ECE 451
Automated Microwave Measurements Laboratory
Experiment No. 1
Detecting RF Power and Using the Slotted Line to Make Impedance Measurements
Introduction
This laboratory experiment explores the beginnings of microwave measurements, those performed in the midtwentieth century. The purpose of using these now-obsolete methods is to teach the student the fundamentals that are otherwise invisible to the user of the modern measurement systems. This experiment has two components. The first part consists of generating a response curve of a basic microwave measurement tool: the square-law detector. In the process of generating this curve, the student comes across a few crucial concepts.
The first is that an engineer must know the exact conditions under which a measurement tool provides reliable data. As will be discussed, a square-law detector is only useful over a specific range of input power levels and a specific range of frequencies. Second, the material properties of a device often motivate why that device is used for a specific task. For example, a crystal detector rectifies a radio signal, converting it from alternating current to a pulsing direct current.
The second part of this lab introduces the slotted line, an instrument that makes use of the reflection phenomenon that is seen at microwave frequencies. The slotted line is used to measure the voltage standing wave ratio (VSWR) generated from terminating it with the device under test. This is useful because directly measuring current and voltage at microwave frequencies is difficult. These VSWR measurements can then used to calculate the unknown impedance of the device under test at the microwave frequency of interest.
The background sections of this lab provide you information on why crystal detectors and slotted lines are used and how they function. Do not be concerned with having a full understanding of these sections. They are merely included to be used as a reference for when questions arise during laboratory work and to help you make conclusions when you write your report.
Purpose
The purpose of this experiment is to determine the square-law range of a crystal detector and to use a slotted line to determine the impedance of passive devices.
Background
Square-Law Detectors
All experimental measurements made in the microwave region depend upon the ability to detect the presence of
RF power. By far, the bulk of these measurements are made by two common detection systems, "square-law detectors" and heterodyne systems. The latter are considered to be linear detectors since the useable output signal is proportional to the electric field, or RF voltage, applied to this detector. Our present concern is with the square-law detectors. These devices find the widest use because of their extreme simplicity of instrumentation.
The square-law devices used for the detection of low-level RF power include the crystal diodes and thermally sensitive devices such as bolometers, barretters and thermistors. They derive their name from the fact that the output, a voltage or a current, is approximately proportional to the square of the input RF voltage or current. Thus, this output is proportional to the applied input power.
Although barretters and thermisters are useful in the absolute measurement of power, for most laboratory measurements it is sufficient to determine the relative power or field strength levels. These relative power measurements may be made on unmodulated signals giving a direct current or voltage from the detector. For the utmost sensitivity the detector should be followed by some signal amplification before the output is presented on an indicator. The problems associated with stable, high gain DC amplifiers can be avoided by using an RF source that is modulated at an audio frequency. The resultant detected, low-level audio signal may be easily amplified in a narrow band, high gain amplifier.
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The detector must be able to follow the modulation envelope of the RF signal to obtain a useable audio output signal. An audio modulation frequency of 1000 Hz is a good compromise frequency. It is sufficiently high that the selective circuits in the amplifier can reject ripple voltages from the power supply (in America, 60 Hz and all of its harmonics), and low enough to be compatible with time constants of the barretters and crystals.
Thermistors, however, have a time constant of approximately 0.1 second and would require a considerably lower frequency of modulation.
The range of power levels that can be detected or accurately measured is limited at higher power levels by the deviation of the device from the square law characteristic and at the very low power levels by the noise produced by the detector itself and the following amplifying system.
The Slotted Line
The Type 874-LBA Slotted Line is designed to measure the voltage standing wave pattern produced by any load connected to it. Its characteristic impedance is 50 ohms. The outer conductor is slotted for a length of approximately 50 centimeters, and a small shielded probe extends into the region between the two conductors.
The probe is mounted on a carriage, which slides along the outside of the outer conductor. The penetration of the probe into the line and, hence, the capacitive coupling between the probe and the line, can be adjusted over a wide range by means of a screw adjustment. Cross-sectional views of the probe arrangment are shown in Fig.
1. Since the probe is capacitively coupled to the line, the voltage induced in the probe circuit is proportional to the voltage existing between the inner and outer conductors of the line at the probe position. The RF voltage induced in the probe circuit is detected by a crystal in the probe carriage.
Figure 1
Tuning the Crystal Detector
The crystal rectifier built into the carriage is tuned by means of the adjustable stub, which is effectively connected in parallel with it in order to increase the sensitivity and to provide selectivity. The stub is adjusted until maximum output is indicated by the detector.
The General Radio Type 874-LBA slotted line and cross-section views of the carriage showing the crystal mount and the probe are shown in Fig. 2.
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Figure 2
It’s critical to ensure that the stub is tuned to the fundamental and not one of the harmonics of the desired signal.
Tuning with a high VSWR on the line may lead to confusion at times as the minima of the harmonics may not be coincident with the minima of the fundamental, consequently, the harmonic content of the signal picked up by the probe may be greater than that present in the local oscillator output. To minimize the possibility of mistuning, the probe should be tuned with a low VSWR on the line, for instance, with the line terminated in a
Type 874-WM 50 Ohm Termination. As a check, the distance between two adjacent voltage mimina when the line is terminated in a short circuit can be measured. If the stub is tuned correctly, the spacing should be a half wavelength.
Probe Penetration Adjustment (for your reference only – the TAs will handle this)
The probe penetration should be adjusted for adequate senstivity as well as insignificant effect on the measured
VSWR. The presence of the probe affects the VSWR because it is a small admittance in shunt with the line. It has the greatest effect at a voltage maximum, where the line impedance is high.
To adjust the probe penetration, remove the tuning stub connected to the left-hand connector and turn the small screw found inside the inner connector (See Fig. 2). Clockwise rotation of the screw increases the coupling. In most cases in which moderate VSWR's are measured, a penetration of about 30% of the distance between the two conductors gives satisfactory results. For this experiment, your probe penetration has been initially adjusted to give approximately 30% coupling as indicated below.
To adjust the coupling to 30%, increase the coupling until the probe strikes the center conductor of the slotted line; then back it off six full turns of the screw. The point of contact between the probe and the center conductor is most easily measured by connecting an ohmmeter between the inner and outer conductors of the line with the standing wave indicator connected as shown in Figure 1. When using the 2000 ohm range on the ohmmeter, its imperative to note the point at which the resistance suddenly drops from a very high value to a reasonably low value. The crystal is in series with this circuit, so the resistance will not drop to zero. No
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indication will be obtained if the crystal has been removed. Do not screw the probe down tight against the center conductor, as it will damage the probe.
The amount of the probe penetration can be visually checked by looking at the probe though the slot from one end of the line.
The effect of the probe coupling on the VSWR can be determined by measurement of the VSWR at two different degrees of coupling. If the measured VSWR is the same in both cases, the probe coupling used has no significant effect on the measurement. If the measured VSWR's are different, additional measurements should be made with decreasing amounts of probe penetration until no difference occurs. However, as pointed out in the previous paragraph, a 30% coupling usually gives satisfactory results except when the VSWR is high, which usually requires a large coupling.
Useful-Links
1.
Slotted-Line Lab video on Youtube http://www.youtube.com/watch?v=ssqlGsPRKik
2.
VSWR 101 video on Youtube http://www.youtube.com/watch?v=w1eE13UXAKs
3.
Slotted-Line Reference Document http://radio.feld.cvut.cz/personal/matejka/download/slm.pdf
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Procedure
Equipment Used:
HP Microwave Source (8350 Sweep-Oscillator)
HP Crystal Detector
HP Voltmeter (3457A)
General Radio Slotted Line
BNC Cable
N-type Microwave Short
N-type 50-ohm termination
2.5 VSWR Standard
5.0 VSWR Standard
1) Arrange the lab equipment as shown in Fig. 3. The crystal detector can be directly connected to the microwave source to minimize the number of cables you need and eliminating the N-type cable. On some sources, the RF output is on the back of the device ( make sure you connect to the one labeled RF, not
AUX ). Obtain data to plot a curve of RF (600 MHz) power 10 dBm to a -50 dBm in 5 dBm steps. Plot the data in Excel with the RF power (in dBm) on the linear x-axis and the DC voltage (in millivolts – take the absolute value) on the y-axis (make y-axis logarithmic scale).
Figure 3
2) From your curve of RF power into the detector vs. DC voltage out of the detector, determine the square law range of your detector. This information will be required in a later experiment. How does the measured curve compare with what you would expect from a detector (see square-law detector background)?
Voltmeter
Microwave Source
0 cm
Figure 4
Slotted Line
50 cm
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3) Replace the square-law detector with a General Radio slotted line terminated with a 50-ohm termination (the line has a crystal detector embedded into the carriage) as shown in Fig. 4. Again measure and plot the DC voltage (in millivolts) as a function of input power (in dBm) over the same range as in Step 1. Does this curve exhibit a similar shape to the previous one? What is the square-law region of this detector?
4) Set the power output of the source to -10 dBm. With the 50-ohm termination still on the end, adjust the single stub tuner to maximize the output on the voltmeter by pulling it in or out (not twisting it). a. Measure a short at 650 MHz. Find more than one minimum and one maximum (if possible); record both the locations on the line and the voltage level at those locations for each minima and maxima. Calculate the distance between the minimum and its adjacent maximum (one quarter wavelength); calculate the frequency and compare the value with frequency set on the signal generator. b. Measure an open at 650 MHz. Find more than one minimum and one maximum (if possible); record both the locations on the line and the voltage level at those locations for each minima and maxima. Calculate the distance between the minimum and its adjacent maximum (one quarter wavelength); calculate the frequency and compare the value with frequency set on the signal generator. How do the locations of these minima and maxima compare to those of the short? c. Measure the VSWR of the calibrated 2.5 and 5.0 VSWR terminations at 650 MHz. d. Measure the impedance of the calibrated 2.5 and 5.0 VSWR terminations at 650 MHz (be sure to use a type N short). Obtain data to calculate the impedance of the calibrated terminations using a formula.
Plot the impedance directly on a Smith Chart. Compare the results of these two techniques by plotting both impedance values for both terminations on the same Smith Chart.
NOTE: The length markings on the slotted line do not have any absolute meaning. Think of them as simply a ruler placed along the length of the slotted line for your convenience. We measure the location of the minimum of the short closest to the short because we know that this point is half of a wavelength from the end of the line
(why?). This provides our absolute reference, relative to which we make all other measurements in order to determine the impedance of unknown terminations. It is recommended that you use a Smith Chart when making your impedance calculations. The impedance calculation formulas will also work, but care must be taken to ensure that the correct value for d min
is used.
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1.
Show Z
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Z
0
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
j
VSWR
VSWR
j
d d min min
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2.
If the gain of our detector was made 4 times larger, how would the slope of its output (in log V) change (if at all)?
3.
How do you measure VSWR with the setup in Fig. 4? Remember that the output of the square-law detector is proportional to power, not voltage.
4.
How can you determine the impedance of an unknown termination (like the 2.5 and 5.0
VSWR standards) from our measurements of the minima and maxima of the shorted slotted line and of the minima and maxima of the unknown termination impedance? Be sure to explain this directly.
5.
What does the wave pattern on the slotted line look like for a perfectly matched load, a short, and an open? (Think about VSWR and what the voltage at the endpoint should look like.)
1.
What is the measured slope of your detector in the square-law region (log V vs. dBm)?
2.
Where and why does this slope become nonlinear (for both small and large log V)?
3.
All of the measurements made with the square-law detector in this experiment have been relative. What would we need to make absolute measurements?
4.
What measurements are required to calculate the impedance of the unknown when it is placed directly on the slotted line?
5.
Why is the slotted line useful? What parameters does it allow you to measure or calculate?
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