Download ElecEng 4/6FJ4 LABORATORY MODULE #2 PIN Diodes I

ElecEng 4/6FJ4 LABORATORY
MODULE #2
PIN Diodes
I. Objectives
The purpose of this module is to help the students get familiar with the PIN diode, one of the
most common low-power switching devices at microwave and millimeter-wave frequencies.
The lectures of ElecEng 4FJ4 do not cover the general topic of microwave diodes. Thus, this
laboratory module is complementary to the lectures in both its content and hands-on experience.
The students must prepare for the 3-hour laboratory beforehand by reading the DISCUSION part
of the PIN Diode LabVolt (https://www.labvolt.com/) exercise, which constitutes MODULE #2.
If the students come to the lab unprepared, it is likely that they will not be able to complete the
exercises on time.
II. Preparing the Lab Report
The student is expected to bring along a printout of this guide. The Lab Report consists of simply
filling in the required information. Note that there are REVIEW QUESTIONS at the end of the
LabVolt exercise. The student must provide answers to these questions, which are closely related
to the DISCUSSION and the PROCEDURE part of the exercise.
The student is expected to hand in the lab report to the teaching assistant (TA) at the end of the
lab session. Take home work should not be necessary.
PLEASE WRITE DOWN YOUR NAME AND STUDENT ID ON THE TITLE PAGE!
III. Grading the Lab Report
•
•
•
•
Total points: 100
Penalty for a missing item in the PROCEDURE part: −5 points
Penalty for a missing plot or table in the PROCEDURE part: −10 points
Penalty for a missing or wrong answer in the REVIEW QUESTIONS part: −10 points
IV. Feedback
We value your opinion. Direct your recommendations, opinions, and criticism to the Instructor
(Prof. Nikolova) at [email protected].
Nikolova
ElecEng 4FJ4 Labs
2014
ElecEng 4/6FJ4 LABORATORY
MODULE #2
PIN DIODES
Student Name:
Student ID:
Student Signature:
Date:
TA Name:
TA Signature:
Date:
REPORT GRADE:
(provided no later than one week after report submission)
Nikolova
ElecEng 4FJ4 Labs
2014
Exercise
17
PIN Diodes
EXERCISE OBJECTIVES
When you have completed this exercise, you will be familiar with the construction
and operation of PIN diodes. You will know how to characterize the Lab-Volt
PIN diode. You will be able to demonstrate that a PIN diode can be used to switch
microwave signals. Finally, you will be able to name various applications for
PIN diodes.
DISCUSSION
Introduction
A PIN diode is a semiconductor device that acts like a variable resistor at microwave
frequencies. The resistance of the diode is controlled by varying the DC current used
to forward bias the diode.
The word PIN is an acronym indicating the three types of materials that form the
diode (see Figure 17-1):
•
A P layer of highly doped P-type material;
•
An I (intrinsic) layer of undoped, or slightly doped semiconductor material
•
An N layer of highly doped N-type material
At DC or low frequencies, a PIN diode acts like an ordinary PN diode: it has a non
linear current-versus-voltage response when forward-biased; and it blocks current
flow when reverse-biased.
Figure 17-1. Cross-sectional view of a PIN diode.
17-1
PIN Diodes
Operation of a PIN Diode When Forward-Biased
A PIN diode is said to be forward-biased when the polarity of its P layer is positive
relative to the polarity of the N layer. In that case, a DC bias current can flow through
the diode.
When an RF signal is applied to the diode, holes and electrons from the P and
N layers move towards the I layer. There, the holes and electrons do not recombine
immediately. This occurs because the I layer increases the number of minority
carriers, thereby increasing the minority carrier lifetime.
If the period of the RF signal is shorter than the minority carrier lifetime, the diode will
lack sufficient time to pass from the "on" (conducting) state to the "off" (blocked)
state. Therefore, the diode will allow the RF signal to pass through during the entire
signal's period, offering a certain resistance to the passing of this signal.
At RF frequencies, therefore, a PIN diode does not behave like an ordinary
PN diode, but like a linear variable resistor, even in the presence of strong
RF signals.
The frequency at which a PIN diode stops acting like a PN diode and starts acting
like a linear variable resistor is called the transition frequency. The transition
frequency is a function of the thickness of the I layer.
The advantage of a PIN diode at or above the transition frequency becomes a
disadvantage at lower frequencies: thus, the PIN diode cannot be used as a rectifier
at lower frequencies.
Typical Resistance-Versus-Bias Current Response of a Forward-Biased
PIN Diode
Figure 17-2 shows the typical resistance-versus-bias current response of a forwardbiased PIN diode at microwave frequencies. The higher the bias current is, the lower
the resistance of the I layer to the passing of microwave signals will be.
As the bias current increases, the resistance of the diode decreases rapidly. Beyond
a certain bias current, the diode resistance becomes quite constant: the PIN diode
can be considered as fully "on" (conducting).
Note that a PIN diode will not conduct if the bias current is below some minimum
level. Below this level, the resistance of the I layer remains very high.
17-2
PIN Diodes
Figure 17-2. Typical resistance-versus-forward bias current response of a PIN diode at microwave
frequencies.
Equivalent RF Circuit of a Forward-Biased PIN Diode
The equivalent RF circuit of a forward-biased PIN diode is dependent upon the way
the diode is constructed (structure and packaging), and the quality of the coupling
between the diode and the waveguide (if it is used in a waveguide).
Figure 17-3 shows the construction of the Lab-Volt PIN Diode, Model 9358, and its
equivalent RF circuit with and without shunt compensation.
•
As Figure 17-3 (a) shows, the Lab-Volt PIN Diode is coupled to the waveguide
through metal posts.
•
The metal posts act like an inductor in series with the diode. This forms a series
RL circuit across the waveguide, as Figure 17-3 (b) shows. jXL is the sum of the
inductance reactances of the metal posts and diode stud.
•
The series RL circuit can be converted into the equivalent parallel circuit shown
in Figure 17-3 (c).
In this condition, XL is not high enough, relative to the characteristic impedance
(Z0) of the waveguide, to prevent significant absorption and reflection losses.
•
To reduce these losses to a minimum, a capacitive metal disk is added to the
diode assembly [see Figure 17-3 (a)].
The addition of the disk connects a capacitive reactance, -jXC, in parallel (shunt)
with the circuit of Figure 17-3 (c), giving the equivalent circuit of Figure 17-3 (d).
•
The capacitive reactance is tuned so that, at the operating (resonant) frequency,
jXC = jX'L. The tuning is performed with the resistance of the PIN diode at the
minimum (forward bias current set to the maximum).
At resonance, X'L = XC, so that the equivalent circuit becomes a shunt impedance,
R', across the waveguide, as Figure 17-3 (e) shows. Since R' is much greater
than Z0, the attenuation provided by the diode is negligible below a minimum
forward bias current.
17-3
PIN Diodes
Note: The explanations and circuits provided in this section apply
specifically to the Lab-Volt PIN Diode, Model 9538. Since PIN diodes
can be mounted in several different ways (e.g. parallel mounting, series
mounting, etc.), their equivalent RF circuits will change accordingly.
Figure 17-3. Equivalent RF circuit of the Lab-Volt PIN Diode (forward-biased) with and without
shunt compensation.
17-4
PIN Diodes
Operation of a PIN Diode When Reverse-Biased
A PIN diode is said to be reverse-biased when the polarity of its P layer is negative
relative to the polarity of the N layer. In this condition, the diode acts essentially like
a capacitance, providing a high resistance independently of the reverse voltage
across it.
Figure 17-4 (a) shows the equivalent RF circuit of the Lab-Volt PIN Diode in the
reverse-biased condition. R is the reverse parallel resistance of the diode. This
resistance is much greater than the diode package capacitance, XC.
The circuit of Figure 17-4 (a) can be converted into the equivalent series circuit of
Figure 17-4 (b). At the operating (resonant) frequency, -jX'C = jXL, so that the
equivalent circuit becomes a shunt impedance, R', across the waveguide, as Figure
17-4 (b) shows. Since R' is much lower than Z0, the attenuation provided by the
diode is very high due to strong reflections produced at the diode.
Figure 17-4. Equivalent RF circuit of the Lab-Volt PIN Diode in the reverse-biased condition.
Attenuation of a Reverse-Biased PIN Diode as a Function of Frequency
The high attenuation provided by a PIN diode in the reverse-biased condition varies
with frequency. This occurs because the structure and packaging of the diode cause
its equivalent circuit to have a frequency-sensitive response. Figure 17-5 shows an
example of an attenuation-versus-frequency curve of a PIN diode in the reversebiased condition.
17-5
PIN Diodes
Figure 17-5. Example of an attenuation-versus-frequency curve of a PIN diode in the reversebiased condition.
Applications of PIN Diodes
PIN diodes are used in numerous microwave and wireless applications, including
those listed below.
17-6
•
They are used for microwave switching.
•
They are used as variable attenuators. PIN diode attenuators can be designed
as matched or reflective.
•
They are used for amplitude modulation and leveling of microwave signals.
•
They are used as microwave phase shifters.
PIN Diodes
Procedure Summary
In the first part of this exercise, you will characterize the Lab-Volt PIN Diode by
plotting the following response curves at 10.525 GHz: the bias current-versus-bias
voltage curve, and the attenuation-versus-bias current curve.
In the second part of the exercise, you will use the PIN Diode as a microwave switch.
To do this, you will modulate the amplitude of the bias voltage applied to the diode
with a 1-kHz square-wave signal.
In the third part of the exercise (optional), an additional curve is plotted to complete
characterization of the PIN Diode: the attenuation-versus-frequency curve of the
diode.
Note: The third part of the exercise section requires the use of the Lab-Volt
Voltage Controlled RF Oscillator, Model 9511, available as an option, and a
power meter able to provide stable power readings to -45 dBm or lower over
9.6 - 10.6 GHz with the required probe adapter.
Information on how to use the LVDAM-MW software to control or monitor the
PIN Diode and the Frequency Meter of the optional Voltage Controlled RF
Oscillator, Model 9511, are provided in the Parameter Settings section of the
Help file in this software.
EQUIPMENT REQUIRED
Refer to the Equipment Utilization Chart, in Appendix F of this manual, to obtain the
list of equipment required to perform this exercise.
PROCEDURE
Nikolova: Please place a "tick" mark in the box of each step of the Procedure once this step is completed.
A missing "tick" mark indicates missed Procedure item, which can result in a penalty of -5 or -10 points.
Characterization of the PIN Diode
G
1. Make sure that all power switches are in the O (off) position. Set up the
modules and assemble the microwave components as shown in
Figure 17-6.
Note: Before connecting the Thermistor Mount, unscrew the matching
screws so that they do not penetrate into the waveguide; the screws do
not need to be removed from the posts.
–
Connect the Thermistor Mount to the analog input of the Data
Acquisition Interface (DAI) dedicated to the LVDAM-MW Power Meter:
MULTI-FUNCTION INPUT 4.
–
Connect the PIN Diode to the output of the Data Acquisition
Interface dedicated to the control of the PIN Diode: the PIN DIODE
BIAS AND MODULATION OUTPUT.
17-7
PIN Diodes
CAUTION!
The PIN DIODE BIAS AND MODULATION OUTPUT of the Data
Acquisition Interface is intended to be connected to the PIN
Diode input only. Connection of this output to other modules
or components might result in damage to the equipment.
17-8
PIN Diodes
Figure 17-6. Computer and module arrangement (showing electrical connections to microwave
components) used to characterize the PIN Diode at 10.525 GHz.
17-9
PIN Diodes
G
2. Make the following settings:
On the Gunn Oscillator Power Supply:
VOLTAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . MIN.
MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DC
METER SCALE . . . . . . . . . . . . . . . . . . . . . . . 10 V
On the Data Acquisition Interface:
BIAS knob . . . . . . . . . . . . . . . turned fully clockwise
G
3. Turn on the Gunn Oscillator Power Supply and the Data Acquisition
Interface (DAI) by setting their POWER switch to the "I" (ON) position.
Set the Gunn Oscillator supply voltage to 10 V. Wait for about 5 minutes to
allow the modules to warm up.
G
4. On the host computer, start the LVDAM-MW software. In the Application
Selection window, make sure the Work in stand-alone box is unchecked,
and click OK.
In the Settings panel of LVDAM-MW, make the following settings:
Gunn Oscillator/VCO Power . . . . . . . . . . . . . . . ON
Function Input 4 . . . . . . . . . . . . . . . . . Power Meter
Gain Input 4 . . . . . . . . . . . . . . . . . . . . . . . . . . 0 dB
PIN Diode Modulation . . . . . . . . . . . . . . OFF (see note)
Note: The field PIN Diode Modulation of the LVDAM-MW Settings
panel must be set to OFF for the user to be able to adjust the bias
voltage at the BIAS AND MODULATION OUTPUT of the Data
Acquisition Interface, using the BIAS knob of this interface.
17-10
G
5. In LVDAM-MW, start the Power Meter and set it to display mW reading.
Then, perform zeroing of the Power Meter.
G
6. On the Thermistor Mount, loosen the knurled lock-nut that holds the
moveable short circuit into place. Adjust the short circuit to the position
nearest the waveguide which gives a maximum reading on the Power
Meter. Then, adjust each matching screw of the Thermistor Mount to
maximize the power reading. Fine tune if necessary. Finally, lock the
moveable short circuit into position.
G
7. In LVDAM-MW, select the Data Table function, which will bring up the Data
Table. In this Table, enter the column titles and figures already recorded in
Table 17-1, and then save your Data Table. (For the cells containing the text
"MAX. =" or "MIN. = ", do not enter anything in the corresponding cells of the
Data Table of LVDAM-MW. Leave these cells empty for now).
PIN Diodes
Nikolova: Hit "refresh" if Current Meter shows 0
(value should be between 5 and 6 mA). Set mode to
"continuous refresh".
Note: No values are specified for the "MAX. =" and "MIN. = " values of
Table 17-1, since these values will be determined experimentally.
Nikolova: Do not forget to zero
the Power Meter every time you
change scale (the Gain on Input
4). With Gain of 40 dB (power
between 0.01 mW and 0.1 mW),
zero before each measurement
and read out value within couple
of seconds.
VOLTAGE
ACROSS THE
DIODE ΔV (V)
MAX. = (note)
PIN DIODE
BIAS
CURRENT (mA)
MAX. =
PIN DIODE
BIAS VOLTAGE
(V)
POWER
METER
READING (mW)
ATTENUATION
(dB)
MAX. =
4.00
3.50
3.00
2.50
2.25
2.20
2.00
1.90
1.80
1.70
1.60
1.50
1.40
1.30
1.25
1.20
1.15
1.10
1.0
0.9
0.8
0.7
0.6
0.5
0.00
-0.25
MIN. = (note)
Table 17-1. Determining the characteristics of the PIN Diode at 10.525 GHz.
Nikolova: You can fill in this table by hand directly in your Report printout. Using the
software to generate the Table is optional.
17-11
PIN Diodes
G
8. In LVDAM-MW, leave the Data Table and the Power Meter open.
Open the PIN Diode Bias Meter and perform zeroing of this meter.
The PIN Diode Bias Meter now indicates the DC voltage currently applied
across the PIN Diode, and the DC current flowing across the diode.
Since the PIN DIODE BIAS knob of the Data Acquisition Interface is set to
the fully clockwise (CW) position, the maximum voltage is currently applied
across the PIN Diode, so that the current flowing through this diode is
maximum. Therefore, the resistance of the diode to the passing of the Gunn
Oscillator's output signal is minimum, causing the Power Meter to read the
maximum power.
In the first row of the Data Table, record
a. the maximum voltage across the diode, as indicated by the PIN Diode
Bias Meter, under the column "VOLTAGE ACROSS THE DIODE, ΔV".
b. the maximum PIN Diode bias current, as indicated by the PIN Diode
Bias Meter, under the column "PIN DIODE BIAS CURRENT".
c. the power absorbed by the thermistor, as indicated by the Power Meter,
under the column "POWER METER READING".
G
9. Fill in the columns "PIN DIODE BIAS CURRENT" and "POWER METER
READING" of the Data Table:
Adjust the voltage across the PIN Diode (by observing the PIN Diode Bias
Meter) to each of the ΔV settings listed in the leftmost column of the table,
using the PIN DIODE BIAS knob of the Data Acquisition Interface(see note).
For each ΔV setting,
a. record the current indicated by the PIN Diode Bias Meter in the column
"PIN DIODE BIAS CURRENT". (At low ΔV settings, the meter can
indicate a very small, negative current. In that case, record 0.00 mA in
the table).
b. record the power indicated by the Power Meter in the column "POWER
METER READING".
Note: When the Power Meter reading comes close to the lower
end of the measuring scale (the indicator bar turns from green to
blue), select the next lower scale by changing Gain Input 4, then
perform zeroing of the Power Meter and continue to fill in the
table.
For the ΔV settings below 0.7 V approximately, the PIN Diode
bias current will be approximately null, and the Power Meter
reading will stay at some low, minimum level (Gain Input 4 will be
set to 40 dB). Perform zeroing of the Power Meter before each
power measurement on this scale.
17-12
PIN Diodes
c. Save your Data Table.
Note: For the last cell of the column "VOLTAGE ACROSS THE
DIODE, ΔV", enter the minimum voltage indicated by the PIN
Diode Bias Meter when the PIN DIODE BIAS knob is turned fully
counterclockwise.
G 10. Fill in the column "PIN DIODE BIAS VOLTAGE" of the Data Table. To do
this, use the data recorded in the two leftmost columns of this table, and the
formula below. Save your Data Table.
Note: The formula permits calculation of the bias voltage actually
applied to the PIN Diode, since it accounts for the voltage drop
caused by a 620-Ω resistor internally connected between the
BNC input of the PIN Diode and the diode itself.
G 11. Using the data recorded under "POWER METER READING" and the
formula below, calculate the attenuation provided by the PIN Diode for each
bias voltage setting. Record your results (absolute value) in the column
"ATTENUATION" of the Data Table.
Save your Data Table.
Maximum Power =
Power Meter reading when the maximum voltage
is present across the diode (recorded in the first
row of the table).
G 12. In LVDAM-MW, select the Graph function of the Data Table and plot the
curve of the bias current-versus-bias voltage curve of the PIN Diode at
10.525 GHz.
Make the following settings in the Graph window:
Scale
X-Axis Scale . . . . . . . . . . . . . . . . . . . . . Manual
X Interval . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2
X Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
X Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -1
Your graph should resemble that shown in Figure 17-7.
Nikolova: You can plot your curve by entering the data from Table 17-1 directly onto Figure 17-7
in your Report printout. Using the software to generate your plot is optional.
17-13
PIN Diodes
Based on the obtained curve, describe how the bias current through the PIN
Diode varies as a function of the bias voltage.
Figure 17-7. Bias current-versus-bias voltage curve of the PIN Diode at 10.525 GHz.
17-14
PIN Diodes
G 13. In the curve you obtained, is there a certain "dead zone" within which
increasing the DC voltage (positive or negative) results in no corresponding
increase in the bias current? Explain.
G 14. Using the Graph function of the Data Table, plot the attenuation-versus-bias
current curve of the PIN Diode at 10.525 GHz.
Make the following settings in the Graph window:
Scale
X-Axis Scale . . . . . . . . . . . . . . . . . . . . . Manual
X Interval . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2
X Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
X Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0
Your graph should resemble that shown in Figure 17-8.
Nikolova: You can plot your curve by entering the data from Table 17-1 directly onto Figure 17-8
in your Report printout. Using the software to generate your plot is optional.
According to the curve obtained, describe how the attenuation provided by
the PIN Diode varies as a function of the bias current and explain why this
occurs.
G 15. Close the Graph and the Data Table. Close the Power Meter.
PIN Diode Used as a Microwave Switch
G 16. In the Settings panel of LVDAM-MW, set the field Gunn Oscillator/VCO
Power to OFF.
17-15
PIN Diodes
Figure 17-8. Attenuation-versus-bias current curve of the PIN Diode at 10.525 GHz.
Set up the modules and assemble the microwave components as shown in
Figure 17-9.
17-16
–
Connect the Crystal Detector to MULTI-FUNCTION INPUT 1 of the
Data Acquisition Interface.
–
Connect the PIN Diode to the output of the Data Acquisition
Interface dedicated to the control of the PIN Diode: the PIN DIODE
BIAS AND MODULATION OUTPUT.
PIN Diodes
Figure 17-9. Computer and module arrangement (showing electrical connections to microwave
components) used to perform microwave switching with a PIN Diode.
17-17
PIN Diodes
G 17. Make the following settings:
On the Variable Attenuator:
Blade Position . . . . . . . . . . . . . . . . . . . . . . . . 10 mm
In the Settings panel of LVDAM-MW:
Gunn Oscillator/VCO Power . . . . . . . . . . . . . . . ON
Filter Input 1 . . . . . . . . . . . . . . . . . . . . . . . > 30 kHz
Function Input 1 . . . . . . . Voltage Probe (Unipolar)
Gain Input 1 . . . . . . . . . . . . . . . . . . . . . . . . . 20 dB
PIN Diode Modulation . . . . . . . . . . . . . . . ON (see note)
Note: When the field PIN Diode Modulation is set to ON, the
amplitude of the bias voltage present at the BIAS AND
MODULATION OUTPUT of the Data Acquisition Interface is
modulated with a 1-kHz square-wave signal internally generated
by the interface.
G 18. Wait for about 5 minutes to allow the modules to warm up.
G 19. In LVDAM-MW, start the Oscilloscope and set it as follows:
Channel 1 (X)
Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 mV/DIV
Time base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 μs/DIV
Trigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ch1 Software
Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous Refresh
Nikolova: This happens when the
blade position is at approx. 2.5 mm.
G 20. Decrease the attenuation provided by the Variable Attenuator until the
Crystal Detector's output voltage appears on the Oscilloscope screen, and
set the attenuator so that the peak amplitude of this voltage is approximately
100 mV, as Figure 17-10 shows.
The Crystal Detector's output voltage is very similar in shape to the 1-kHz
square-wave signal produced by the Data Acquisition Interface to modulate
the bias voltage applied to the PIN Diode.
This occurs because
–
17-18
when the 1-kHz square-wave modulating signal is at logic level 1, a high
forward bias current flows through the PIN Diode. In this condition, the
impedance of the PIN Diode's equivalent RF circuit (that is shunt
resistor R') is much higher than the characteristic impedance of the
waveguide, Z0, causing the reflections produced at the diode, and
therefore the attenuation of the diode, to be negligible. The Crystal
Detector's output voltage, therefore, is at some maximum level.
PIN Diodes
–
when the 1-kHz square-wave modulating signal is at logic level 0, no
current flows through the PIN Diode. In this condition, the impedance of
the PIN Diode's equivalent circuit (that is shunt resistor R') is much
lower than Z0, causing the attenuation of the diode to be very high due
to reflections produced at the diode.
Figure 17-10. Crystal Detector's output voltage.
Close the Oscilloscope.
END OF PROCEDURE
17-19
PIN Diodes
DO NOT PERFORM OPTIONAL SECTION - VCO NOT AVAILABLE
Attenuation-Versus-Frequency Curve of the PIN Diode (Optional Section)
Note: This section requires the use of the Lab-Volt Voltage
Controlled RF Oscillator, Model 9511, offered as an option, and
a power meter able to provide stable power readings down to
!45 dBm or lower, with the required probe adapter.
G 21. In
the Settings panel of
Oscillator/VCO Power to OFF.
LVDAM-MW,
set
the
field
Gunn
Set up the modules and assemble the microwave components as shown in
Figure 17-11.
–
Connect the PIN Diode to the output of the Data Acquisition
Interface dedicated to the control of this diode: the PIN DIODE BIAS
AND MODULATION OUTPUT.
–
Connect the supply input of the Voltage-Controlled RF Oscillator to the
RF OSCILLATOR SUPPLY AND CONTROL OUTPUT of the Data
Acquisition Interface. Connect the prescaler output (BNC output) of the
Voltage-Controlled RF Oscillator to the RF OSCILLATOR FREQUENCY
MONITORING INPUT of this interface.
–
In the setup of Figure 17-11, an Anritsu™ Model 2437A power meter is
used. It is connected to the microwave circuit via a Lab-Volt waveguideto-coaxial transition, Model 28631.
Note: If you want to use another power meter model, this meter
must be able to provide stable power readings to !45 dBm or
lower over a frequency range of 9.6 - 10.6 GHz.
17-20
PIN Diodes
Figure 17-11. Computer and module arrangement (showing electrical connections to microwave
components) used to plot the attenuation-versus-frequency curve of the PIN Diode.
17-21
PIN Diodes
G 22. In LVDAM-MW, start the Frequency Meter.
In the Settings panel of LVDAM-MW, make the following settings:
PIN Diode Modulation . . . . . . . . . . . . . . . . . . . . OFF
RF Oscillator (VCO)
Frequency Control . . . . . . . . . . . . . . . MANUAL
Amplitude Modulation . . . . . . . . . . . . . . . . . OFF
Control Voltage (0-10 V) . . . . . . . . . . . . . . . . . 0
Wait for about 5 minutes to allow the modules to warm up.
G 23. The Frequency Meter displays the current operating frequency of the
Voltage-Controlled RF Oscillator.
Since the field Frequency Control of the RF Oscillator (VCO) section is set
to Manual, the frequency of the RF oscillator can be varied, over the
nominal range of 9.6 to 10.6 GHz, by changing the value of the field Control
Voltage (0-10 V).
Adjust this Control Voltage until the frequency of the RF Oscillator's output
signal indicated on the Frequency Meter is 9.6 GHz. 9.6 GHz is the nominal
minimum frequency of the RF oscillator's output signal.
G 24. In LVDAM-MW, select the Data Table function, which will bring up the Data
Table. In this Table, enter the column titles and figures already recorded in
Table 17-2, and then save your Data Table.
RF OSCILLATOR
OUTPUT SIGNAL'S
FREQUENCY (GHz)
MINIMUM POWER
(dBm)
MAXIMUM POWER
(dBm)
PIN DIODE
MAXIMUM
ATTENUATION (dB)
9.6
9.7
9.8
9.9
10.0
10.1
10.2
10.3
10.4
10.5
10.6
Table 17-2. Maximum PIN Diode attenuation at different frequencies.
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PIN Diodes
G 25. With the RF Oscillator's output signal frequency set to 9.6 GHz, measure
the maximum attenuation that the PIN Diode can provide, using the steps
below:
a. On the Data Acquisition Interface, set the PIN DIODE BIAS knob to the
fully counterclockwise position to apply the minimum bias voltage. This
sets the attenuation provided by the PIN Diode to the maximum.
b. Note and record the RF power indicated by the power meter in the
"9.6 GHz" row of the Data Table, under the column "MINIMUM
POWER".
c. On the Data Acquisition Interface, set the PIN DIODE BIAS knob to the
fully clockwise position. This sets the attenuation provided by the
PIN Diode to the minimum.
d. Note and record the RF power indicated by the power meter in the row
"9.6 GHz" of the Data Table, under the column "MAXIMUM POWER".
e. From the data recorded, calculate the maximum attenuation provided
by the PIN Diode at 9.6 GHz, using the formula below. Record your
result under the column "PIN DIODE MAXIMUM ATTENUATION" of the
Data Table.
G 26. In the Gunn Oscillator (VCO) section, adjust the Control Voltage until the
frequency of the RF Oscillator's output signal indicated on the Frequency
Meter is 9.7 GHz.
Then, fill in the corresponding row of the Data Table by repeating step 25.
G 27. Fill in the remainder of Table 17-2 by adjusting the RF Oscillator’s output
signal to each of the other frequencies in this table.
After each new frequency setting, repeat step 25.
G 28. From the results recorded in the Data Table, plot the maximum attenuation
of the PIN Diode as a function of the frequency.
Make the following settings in the Graph window:
Scale
X-Axis Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Manual
X Interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.2
X Max . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.7
X Min . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6
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PIN Diodes
Your graph should resemble that shown in Figure 17-12.
According to the curve obtained, does the maximum attenuation provided
by the PIN Diode vary over the frequency range of 9.6 to 10.6 GHz?
Explain.
Figure 17-12 . Maximum attenuation-versus-frequency curve of the PIN Diode (example).
G 29. Turn off the Gunn Oscillator Power Supply and the Data Acquisition
Interface by setting their POWER switch to the O (OFF) position.
Disassemble the setup and return all components to their storage location.
G 30. Close the LVDAM-MW software.
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PIN Diodes
CONCLUSION
In this exercise, you were introduced to PIN diodes. You learned that a PIN diode is
a semiconductor device that acts like a variable resistor at microwave frequencies.
The resistance of the diode is controlled by varying the DC current used to forward
bias the diode.
You learned that when a PIN diode is forward-biased, the diode allows RF signals
having a period shorter than a certain time constant (τ, or minority carrier lifetime) to
pass through it, because it lacks sufficient time to pass from the "on" (conducting)
state to the "off" (blocked) state.
You saw that as the bias current is increased, the resistance of the diode decreases
rapidly, following a 1/x function. Beyond a certain bias current, the diode resistance
becomes quite constant: the PIN diode can be considered as fully "on" (conducting).
Conversely, you learned that when a PIN diode is reverse-biased, it acts as a
capacitance, providing a high resistance independently of the reverse voltage across
it. You saw that the high attenuation provided by a reverse-biased PIN diode varies
with frequency, as the structure and packaging of the diode cause its equivalent
circuit to be frequency sensitive.
Finally, you learned that PIN diodes are used in numerous microwave applications,
including microwave switching, variable attenuation, amplitude modulation and
leveling, and wireless communication systems.
REVIEW QUESTIONS
1. What is a PIN diode? What does the acronym "PIN" stand for?
2. Briefly describe the typical resistance-versus-bias current response of a forwardbiased PIN diode at microwave frequencies.
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PIN Diodes
3. How does a PIN diode behave when it is reverse-biased?
4. What is meant by the "transition frequency" of a PIN diode? Explain.
5. List microwave applications in which PIN diodes are used.
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