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1-1 ECE 451 Automated Microwave Measurements Laboratory

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1-1 ECE 451 Automated Microwave Measurements Laboratory 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 mid- twentieth 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. 1-1 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. 1-2 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 1-3 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.
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