08-05-2013, 04:09 PM
Microwave Devices
[attachment=53613]
Introduction
Microwaves have frequencies > 1 GHz approx.
Stray reactances are more important as frequency increases
Transmission line techniques must be applied to short conductors like circuit board traces
Device capacitance and transit time are important
Cable losses increase: waveguides often used instead
Waveguides
Pipe through which waves propagate
Can have various cross sections
Rectangular
Circular
Elliptical
CaAn be rigid or flexible
Waveguides have very low loss
ModAes
Waves can propagate in various ways
Time taken to move down the guide varies with the mode
Each mode has a cutoff frequency below which it won’t propagate
Mode with lowest cutoff frequency is dominant mode
Rectangular Waveguides
Dominant mode is TE10
1 half cycle along long dimension (a)
No half cycles along short dimension (b)
Cutoff for a = c/2
Modes with next higher cutoff frequency are TE01 and TE20
Both have cutoff frequency twice that for TE10
Passive Compenents
Bends
Called E-plane or H-Plane bends depending on the direction of bending
Tees
Also have E and H-plane varieties
Hybrid or magic tee combines both and can be used for isolation
Circulator and Isolator
Both use the unique properties of ferrites in a magnetic field
Isolator passes signals in one direction, attenuates in the other
Circulator passes input from each port to the next around the circle, not to any other port
Microwave Tubes
Used for high power/high frequency combination
Tubes generate and amplify high levels of microwave power more cheaply than solid state devices
Conventional tubes can be modified for low capacitance but specialized microwave tubes are also used
[attachment=53613]
Introduction
Microwaves have frequencies > 1 GHz approx.
Stray reactances are more important as frequency increases
Transmission line techniques must be applied to short conductors like circuit board traces
Device capacitance and transit time are important
Cable losses increase: waveguides often used instead
Waveguides
Pipe through which waves propagate
Can have various cross sections
Rectangular
Circular
Elliptical
CaAn be rigid or flexible
Waveguides have very low loss
ModAes
Waves can propagate in various ways
Time taken to move down the guide varies with the mode
Each mode has a cutoff frequency below which it won’t propagate
Mode with lowest cutoff frequency is dominant mode
Rectangular Waveguides
Dominant mode is TE10
1 half cycle along long dimension (a)
No half cycles along short dimension (b)
Cutoff for a = c/2
Modes with next higher cutoff frequency are TE01 and TE20
Both have cutoff frequency twice that for TE10
Passive Compenents
Bends
Called E-plane or H-Plane bends depending on the direction of bending
Tees
Also have E and H-plane varieties
Hybrid or magic tee combines both and can be used for isolation
Circulator and Isolator
Both use the unique properties of ferrites in a magnetic field
Isolator passes signals in one direction, attenuates in the other
Circulator passes input from each port to the next around the circle, not to any other port
Microwave Tubes
Used for high power/high frequency combination
Tubes generate and amplify high levels of microwave power more cheaply than solid state devices
Conventional tubes can be modified for low capacitance but specialized microwave tubes are also used