Download Active Microwave Devices

Semiconductor Microwave Devices
Most microwave devices are fabricated on a GaAs substrate because of its high
mobility. A silicon substrate, on the other hand, has the advantages of low cost
and high yield. The following table summarizes the various microwave solidstate devices and their applications.
Device
Frequency
Limitation
Substrate
Material
Major Applications
IMPATT
< 300 GHz
Si, GaAs, InP
Transmitters Amplifiers
Gunn
< 140 GHz
GaAs, InP
FET&HEMT < 100 GHz
GaAs, InP
p-i-n
< 100 GHz
Si, GaAs
Varactor
< 300 GHz
GaAs
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Local oscillators, Amplifiers
Transmitters
Amplifiers , Oscillators, Switches,
Mixers, and Phase shifters
Switches, Limiters, Phase shifters,
Modulators, and Attenuators
Multipliers, Tuning, Phase shifters,
and Modulators
Microwave Engineering/Active Microwave Devices
9-13 September 2006
Microwave Diodes
A microwave diode is much more than just a two-element device which
has limited capabilities. It is a complex device which an integral part of
many sophisticated microwave systems. Many devices have been
developed using the non-linear I-V and C-V characteristics of the p-n or
Schottky-barrier junction. Various applications are summarized below
2
Non-linear I-V Characteristics
Non-linear C-V Characteristics
Frequency mixing
Frequency multiplication
Harmonic generation
Voltage Controlled Oscillator
Switching
Voltage tuned filter
Modulation
Frequency conversion
Limiting
Harmonic generation
Detection
Parametric amplification
Microwave Engineering/Active Microwave Devices
9-13 September 2006
Non-Linear Characteristics of p-n and Schottky
diodes
I
VB
C
Is
VB
Vbi
V
Non-linear I-V
Characteristics of a
diode
3
V
Non-linear I-V
Characteristics of a diode
Microwave Engineering/Active Microwave Devices
9-13 September 2006
Varactor Devices and Circuits
Semiconductor p-n junction, or Schottky-barrier
n-type semiconductors with p-type diffusion
Important parameters:
Q factor
Cutoff frequency
Breakdown voltage
Sensitivity.
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9-13 September 2006
Applications:
(1) Voltage controlled Oscillator VCO:
 FM systems and frequency agile systems
 Instrumentation
 Electronic warfare (EW)
 Electronic counter measurement (ECM) systems.
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Microwave Engineering/Active Microwave Devices
9-13 September 2006
(2) Multiplier and harmonic generation
Feasible alternative for the generation of high
frequency signal
Zo
LPF and
matching
Cj(V)
Rs
6
BPF and
matching
Zo
Varactor
Microwave Engineering/Active Microwave Devices
9-13 September 2006
(3) Parametric Amplifiers:
Provide very low noise amplification
Output
Circulator
Combiner
and
Varactor
Input
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9-13 September 2006
Pump
signal
p-i-n Diodes
Similar to the pn diode with smaller junction capacitance
Very useful for a diode used a microwave switch
P+
i
Ls
n+
Parasitics
Weakly doped
P-i-n structure
Rp
r.b
.
Cj(V)
Rs Ls~ 0.1 nH
f.b Cp~ 0.3 pF
. Rs~ 0.3 W
Rj(V)
Equivalent circuit
of p-i-n
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Microwave Engineering/Active Microwave Devices
9-13 September 2006
Switches Applications
Output
Source
Switch
t
Bias
(1) Modulators in communication systems
Wideband
switch
.
(2) Switch in wide. band system
.
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9-13 September 2006
Tx
Rx
(3) To protect receiver from the transmitter (such as in radar system)
(4) Channel selection in wideband system
(5) Signal path control in measurement systems
As a switch the main important p-i-n diode parameters are Isolation and
Insertion loss
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Microwave Engineering/Active Microwave Devices
9-13 September 2006
p-i-n Diode Attenuator
p-i-n diode attenuator circuits are used extensively in
automatic gain control (AGC) and RF leveling applications as
well as in electronically controlled attenuators and modulators
Zo
Zo
Reflective type
A = 20log (1 + Zo/2Rs)
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9-13 September 2006
Matched attenuator
3-dB quadrature
coupler
p-i-n
diode
Input
Bias
Output
Zo
p-i-n
diode
Zo
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9-13 September 2006
p-i-n Phase Shifters
Input
Output
3-dB, 90o
l/4
Input
Output
Hybrid
Zo
B1
Hybrid coupler phase shifter.
Uses the fewest diodes. Any
phase shift increment can be
obtained with proper design
of the terminating circuit.
13
B1
B2
Diode
The loaded line phase shifter
Microwave Engineering/Active Microwave Devices
9-13 September 2006
B2
Diode
Switched line phase shifter
L1
Bias
L2
Switching action is used to obtain insertion phase by
providing alternative transmission paths, the difference in
electrical length being the desired phase shift
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9-13 September 2006
Limiter p-i-n Diodes
Used for protection applications
Limiter
3 dB
Coupler
3 dB
Coupler
Limiter
Transmitter
15
Microwave Engineering/Active Microwave Devices
Receiver
9-13 September 2006
Pout
Pin
Maximum
Isolation
p-i-n diode
Pout
Insertion loss
Pin
Passive Limitation. No exterior control is needed
and the incident microwave power is responsible for
switching from the high impedance state to low
impedance state of the diode
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9-13 September 2006
p-i-n
diode
Pin
Control
p-i-n
diode
Pin
Pout
pulse
Pout
Controlled limitations. This
method gives lower losses,
better isolation, but require
a delicate control circuit.
Any loose of control affect
receiver protection
17
Schottky diode
Controlled limitations. A
small part of the incident
signal is sampled and detected
by Schottky diode whose the
rectified current biases the
diode in the forward state. The
losses at low level are slightly
higher, adjustments are very
difficult
Microwave Engineering/Active Microwave Devices
9-13 September 2006
Gunn Diodes
Single piece of GaAs or Inp and contains no junctions
Exhibits negative differential resistance
Applications:
low-noise local oscillators for mixers (2 to 140 GHz).
Low-power transmitters and wide band tunable sources
Continuous-wave (CW) power levels of up to several hundred mill watts
can be obtained in the X-, Ku-, and Ka-bands. A power output of 30 mW
can be achieved from commercially available devices at 94 GHz.
Higher power can be achieved by combining several devices in a power
combiner.
Gunn oscillators exhibit very low dc-to-RF efficiency of 1 to 4%.
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9-13 September 2006
Varactor Tuned Gunn Oscillators Circuits
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9-13 September 2006
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9-13 September 2006
IMPATT Devices and Circuits
IMPact Ionization Transit Time
IMPATT devices can be used for oscillator and amplifier applications
They can be fabricated with Si, GaAs, and InP
Can be used up 400 GHz.
Noisy oscillator
In general, IMPATTs have 10 dB higher AM noise than that of Gunn
diodes
IMPATT diode is not suitable for use as a local oscillator in a receiver.
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9-13 September 2006
Some IMPATT Circuits
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9-13 September 2006