Laboratory Experience #5: Digital Spectrum Analyzer Basic use

Transcription

1 TELECOMMUNICATION ENGINEERING TECHNOLOGY PROGRAM TLCM 242: INTRODUCTION TO TELECOMMUNICATIONS LABORATORY Laboratory Experience #5: Digital Spectrum Analyzer Basic use 1.- INTRODUCTION Our normal frame of reference is time. We note when certain events occur. This holds for electrical events too, as for example we use oscilloscopes to view the instantaneous value of an electrical signal, that is how this signal changes with time. With the use of transducers we can extend the range of applications to nonelectrical signals as the transducer will appropriate convert them into electrical signals. In other words, we view the waveform of a signal in the time domain. And now it comes Fourier. Fourier's theorems tell us that any time-domain signal is made up of one or more sine waves of appropriate amplitude, frequency and phase. Thus, with a proper instrument we can deconvert any complex electrical signal into a series of separate sine waves or spectral components that can be evaluated independently. In the majority of applications, we will only focus on the frequency and amplitude of these sine waves and will not pay attention to their phase. The knowledge that we would gain by keeping into account their phase does not justify the increase in mathematical complexity that is required. To properly make the transformation from time to frequency the signal must be evaluated over all the time, that is from zero to infinity time. This operation that is easy to perform mathematically does not make sense from a physical, practical point of view. We will take a much shorter, more practical time and assume that the signal behavior over several milliseconds is indicative of the overall characteristics of the signal. Then, we can define the spectrum as the collection of sine waves with specific values of frequency and amplitude that make up the original signal that we are analyzing. Measuring signals in the frequency domain has strengths although this does not imply that measurements in the time domain are out. Time-domain measurements are essential, for example, to evaluate rise and fall times of signals, overshoot and ringing. The strength of frequency-domain measurements is that they show the whole picture of the signal, that is, what are the signals that are involved in the formation of the signal that is object of our analysis. Fortunately, we have an electronic instrument that allows us to see the spectrum of signal, that is, the signal in the frequency domain. This instrument is called The Spectrum Analyzer and has become an essential tool for multiple applications, in particular those involved in telecommunications because signals and services are normally defined in terms of frequency. In this laboratory exercise we will learn the use of a digital, medium frequency (up to 1.5 GHz) spectrum Page 1

2 In this laboratory exercise we will learn the use of a digital, medium frequency (up to 1.5 GHz) spectrum analyzer. In an upcoming laboratory exercise we will work with an analog spectrum analyzer with a wider frequency range (up to 60 GHz). My goal is that after the experimental work is finished, students are able to use a spectrum analyzer properly and are able to choose which one would better fit any specific application. Question 1. Describe an application where time-domain measurements will provide better results than frequency-domain measurements. Question 2. Describe two applications where the use of an spectrum analyzer will help the engineers and technicians working in that area. 2.- BRIEF DESCRIPTION OF THE SPECTRUM ANALYZER The output of the spectrum analyzer is an X-Y display on a CRT screen, similar to the well known oscilloscope screen. The horizontal axis is calibrated in frequency that increases linearly from left to right. The vertical axis is calibrated in amplitude that can take several possibilities. The most common one is the vertical axis being calibrated in db. This analyzer also offers the possibility of the vertical axis being calibrated in linear units such as Volts (or micro Volts) or power units. Page 2

3 There are three main buttons in the Spectrum Analyzer: Frequency, Span and Amplitude. By pressing the Frequency button we enter in the menu for frequency selection. This is normally a two-step process and can take several options. We can specify either the low and higher frequency that we want to visualize on the screen. Alternatively, we can specify the center frequency and the span over this center frequency. Once we have focused on the section of the spectrum that we want to analyze, we can change how much we want to see in a convenient way with the Span button that will bring the menu related to the total span of frequency seen on screen. Finally, the Amplitude button will put the Analyzer into the amplitude menu where we can select the type of units shown on the screen. We can enter the values for frequency, amplitude or span by enter the values directly using the numeric keypad, or by using the knob located underneath the main selection keys. Another special key that is useful for analysis is the Marker key (MKR) and the peak search (Peak Search). The Marker displays on the CRT screen the value of amplitude in the units selected for a given frequency. The marker position can be changed by rotating the main knob. The Peak Search button will position the Marker at the frequency with the strongest signal and will enter a submenu to choose additional options. 3.- EXPERIMENTAL PROCEDURE Before connecting any signal to the Spectrum Analyzer be sure that the signal will not damage the very sensitive electronic circuitry of the Spectrum Analyzer. The Maximum rates are specified under the connector input. Question 3. What are the absolute maximum ratings for the Analyzer's input? Page 3

4 Turn on the Spectrum Analyzer. If the Analyzer was already on, press the green Preset button. Connect the CAL OUTPUT BNC connector to the RF INPUT connector. Question 4. Describe and sketch the signal that appears on the screen. Be sure to annotate the frequency and amplitude of those signals that you believe are essential. Question 5. What is the range of frequencies displayed by the analyzer? Question 6. Write down and describe the other parameters displayed on the CRT Page 4

5 The previous configuration should have displayed the full spectrum, what is useful to obtain a general idea to understand the type of signal being analyzed although most of the times the frequency range is too wide to perform accurate measurements. Configure the Spectrum Analyzer to display from 200 MHz to 500 MHz Question 7. Sketch the waveform on the screen Question 8. Compare this waveform what the one obtained in question 4. What is different? What is similar? Page 5

6 Use the Peak Search feature to find the characteristics (frequency/amplitude) of the peak with higher energy. Question 9. What are the readings for the peak? Question 10. What are the expected frequency and amplitude for this signal? Why? Modify the configuration of the analyzer until the main peak is centered in the middle of the screen. Press the SPAN button and rotate the main knob until you achieve a total span of 200 MHz. Repeat the procedure for spans of 70 MHz and 19 MHz Question 11. How does the CRT screen change for each one of these configurations? Page 6

7 Question 12. What additional parameters also change? How can you explain those changes? Configure the CRT screen to view the range of frequency from 100 MHz to 1500 MHz. Question 13. Look at the spectrum on the CRT screen. What type of signal do you think is being applied to the Analyzer's input? Why? Question 14. What is the amplitude of the main peak in dbm? Question 15. Convert the value from the previous question to linear power units (Watts). Use the corresponding conversion formulas. Page 7

8 Question 16. Calculate the value of rms voltage to produce the energy found in Question 14. Question 17. Change the display units on the Analyzer's CRT screen to Watts (or a sub-multiple) and Volts. Compare the measured results on the screen with the theoretical values calculated on questions 15 and 16. Are they different? If so, why? Configure the CRT screen to view a frequency range between 100 MHz and 500 MHz. Change the measurement bandwidth to all the possible options allowed. Question 18. Does the reference level change when the bandwidth changes? Explain your results. Page 8

9 Press the green Preset button Question 19. What happened? Why did the manufacturer of the Spectrum Analyzer include this key? Question 20. What is the reason for the high-amplitude peak located at the origin (left side) of the screen? Question 21. How can this peak be avoided? Question 22. Comment on this laboratory exercise: (how appropriate it was, difficult/easy, interest) with special emphasis on what you have learned. Submit this lab with the answers to your instructor. Use as many additional sheets of paper as needed. Albert Lozano. May 2004 Page 9