MICROWAVE DEVICES
IRINEO P. QUINTOECE / REE
MICROWAVE TUBES
• KLYSTRON AMPLIFIER– AMPLIFIES
MICROWAVE SIGNAL USING VELOCITY MODULATION
– FORMS HIGH VELOCITY ELECTRONS
– TYPICAL EFFICIENCY: 30-45%; MAX=70%
– 0.5 – 6.4 GHz
BUNCHER CAVITY CATCHER CAVITY
DRIFT SPACE
MICROWAVE TUBES
• REFLEX KLYSTRON– LOW POWER, LOW
EFFICIENCY MICROWAVE OSCILLATOR
– 4-200 GHz– <10% EFFICIENCY– TYP Po=100 mW
T = n + 3/4
T – transit time
n – any integer
REPELLER
OUTPUT
ANODE
MAGNETRON• A DIODE WHICH USES THE
INTERACTION OF MAGNETIC AND ELECTRIC FIELDS IN A COMPLEX CAVITY TO PROVIDE OSCILLATIONS
• 10 MW – UHF; 2MW – X-BAND; 80 kW – 95 GHz
• EFFICIENCY OF 50-60%
• TYPES OF:– HOLE & SLOT– VANE– RISING SUN– COAXIAL
MICROWAVE TUBES
• TRAVELING WAVE TUBE (TWT)– THE INTERACTION BETWEEN THE BEAM
AND THE RF FIELD IS CONTINUOUS– CAN BE USED AS A LOW-LEVEL, LOW
NOISE AMPLIFIER OR AS A HIGH POWER ONE, EITHER CW OR PULSED
– 2 – 16 GHz, 30-45 dB GAIN, F=4-10 dB, 10-100 mW
– CW 1-100 GHz, UP TO 10 kW, 25-35% effy
– PULSED 2-40 GHz,1-250 kW
• CROSS FIELD AMPLIFIER (CFA)– CROSS BETWEEN THE TWT &
MAGNETRON– PULSED TYPE 1-18 GHz, 5MW –
UHF, 70%; 1 MW – X BAND, 55%• BACKWARD WAVE OSCILLATOR
(BWO)– SHORTER & THICKER TWT– MICROWAVE CW OSCILLATOR– 1-1000 GHz
Microwave Tubes
1 MW
1 KW
1W
1mW
0.3 1 3 10 30 100 300 Frequency (GHz)
Av
era
ge
po
wer
Microwave tubes
Microwave semiconductor
devices
• Lower weight
• Smaller size
• Longer life time
• Higher power
• Limited life time
• High vacuum
• High potential
Two possible methods of achieving high output power in microwave system
Low power semiconductor
oscillator
High power tube amplifier
High power tube
oscillator
Important Parameters • Peak power • Average power• Efficiency • Gain • Bandwidth • Frequency • Harmonic and spurious power • Intermodulation products • Manufacturability at low cost
TypeRelative BW (%)
(%)Gain (dB)
Relative spurious
level
Relative operating
voltage
Relative complexity
of operation
Gridded tube 1-10 20-50 6-15 2 Low 1
Klystron 1-5 30-70 40-60 1 High 2
Helix tube 30-120 20-40 30-50 3 High 3
Coupled cavity tube
5-40 20-40 30-50 3 High 3
10 MW
1 MW
100 KW
10 KW
1 KW
100 W
0.3 1 3 10 30 100 300 Frequency (GHz)
Average power
Klystron
Coupled cavity TWT
Helix TWT
Gridded tube
1000 MW
100 MW
10 MW
1 MW
100 KW
10 K W
0.3 1 3 10 30 100 300 Frequency
(GHz)
Peak power
Coupled cavity TWT
Klystron
Gridded tube
Helix TWT
Klystron
Electron beam
Electron Gun
Intermediate cavity
Beam collector
Microwave input
Microwave output
TWT
Major applications for TWTs include:Amplifiers:• Space applications• Radar • Electron Counter Measure • Missile
Driver for other high power RF amplifiers
Missile TWTs for Active Seekers
Features that influence the design include:
• Minimal size and weight;
• Narrow-to-moderate bandwidths;.
• Off-to-fully-operational turn-on times of one second or less;
• High efficiency;
• High reliability after long inactive storage periods.
Normally, these TWTs are of the periodic-permanent-magnet (PPM) focused helix variety. They normally utilize unique cathode-heater designs to provide the very fast warm-up required. They typically have multiple stage depressed collectors with conduction cooling.
Microwave Ovens
5. Microwave ovens
Source:NPL
Electromagnetic spectrum
• Microwaves• From
– 0.8 GHzto
– 1000 GHz
1 101 102 103 104 105 106 107 108 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022
Radio & TVInfra Red
Microwaves
Gamma-Rays
X-Rays
Ultra Violet
Microwaves
Conventional Oven
Microwave Oven
Absorption (1)Too strong
• If power were absorbed too strongly,– Microwaves would only penetrate a short distance– Surface would be heated– Inside would remain uncooked
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.01 0.1 1 10 100
Lo
ss o
f en
erg
y in
wa
ter
at 0
C
Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Absorption (2) Too weak
• If power were absorbed too weakly,– Microwaves would go right through – No cooking
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.01 0.1 1 10 100
Lo
ss o
f en
erg
y in
wa
ter
at 0
C
Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Absorption (3)
• If power is absorbed just right,– Microwaves penetrate about 5 cm (2 inches) – Cooks the outer 5 cm of the food– Good enough for most cases
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.01 0.1 1 10 100
Lo
ss o
f en
erg
y in
wa
ter
at 0
C
Frequency (GHz)
Microwave ovens operate at 2.45 GHz
Microwave OvensSummary
• A microwave oven cooks food by heating it• The heating comes from
– intense waves at 2.45 GHz • rather than
– a wide spectrum of waves at infra red frequencies.– Frequency chosen because of absorption properties
of water molecules at that frequency.
Microwave GenerationCavity Magnetron Valve
Microwave Ovens
Magnetron
Microwave Ovens
Inside a microwave oven
Microwave Power
• Power– This is a 700 watt oven– Think of 7 x 100 watt light bulbs
MicrowaveElectric field
• 700 watts – Around 140 000 volts per metre– Look what happens to a CD
Microwave Intensity
• 700 watts – transmitted into an area of around 1/25th square metre– Between 104 to 105 watts per square metre– (Most intense sunlight around 103 watts per square metre)– Very Dangerous– Could I have a stupid volunteer please?
A Cautionary Tale
Don’t try this at home!
August 14, 2002 I don't want to sound like I know everything in the world or even like I know quite a lot. But you had a question regarding “If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP” Well ..Having the thought process that I have, kinda how should I put it? ...Stupid? or inventive or even in-between. Well, my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I wondered if it would burn fast or would it gradually warm up. I slid my finger between...You probably dying to hear what happened... But it didn't gradually warm up at all. It was instant heat! It didn't scar me or anything like that, but sure scared the H*** out of me to find out it got so hot so quick. I didn't get any blisters either. But it just burned like touching something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, "You learn from your mistakes", stands true. lol -
Microwave OvenSAR inside oven
• Inside – 700 watts:– Absorbed in 1 kg of water: SAR = 700 watts per kg– Your brain weighs about a kilogram!
• QUESTION: After 1 minute, what temperature rise results from an SAR of 700 watts per kg?
Microwave Ovens (13) SAR
SAR Wattsper kilogram
Temperature Rise in 1 kg of ‘Brain Fluid’
Microwave 700 8 ºC (ish)
Mobile Phone 1 Can’t be measured directly
Expect 1/700 of microwave temperature riseThe effects of blood flow reduce this further
Semiconductor Microwave Devices
SEMICONDUCTOR MICROWAVE DEVICES
• TRANSISTOR– 2-4 GHz, 9W, 12-8 dB– 29.5-32.5 dB at 4-6 GHz, 15
mW– FET – G=10dB, 9-15 GHz, F=7-
14 dB• PARAMETRIC AMPLIFIER
– USES A DEVICE WHOSE REACTANCE VARIES SUCH THAT AMPLIFICATION RESULTS
• GUNN DIODE– WORKS ON THE PRINCIPLE OF
TRANSFERRED ELECTRON EFFECT
– MADE WITH GaAs AND InP– CW 4-75 GHz (1.5W – 50
mW), 12-2% (TYP EFFY: 2.5-5%)
SEMICONDUCTOR MICROWAVE DEVICES
• IMPACT AVALANCHE & TRANSIT TIME DIODE (IMPATT)
– MADE OF Si OR GaAs– WORKS LIKE TUNNEL DIODE / GUNN
DIODE– MADE OF 4 LAYERS
• TRAPPED PLASMA AVALANCHE TRIGERRED TRANSIT TIME DIODE (TRAPATT)– PULSED – 600 W, 1 GHz, 75%
(TYP:30%)
• OTHER MICROWAVE DIODES– TUNNEL– VARACTOR– SCHOTTKY BARRIER– PIN
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Semiconductor Microwave Devices
DeviceFrequency Limitation
Substrate Material
Major Applications
IMPATT < 300 GHz Si, GaAs, InP Transmitters Amplifiers
Gunn < 140 GHz GaAs, InPLocal oscillators, Amplifiers
Transmitters
FET&HEMT < 100 GHz GaAs, InPAmplifiers , Oscillators, Switches,
Mixers, and Phase shifters
p-i-n < 100 GHz Si, GaAsSwitches, Limiters, Phase shifters,
Modulators, and Attenuators
Varactor < 300 GHz GaAsMultipliers, Tuning, Phase shifters,
and Modulators
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 solid-state devices and their applications.
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Microwave Diodes
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
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
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Non-Linear Characteristics of p-n and Schottky diodes
V
I
IsVB
Non-linear I-V Characteristics of a diode
V
C
VB
Non-linear I-V Characteristics of a diode
Vbi
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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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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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(2) Multiplier and harmonic generation
Feasible alternative for the generation of high frequency signal
Zo
Zo
LPF and matching
BPF and matching
Cj(V)
Rs Varactor
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(3) Parametric Amplifiers:
Provide very low noise amplification
Pump signal
Input
Output
CirculatorCombiner and
Varactor
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p-i-n Diodes
Similar to the pn diode with smaller junction capacitance
Very useful for a diode used a microwave switch
P+ n+i
Weakly doped f.b. r.b.
Rj(V) Cj(V)
Rp
Ls
Rs
P-i-n structure
Equivalent circuit of p-i-n
Parasitics Ls~ 0.1 nH Cp~ 0.3 pF Rs~ 0.3
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Switches Applications
t
Switch
Bias
Source
Output
(1) Modulators in communication systems
.
.
.
Wideband switch
(2) Switch in wide band system
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(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
(3) To protect receiver from the transmitter (such as in radar system)
Rx
Tx
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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
A = 20log (1 + Zo/2Rs)
Reflective type
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p-i-n diode
p-i-n diode
Zo
Zo
Bias
Input
Output
3-dB quadrature coupler
Matched attenuator
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p-i-n Phase Shifters
3-dB, 90o
Hybrid
B2B1 B2B1
DiodeDiode
/4
Zo
Hybrid coupler phase shifter. Uses the fewest diodes. Any phase shift increment can be obtained with proper design of the terminating circuit.
The loaded line phase shifter
Input Output
Input Output
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Switched line phase shifter
L1
L2
Bias
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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Limiter p-i-n Diodes
Used for protection applications
3 dB Coupler
3 dB Coupler
Limiter
TransmitterReceiver
Limiter
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Pin
Pout
Insertion loss
Maximum Isolation
Pin Pout
p-i-n diode
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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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
Controlled limitations. This method gives lower losses, better isolation, but require a delicate control circuit. Any loose of control affect receiver protection
Pin Pout
Schottky diodep-i-n diode
Pin Pout
Control pulse
p-i-n diode
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Gunn DiodesSingle 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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Varactor Tuned Gunn Oscillators Circuits
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IMPATT Devices and Circuits
IMPact Ionization Transit Time
IMPATT devices can be used for oscillator and amplifier applicationsThey 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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Some IMPATT Circuits