplier
Mechanical tuned Resonators
Phase noise
The phase noise is defined as the power in 1-Hz bandwidth at
a frequency fm from the carrier, and measured in dB below the
carrier power.
The phase noise of microwave oscillator is 60 to 120 dB
below the carrier power.
Phase noise
(dB)
1
0
1
KHz
KHz
Effect of Phase Noise on Receiver Performance
How much phase noise can be tolerated in a given system
design ?
Desired
signal
Desired LO
Phase noise
undesired
signal
IF
IF
IF
Noisy LO
How phase noise affect false detection for radar applications
40 dB
Main signal
Mountain
Noise
sideband
Airplane
Received signal
40 dB
Main signal
Noise
sideband
Transmitted signal
Some Oscillator Configurations
Varactor Tuned Oscillators:
Electronically Tuned
Some IMPATT Oscillators
Mechanically Tuned
Transistors Based Oscillators Configurations
Harmonic Multiplier
Varactor
Duroid
substrate
LPF
Matching
Network
Frequency Synthesizers
Precisely frequency controlled oscillator derived from a
stable oscillator
The stable reference source is usually a crystal controlled
oscillator,
Can be be housed in a temperature controlled environment for
even greater stability.
Eliminates the need for many independent crystal oscillators
in a multichannl system (ex. GSM system)
Can be easily implemented using a variety of available
integrated circuits. There are three basic methods that can be
used, the most popular method is the phase-locked loop.
Phase locked loops
Loop
Phase
detector amplifier
Loop Filter
fo
Reference
Frequency
N
Frequency
divider
VCO
Outpu
t
Nfo
PLL consists of the following circuits:
Reference oscillator
Phase detector produces an output voltage proportional to the
difference in phase of the two inputs
Loop amplifier and filter
VCO operating at the desired output frequency
Frequency divider
In operation, the output of the voltage controlled oscillator is divided by
N to match the frequency of the reference oscillator. The phase detector
produces a voltage proportional to the difference in phase of these two
signals and is used to make small corrections in the frequency of the the
VCO in order to align the phase of the VCO with that of the reference
source. There are