20-07-2013, 02:47 PM
Measurement of frequency, guide wavelength, power, VSWR, attenuation in a microwave test bench
AIM:-
To plot the V-I characteristics of Gunn diode and to measure the Guide wavelength and operating
frequency and VSWR of a Gunn Diode.
COMPONENTS REQUIRED
Gunn Power supply
Gunn oscillator
Isolator
Pin modulator
Variable Attenuator
frequency meter
Slotted probe carriage
Tunable detector
VSWR meter
CRO
THEORY:-
In 1963, J.B. Gunn discovered the bulk TRANFERRED ELECTRON EFFECT in Gallium Arsenide which
is a semiconductor material.Si & Ge are called INDIRECT GAP semiconductors because the bottom of the
conduction band does not lie directly above the top of the valence band.In GaAs the conduction band lies
directly above the top of the valence band. The lowest energy conduction band in ga as is called as
PRIMARY VALLEY. Gunn while measuring the current density J as a junction of electric field E in a
Gallium Arsenide n-type specimen discoveredthat after a threshold field Eth is reached, the current in the
specimen suddenly becomes oscillatory w.r.t. time and these oscillations are in the microwave frequency
range. This effect is called “GUNN EFFECT”.
PROCEDURE:-
(a) TO FIND V-I CHARACTERISTICS
Set the Gunn diode oscillator micrometer screw for a suitable frequency to get a square wave.
Bias the gunn diode and Pin modulator from Gunn power supply.
Select the internal modulation in Gunn power supply.
Set modulating frequency and pin bias knobs to approximately half.
Change Gunn Bias in steps of 0.5v from 0to 7v and note down the corresponding current.
Plot V-I characteristic graph and find Threshold Voltage from the graph.