ElecEng 4/6FJ4 LABORATORY

MODULE #3

Microwave Detection with a Crystal Detector I. Objectives The purpose of this module is to help the students get familiar with the “crystal” detector, one of the simplest power detectors at radio, microwave and millimeter-wave frequencies. The crystal detector uses a simple circuit, consisting of a diode, a RF (radio-frequency) choke and a low-pass filter (often, this is simply a capacitor) to rectify the high-frequency signal. The lectures of ElecEng 4FJ4 do not cover the general topic of microwave diodes and their applications. 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 Detection of Microwave Signals LabVolt (https://www.labvolt.com/) exercise, which constitutes MODULE #3. 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 exercise. The student must provide answers to these questions, which are closely related to the DISCUSSION and the PROCEDURE parts 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 nikolova@ieee.org.

Nikolova ElecEng 4FJ4 Labs 2014

ElecEng 4/6FJ4 LABORATORY

MODULE #3

MICROWAVE DETECTION WITH A CRYSTAL DETECTOR 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

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Exercise 5

Detection of Microwave Signals

EXERCISE OBJECTIVES

When you have completed this exercise, you will be familiar with the operation of acrystal detector operating in the square-law region. You will be able to plot thesensitivity curve of a crystal detector and to evaluate its tangential sensitivity.

DISCUSSION

Introduction to Crystal Detectors

Microwave signals are detected by converting the microwave signal to a DC currentor DC voltage whose amplitude is a function of the amplitude of the microwavesignal. By doing so, conventional low-frequency instruments can be used to measuremany of the parameters of the microwave signal, or just to detect its presence.

Figure 5-1 represents a simplified microwave crystal detector circuit and its symbolicrepresentation. Crystal detectors are basically a type of diode rectifier. Because adiode is a non-linear element, harmonic components of the input signal aregenerated in the detection process. These harmonic components are attenuated bythe lowpass filter located before the detector output, leaving only the DC outputsignal.

Figure 5-1. Simplified representation of a detection circuit and the symbolic representation of acrystal detector.

Typical Sensitivity Curve of a Crystal Detector

Figure 5-2 shows how the output voltage of a detector varies as a function of themicrowave signal power at its input. This curve represents a typical sensitivitycurve for crystal detectors.

• When the microwave signal power remains low, the detector output voltage isproportional to the square of the microwave signal voltage and, therefore, to themicrowave signal power. In this condition, the detector is said to be operating inits square-law, or quadratic region.

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• When the microwave signal power is higher than about -15 dBm, the detectoroutput voltage tends to be directly proportional to the microwave signal voltage.In this condition, the detector is said to be operating in its linear region; that is,it rectifies the applied signal.

Figure 5-2. Typical output voltage-versus-microwave input power curve of a crystal detector.

Voltage Sensitivity

The voltage sensitivity is an indication of a detector output voltage in the square-lawregion when a microwave signal of a given power is applied to its input.

The voltage sensitivity can be calculated by using the equation below.

The voltage sensitivity is an approximation. Its main use is to compare theperformance of different detectors.

Amplification of a Crystal Detector's Output Signal

As Figure 5-3 shows, a crystal detector can be used to detect very small signalswhen followed by a low-noise high-gain amplifier. The output level, amplified to alevel high enough to be easily measured, is then applied to an indicator.

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Figure 5-3. Amplification of a crystal detector's output signal.

This method can be used for non-modulated and amplitude-modulated microwavesignals. Usually, the microwave signal is amplitude modulated by a 1-kHz squarewave signal. This permits the use of low-frequency instruments and circuits with lownoise level to measure several parameters of the microwave signal, and thedetection of even smaller signals.

Measuring the Tangential Sensitivity

The tangential sensitivity of a detector is a measure of the detector's ability to detectlow-level signals. It corresponds to the smallest microwave signal level that can bedetected above the noise level.

One way to measure the tangential sensitivity of a detector is to apply a microwavesignal amplitude modulated by a 1-kHz square-wave signal to the detector input:

• The level of the microwave signal is adjusted so that the peak-to-peak voltage ofthe 1-kHz detected signal is equal to the peak-to-peak voltage of the noise at thedetector output, as Figure 5-4 shows.

• The applied microwave signal power corresponds to the tangential sensitivity ofthe crystal detector, as long as the percentage of modulation is 100%.

For the measurement setup, an oscilloscope, and an amplifier with a bandwidth wideenough to prevent distortion of the shape of the 1-kHz square-wave signal arerequired.

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Figure 5-4. Oscilloscope screen showing a 1-kHz detected signal for tangential sensitivitymeasurements.

Generally, crystal detectors will have a tangential sensitivity in the order of -50 dBmfor a bandwidth of 1 MHz. Since the noise power at the amplifier output isproportional to the bandwidth of the amplifier, the tangential sensitivity is better whenthe bandwidth is reduced.

Procedure Summary

In this exercise, you will measure the power of a microwave signal, using theThermistor Mount and the LVDAM-MW Power Meter. This will be the maximumpower used in the exercise, and will be the reference power level.

You will then replace the Thermistor Mount by a Crystal Detector and measure thedetector output voltage with the LVDAM-MW Oscilloscope. You will then attenuatethe microwave signal by steps and measure the Crystal Detector output voltage foreach attenuation setting. This will allow you to plot the sensitivity curve of the CrystalDetector.

Finally, you will amplitude modulate the Gunn Oscillator's output signal with a 1-kHzsquare wave and measure the tangential sensitivity of the Crystal Detector.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart, in Appendix F of this manual, to obtain thelist of equipment required to perform this exercise.

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PROCEDURE

Measuring the Maximum Power of the Microwave Signal

G 1. Make sure that all power switches are in the O (off) position. Set up themodules and assemble the microwave components as shown in Figure 5-5.

Note: Before connecting the Thermistor Mount, unscrew thematching screws so that they do not penetrate into thewaveguide; the screws do not need to be removed from theposts.

G 2. Make the following settings on the Gunn Oscillator Power Supply:

VOLTAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MIN.MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DCMETER SCALE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 V

G 3. Turn on the Gunn Oscillator Power Supply and the Data AcquisitionInterface (DAI) by setting their POWER switch to the "I" (ON) position.

Set the Gunn Oscillator supply voltage to 8.5 V. Wait for about 5 minutes toallow the modules to warm up.

G 4. On the host computer, start the LVDAM-MW software. In the ApplicationSelection 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ONFunction Input 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Power MeterGain Input 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 dB

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Figure 5-5. Computer and module arrangement (showing electrical connections to microwavecomponents), and microwave setup.

G 5. Wait for about 5 minutes to allow the modules to warm up. In LVDAM-MW,start the Power Meter and set it to display dBm readings. Enter the power

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factor indicated (η) on the Thermistor Mount’s waveguide. Then, performzeroing of the Power Meter.

G 6. On the Thermistor Mount, loosen the knurled lock-nut that holds themoveable short circuit into place. Adjust the short circuit to the positionnearest the waveguide which gives a maximum reading on the PowerMeter. Then, adjust each matching screw of the Thermistor Mount tomaximize the power reading. Fine tune if necessary. Finally, lock themoveable short circuit into position.

G 7. Note and record the reading of the Power Meter. This will be, in the contextof this exercise, the maximum power of the microwave signal.

Maximum power of the microwave signal = dBm

Close the Power Meter of LVDAM-MW.

Determining the Sensitivity Curve of the Crystal Detector

Note: A first microwave circuit [Figure 5-6 (b)] will be used toperform the measurements requiring an attenuation lower than30 dB. A 30-dB Fixed Attenuator will then be added to themicrowave circuit to perform the rest of themeasurements—which require an attenuation equal to or higherthan 30 dB.

G 8. In the Settings panel of LVDAM-MW, set the field Gunn Oscillator/VCOPower to OFF.

Set up the modules and assemble the microwave components as shown inFigure 5-6.

Connect the Crystal Detector to MULTI-FUNCTION INPUT 1 of the DataAcquisition Interface.

Note: Connecting the output of the Crystal Detector to one of theDAI Inputs (in this case MULTI-FUNCTION INPUT 1) will allowyou to monitor the Crystal Detector output voltage with theOscilloscope of LVDAM-MW.

G 9. Set the blade of the Variable Attenuator to 0.0 mm.

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Figure 5-6 Computer and module arrangement (showing electrical connections to microwavecomponents), and microwave setup for attenuations lower than 30 dB.

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Note: It is probable that you will not need the 60-dB Amplifier inthis section of the exercise, since the level of the signal producedat the output of the Crystal Detector is usually strong enough forthe acquisition process to perform the calculations and display theproper signal on the Oscilloscope. However, if the signal is tooweak, connect the 60-dB Amplifier between the output of theCrystal Detector and Multi-Function Input 1 of the DataAcquisition Interface (as shown in Figure 5-9 of this exercise).

G 10. In LVDAM-MW, select the Data Table function, which will bring up the DataTable. In this Table, manually enter the column titles and figures alreadyrecorded in Table 5-1 below. Save the Data Table.

ATTENUATIONSETTING (dB)

ATTENUATORBLADE'S

POSITION (mm)

DELIVEREDPOWER (dBm)

DETECTORVOLTAGE (V)

20 logDETECTOR

VOLTAGE (dB)

0 0

5

10

15

20

25

30 0

35

40

Table 5-1. Determining the sensitivity curve for the Crystal Detector.

G 11. Fill in the column "ATTENUATOR BLADE'S POSITION" of the Data Table:

By referring to the attenuation-versus-blade position curve (or thecorresponding Data Table) of the Variable Attenuator obtained inExercise 4, determine as precisely as you can the attenuator blade'sposition required to obtain each of the attenuation settings listed in the DataTable. Record your results in the table and save your work.

Note: For the attenuation settings equal to or higher than 30 dB,subtract 30 dB from the attenuation setting and determine theVariable Attenuator's blade position required to obtain theresulting difference.

For example, an attenuation setting of 30 dB requires that theVariable Attenuator's blade position be returned to 0 mm; anattenuation setting of 35 dB requires that the blade position be setto provide a 5-dB attenuation, and so on.

This must be done because, as already mentioned, a 30-dB FixedAttenuator will be added to the current microwave circuit when therequired attenuation is equal to or higher than 30 dB.

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G 12. Fill in the column "DELIVERED POWER" of the Data Table. For eachattenuation setting, calculate and record the power delivered to the CrystalDetector, using the formula below. Save your work.

where Maximum Power = Maximum power of the microwave signal,as recorded in step 7 of this exercise

G 13. In the Settings panel of LVDAM-MW, make the following settings:

Gunn Oscillator/VCO Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ONFunction Input 1 . . . . . . . . . . . . . . . . . . . . . Voltage Probe (Unipolar)Gain Input 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 dB

Start the LVDAM-MW Oscilloscope and set it as follows:

Channel 1 (X)Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 mV/DIV

Time base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200 µs/DIVTrigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ch1 SoftwareMode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous Refresh

G 14. Fill in the column "DETECTOR VOLTAGE" for each of the attenuationsettings between 0 and 25 dB listed in the Data Table. To do this, performthe steps below.

a. Adjust the Variable Attenuator's blade to the position required to obtainthe attenuation setting.

b. Measure the voltage on channel 1 of the Oscilloscope screen andrecord it in the column "DETECTOR VOLTAGE".

It will be necessary to increase the sensitivity of Oscilloscope Channel 1when the vertical deflection becomes too low. This is performed bychanging the Scale of Channel 1, and by increasing Gain Input 1 asnecessary.

Note: Changing the Gain on the Input associated with anOscilloscope channel will change the scales available on thischannel:– An Input Gain of 0 dB provides the following scales on the

associated Oscilloscope channel: 1 V/div, 500 mV/div, and200 mV/div.

– An Input Gain of 20 dB provides the following scales on theassociated Oscilloscope channel: 100 mV/div, 50 mV/div,and 20 mV/div.

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– An Input Gain of 40 dB provides the following scales on theassociated Oscilloscope channel: 10 mV/div, 5 mV/div, and2 mV/div.

Note: The dynamic range of measurement, on each scale of theOscilloscope, is not the same as with an ordinary oscilloscope. This occursbecause the dynamic range of measurement is limited by the convertersof the Data Acquisition Interface. Therefore, the maximum voltage that youcan measure, on each scale of the Oscilloscope, is as follows:– With Function Input set to Voltage Probe (Unipolar): Gain of 0 dB: 3 V;

Gain of 20 dB: 300 mV; Gain of 40 dB: 30 mV.– With Function Input set to Voltage Probe (Bipolar): Gain of 0 dB: ±

1.5 V; Gain of 20 dB: ± 150 mV; Gain of 40 dB: ± 15 mV.

G 15. Fill in the remainder of the column "DETECTOR VOLTAGE" for theattenuation settings of 30 to 45 dB:

a. In the Settings panel of LVDAM-MW, set the field Gunn Oscillator/VCOPower to OFF.

b. Insert the 30-dB Fixed Attenuator between the 6-dB Fixed Attenuatorand the Variable Attenuator, as Figure 5-7 shows. Do not modify theremainder of your setup.

c. In the Settings panel of LVDAM-MW, set the field Gunn Oscillator/VCOPower to ON.

d. Adjust the Variable Attenuator's blade to the position required to obtaineach of the remaining attenuation settings listed in the Data Table. Foreach attenuation setting, measure the voltage on channel 1 of theOscilloscope screen and record it in the column "DETECTORVOLTAGE".

Note: You may not be able to fill in the last row of the Data Tabledue to the very low voltage level measurement required to do so.

Figure 5-7. Microwave setup for attenuations equal to or higher than 30 dB.

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G 16. Fill in the last column, "20 log DETECTOR VOLTAGE (dB)", of the DataTable. For each attenuation setting, calculate the detector output voltage,in dB, based on the detector voltage (V) measured for each of thesesettings, and record your results. Save your work.

G 17. In LVDAM-MW, select the Graph function of the Data Table and plot thesensitivity curve of the Crystal Detector by selecting "DELIVERED POWER"for the X axis and "20 LOG DETECTOR VOLTAGE" for the Y axis.

The obtained curve should resemble that shown in Figure 5-8.

Print your graph. Then, determine the square-law region of the sensitivitycurve by plotting a straight line tangential to this curve.

As Figure 5-8 shows, in the square-law region, a variation of 1 dBm in theinput power to the Crystal Detector produces a variation of approximately2 dB in the Crystal Detector output voltage.

This occurs because the Crystal Detector output voltage is proportional tothe square of the amplitude of the microwave voltage. Since the X-axis ofthe graph in Figure 5-8 indicates the microwave power (power unit in dBm),and since the Y-axis indicates the detector output voltage (in dB), avariation of 1 dBm in the microwave power produces a variation of 2 dB inthe detector output voltage; a variation of 4 dBm in the supplied microwavepower produces a variation of 8 dB in the detector output voltage; and soon.

Note: The lower section of your curve may deviate from thestraight line, due to inaccuracies of measurements at low signal-to-noise ratios. However, consider this section as making part ofthe square-law region.

Save your work, and then close the Data Table.

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Figure 5-8. Sensitivity curve of the Crystal Detector.

Measuring the Tangential Sensitivity of the Crystal Detector

G 18. In the Settings panel of LVDAM-MW, set the Gunn Oscillator/VCO Powerto OFF, so that there is no microwave signal injected into the waveguide.

G 19. As Figure 5-9 shows, modify your circuit so as to connect the CrystalDetector to MULTI-FUNCTION INPUT 1 of the Data Acquisition Interface,via the 60-dB Amplifier, Model 9593. This will allow you to monitor theamplified Crystal Detector output voltage on the Oscilloscope.

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As Figure 5-9 shows, the supply cable for the 60-dB Amplifier must beconnected to the DB-9 male connector on the bottom of the Data AcquisitionInterface.

Figure 5-9. Computer and module arrangement (showing electrical connections to microwavecomponents), and microwave setup.

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G 20. Make the following settings:

On the Gunn Oscillator Power Supply:

MODE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 kHz

On the Variable Attenuator:

Blade Position . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 mm

Note: Setting the MODE switch of the Gunn Oscillator Power Supply to1 kHz will cause the Gunn Oscillator's output signal to be amplitudemodulated by a 1-kHz square wave.

G 21. In the Settings panel of LVDAM-MW, make the following settings:

Gunn Oscillator/VCO Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ONFilter Input 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . > 30 kHzFunction Input 1 . . . . . . . . . . . . . . . . . . . . . . Voltage Probe (Bipolar)Gain input 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 dB

Set the LVDAM-MW Oscilloscope as follows:

Channel 1 (X)Input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 mV/DIV

Time base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 µs/DIVTrigger Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ch1 SoftwareMode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Continuous Refresh

G 22. Gradually decrease the attenuation provided the Variable Attenuator (turnits adjustment screw counterclockwise) until you obtain a waveform similarto that shown in Figure 5-4 of the DISCUSSION on the oscilloscope screen(that is, a 1-kHz detected signal having a peak-to-peak amplitudeapproximately equal to the peak-to-peak voltage of the noise in this signal.)

Figure 5-10 shows an example of signal obtained on the Oscilloscope.

At that point, the level of the 1-kHz detected signal will be very low. You canswitch the 1-kHz MODE switch of the Gunn Oscillator Power Supplybetween the "DC" and "1 kHz" positions to better see and evaluate the levelof the noise signal alone and that of the 1-kHz detected signal to see if theyare approximately equal.

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Figure 5-10. Example of a 1-kHz detected signal on the Oscilloscope.

G 23. Record the position of the Variable Attenuator's blade below.

Variable Attenuator's blade position: mm

Referring to the attenuation-versus-blade position curve (or thecorresponding Data Table) of the Variable Attenuator obtained inExercise 4, determine the attenuation provided by the Variable Attenuatorat this blade position.

Attenuation: dB

Calculate the total attenuation by adding 30 dB to the attenuation obtainedfrom the Variable Attenuator's curve.

Total attenuation: dB

Calculate the power of the Crystal Detector's input signal (tangentialsensitivity) by subtracting the total attenuation from the maximum power ofthis signal (as recorded in step 7 of this exercise).

Tangential sensitivity: dBm

G 24. Turn off the Gunn Oscillator Power Supply and the Data AcquisitionInterface by setting their POWER switch to the O (OFF) position.Disassemble the setup and return all components to their storage location.

G 25. Close the LVDAM-MW software.

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CONCLUSION

In this exercise, you were introduced to the principles of microwave signal detectionand to the operation of crystal detectors.

You plotted the sensitivity curve of the Crystal Detector by measuring its outputvoltage for different levels of the microwave signal at its input.

You measured the tangential sensitivity of the Crystal Detector, by amplitudemodulating the Gunn Oscillator's output signal with a 1-kHz square wave.

REVIEW QUESTIONS

1. What do microwave detectors allow you to do?

2. What characterizes the square-law region of a detector?

3. What does the sensitivity curve of a crystal detector represent?

4. A crystal detector generates a signal of 10 mV for an incident microwave powerof -25 dBm. What is the detector sensitivity (in mV/mW)?

5. Why is it often useful to amplitude modulate a microwave signal with a 1-kHzsquare wave before applying it to a crystal detector?

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