Amrutvahini College of Engineering, Sangamner

Department of Electronics and Telecommunication Engineering

Experiment No.:
Date of Performance:

AIM: To measure VI characteristics of Gunn Diode and study of PIN modulator.

APPARATUS:
1. Gunn Diode Power Supply
2. Gunn Diode
3. Pin Modulator
4. Isolator
5. Variable Attenuator
6. Frequency meter
7. Power Detector
THEORY:

Gunn diode oscillator normally consist of a resonant cavity, an arrangement for coupling
diode to the cavity a circuit for biasing the diode and a mechanism to couple the RF power from
cavity to external circuit load. A co-axial cavity or a rectangular wave guide cavity is commonly
used. The circuit using co-axial cavity has the Gunn diode at one end at one end of cavity along
with the central conductor of the co-axial line. The output is taken using a inductively or
capacitively coupled probe. The length of the cavity determines the frequency of oscillation. The
location of the coupling loop or probe within the resonator determines the load impedance
presented to the Gunn diode. Heat sink conducts away the heat due to power dissipation of the
device.
 The Gunn oscillator is based on negative differential conductivity effect in bulk
semiconductors which has two conduction band minima separated by an energy gap .A
disturbance at cathode gives rise to high field region which travel’s towards the anode .When this
high field domains reaches the anode, it disappears and another domain is formed at the cathode
and starts moving anode and so on. The time required for domain to travel from cathode to anode
gives oscillation frequency. Above some critical voltage current becomes function of time .It was
discovered by scientist J. B. Gunn. It uses bulk property of semiconductor only. So it must be
associated with electron rather than holes .The voltage applied is proportional to the sample
length and hence electric field is expressed in v/m. When DC voltage is applied across the side
of Gunn diode, cadmium ferrite in negative resistance property across at particular voltage range
value .As the applied potential increases i.e. greater than electric field higher electron present,
hence higher current across the line in short time so acceleration occur in microwave range.
Actually Frequency of oscillation in determined the time so acceleration occurs in microwave
range. Actually frequency of oscillation is determined the time that the bunches of electron form
and arrive at the end

OPERATING MODES OF GUNN DIODE

1. Gunn Oscillation mode.
2. Stable amplifier mode.
3. Limited Space charge accumulation mode(LSA).
Modes depend upon material parameters &operating conditions.
Gunn mode is divided into
a) Transit time mode
b) Quenched mode
c) Delayed mode

APPLICATIONS
1. As low and medium power oscillator in microwave receivers and instruments.
2. In parametric amplifiers as pump source
3. Have advantage over IMPATT diodes having much lower noise and hence used in pump
oscillator.
4. The high power Gunn oscillators (250-2000mw) are used as power output oscillators and
are frequency modulated in a wide variety of low power transmitter applications
These includes police radar, CW Doppler radar, burglar alarms and aircraft rate-of-climb
Indicators.

GUNN OSCILLATOR
 Figure 1 : Gunn oscillator using coaxial cavity

It normally consists of a resonant cavity, an arrangement for coupling diode to the cavity, a
circuit for biasing a diode and a mechanism to couple RF power the diode and a mechanism to
couple RF power from the cavity to the external circuit / load. A co-axial or rectangular cavity is
commonly used. Gunn diode is mounted at one end of the cavity and is in continuation with the
central conductor of the coaxial line. The output is taken using a inductively or capacitively
coupled probe. The length of the cavity determines the frequency of oscillation.
It can be easily fabricated but low Q of coaxial resonator & oscillations at harmonics of the
desired frequency are the disadvantages.
 Figure 2 : Gunn oscillator using waveguide cavity

It is more popular. It consists of a waveguide section separated from the output waveguide by an
iris. Gunn diode is mounted in a post across the narrow dimension in the centre of the
waveguide. The diode post acts as a large inductive suceptance and iris is also inductive. Hence
the resonant freq is lower than that for which the length l is dielectric tuning rod is used to adjust
the freq mechanically.

Figure 3 : Gunn Diode

PIN DIODE

PIN diode acts as a low frequency rectifier that could rectify more power than an ordinary p-n
junction diode. Up to about 100MHz operation is similar to an ordinary p-n junction diode acts
like a variable resistance. Under zero and reverse bias, the diode has a very high impedance at
microwave frequency and very low impedance for small forward currents i.e. with bias variation
on PIN diode, its resistance changes nearly 5-10KΩ under negative bias to 5 Ω under positive
bias i.e. it behaves as a switch. When diode is mounted across a 50 Ω coaxial line, it will not load
the line under back bias. It reflects most of the power under forward bias due to mismatch &
loading. To improve power handling capability several diodes can be used in parallel. The shunt
depletion capacitance limits the upper frequency operation.

Resistance variation with bias

Figure 4: (a) Construction of PIN diode (b) Equivalent Circuit of PIN diode (c) Resistance
variation with bias
 OPERATION OF PIN DIODE:
Zero Bias: The diffusion of the holes &electrons across the junction causes space charge
(density) region of thickness inversely proportional to impurity concentration. An ideal I
layer has no depletion region i.e. p layer & n layer has fixed negative and positive charge
resp.

Reverse Bias: The space charge regions in the p & n layers will become thicker. The reverse
resistance will be very high & almost constant.

Forward Bias: Carriers will be injected into the I layer & p & n space charge regions will
become thinner. Due to this carrier concentration in the I layer is raised above equilibrium
levels & resistivity drops as FB increased and low resistance is offered.

Figure 5: Operation of PIN diode

SPECIFICATIONS:
1. Diodes are available with resistive cutoff of 700GHz.
2. Operating frequency do not exceed 1 tenth of above cutoff.
3. Individual diodes may handle up to 200KW peak.
4. Several diodes may be combined to handle as much as 1MW peak.
5. Actually switching times vary approximately 40ns for high-power limiters to as little as
1ns at lower powers.
APPLICATIONS:
1. PIN diode as an Amplitude Modulator.
2. PIN diode as a phase shifter.
3. PIN diode as a limiter.
4. PIN diode as switch.

BLOCK DIAGRAM:

Gunn Power
Supply

Gunn PIN Isolator Frequency Variable Detector

Oscillator Modulator Meter Attenuator Mount

CRO or
VSWR
Meter
Figure: Block diagram of Gunn diode microwave bench

PROCEDURE:

1. Set the components and equipments as shown in Figure.

2. Initially set the variable attenuator for minimum attenuation.
3. Keep the control knobs of Gunn power supply as below
Meter switch – “OFF”
Gunn bias knob – Fully anti clock wise
PIN bias knob – Fully anti clock wise
PIN mode frequency – any position
4. Set the micrometer of Gunn oscillator for required frequency of operation.
5. Switch “ON” the Gunn power supply.
6. Measure the Gunn diode current to corresponding to the various Gunn bias voltage through
the digital panel meter and meter switch. Do not exceed the bias voltage above 10 volts.
7. Plot the voltage and current readings on the graph.
8. Measure the threshold voltage which corresponding to maximum current.
Note: Do not keep Gunn bias knob position at threshold position for more than 10-15 sec.
readings should be obtained as fast as possible. Otherwise due to excessive heating Gunn diode
may burn.

OBSERVATION TABLE:

Table 1

Sr.No. Gunn Bias Voltage(V) Gunn Diode Current(mA)

EXPECTED GRAPH:

CONCLUSION:

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REFERENCES:
Questions: