ECE 2111 Signals and Systems Fall 2012, UMD Experiment 3: The Spectrum Analyzer

Transcription

1 ECE 2111 Signals and Systems Fall 2012, UMD Experiment 3: The Spectrum Analyzer Objective: To gain an understanding of the basic controls and measurement techniques of the Rohde & Schwarz Handheld Spectrum Analyzer FSH3 Equipment: - Rohde & Schwarz FSH3 Spectrum analyzer - Agilent 33120A Function Generator BACKGROUND Please refer to the text book in Chapter 4 section 4.1 for an understanding of fundamental concepts of signals. Periodic Signal All periodic signals are made up of two or more exponential signals (harmonics) of differing amplitude and frequency. One is able to see the shape of these signals with the aid of an oscilloscope, which graphs the signals voltage over an interval of time. Alternatively, one can observe the amplitude and the frequency of the different harmonics that make up the signal with the use of a Spectrum Analyzer. The type of graph that the spectrum analyzer produces is called an Amplitude Spectrum. The amplitude spectrum is comprised of vertical spikes that represent harmonics. These spikes begin at various places along the horizontal (frequency) axis and rise in the positive direction to various heights (amplitudes). What is a Spectrum Analyzer? Spectrum analyzers are of several types. In this course we will use the heterodyne or scanning analyzer. A scanning spectrum analyzer is essentially a radio receiver. Imagine tuning a conventional FM broadcast receiver from one end to the other, plotting the reading of the tuning meter versus frequency. The graph you would produce is called a Frequency Domain representation, or Spectrum, of the FM broadcast band; it tells you at what frequencies signals occur and how strong they are. If stations are too close together, you will hear them simultaneously and you will not be able to get an independent meter reading for each. This is because the Intermediate Frequency (IF) filter of the receiver has a bandwidth too wide to resolve, or separate, the stations. Page 1 of 10

2 In this hypothetical example, you are manually tuning, or scanning, the FM broadcast band with a Resolution Bandwidth (RBW) equal to the bandwidth of the IF filter in your receiver. Suppose you plot your measurements on graph paper with one centimeter divisions, making each division equal to one MHz. The span/division of the resulting plot is then 1 MHz/division. When you stop tuning, your receiver is no longer scanning; it is in zero span mode. The output of the receiver is now a time domain representation (signal amplitude vs. time) of the signal coming through the IF filter at the frequency to which the receiver is presently tuned. After detection, the signal appears at the receiver's speaker as the sound you hear. A spectrum analyzer performs similarly except that the scan is performed automatically and a selection of IF bandwidths or RBW's are available to choose from. Multiple RBW's needed because in some cases you will want to separate closely-spaced, narrowband signals, while in other cases you will want to examine signals with larger bandwidths. There is a maximum speed at which a band can be accurately scanned with a RBW of a given width; generally, the smaller the RBW, the slower the speed. The spectrum analyzer in the lab can automatically select the fastest available speed for you. Often when looking at the amplitude spectrum it is helpful to observe harmonics with very large and very small amplitudes at the same time. The spectrum analyzer has a logarithmic vertical scale with 8 divisions of 10 db/div (can be changed). This enables the display of signals within an 80 db range, a 10,000:1 ratio. A linear scale such as that of an oscilloscope would require 2000 vertical divisions to display this range. The standard vertical scale of an oscilloscope has 8 divisions each made up of 5 smaller divisions. On such a scale, the largest and smallest signals that could be viewed at the same time would only be of a ratio of 40:1. In db the ratio can be calculated by: Therefore this ratio in db is: Page 2 of 10

3 The FSH3 spectrum analyzer displays voltage along the vertical scale in dbm, and its definition is: where R = spectrum analyzer impedance v(t) = a periodic voltage signal T = the period of the signal V rms = the root mean-square voltage. For sine waves with no DC components, the root mean-square (rms) voltage is given by: For square waves with no DC components, the rms voltage is given by: The difference between two signals peaks vertically is measured in db. This difference represents the voltage ratio between the two signals in db, this is: READ THIS ABOUT THE FSH3!!! The maximum permissible continuous power at the RF inputs is 20 dbm (100 mw). It can be loaded with up to 30 dbm (1 W) for at most 3 seconds. Instrument will be destroyed if operated at this power for longer than 3 seconds. For measurements in circuits with voltages V rms > 30 V, suitable measures should be taken to avoid any hazards using appropriate measuring equipment. Maximum Permitted DC voltage is 50 V. Page 3 of 10

4 In its preset format the FSH3 displaying frequency spectrum from 100 khz to 3 GHz, the FSH3 s maximum frequency span. At 100 MHz, the generator signal is displayed as a vertical line. Generator harmonics can also be seen as lines at frequencies which are integer multiples of 100 MHz. The Rohde & Schwarz Spectrum Analyzer FSH3 Spectrum analyzers measure how the power of an input signal is distributed in frequency. Basic facts about the FSH3: Frequency range Resolution bandwidths Video bandwidths 100 khz to 3 GHz 1 khz to 1 MHz 10 Hz to 1 MHz Page 4 of 10

5 Displayed average noise level Reference level -116 dbm (1kHz) typ. -80 dbm to +20 dbm The function Keys: The large oval keys are the function keys, used to activate corresponding menu. All basic instrument settings are covered with the FREQ, SPAN and AMPT function keys. Pressing the FREQ key opens the frequency menu with the soft key F1 turning red. The center frequency can be adjusted by either: - Alphanumeric keypad - Fine tuning with the Rotary knob - Stepping Up/Down with the Cursor keys The SPAN key activated the F1 key and lets you change the Span of the signal manually. You can adjust the span of the signal by the cursor keys. The AMPT key is the Amplitude reference key used to increase sensitivity. A lower level offers more accuracy while viewing and measuring low power levels. The reference level can be adjusted from +20 dbm to 80 dbm. I. - Measuring frequency components of a square wave Procedure: 1. Make sure no signal source is connected to the spectrum analyzer. Make sure the power chord is connected to the FSH3. Connect the RF output of the signal generator to the RF input of the FSH3. Note: Impedance setting is needed for function generators. Function Generator 33250A: UTILITY->OUTPUT SETUP->change LOAD to High Z Function Generator 33120A: MENU (shift+enter)->sys Menu-> 1: Out Term-> Parameter->change 50 ohm to High Z 2. Set the signal generator to a square wave with no DC component and frequency of 10 MHz with amplitude 0.5 V pp. Page 5 of 10

6 3. Set the FSH3 to its default settings to show all the operating steps that are required. You can do this by pressing the PRESET key. The analyzer displays the frequency spectrum from 100 khz to 3 GHz the FSH3 s maximum Frequency Span. At the 10 MHz, the generator signal is displayed as a vertical line. Generator harmonics can also be seen as lines at frequencies which are integer multiples of 10 MHz. 4. Display of function generator signals: To analyze the generator signal at 10 MHz in more detail, reduce the Frequency Span. Set the FSH3 s Center Frequency to 10 MHz and reduce the span to 1 MHz. Press the FREQ key. Enter 10 using the numeric keypad and confirm the entry with the MHz key. Press the SPAN key. Enter 1 using the numeric keypad and confirm the entry with the MHz key. The FSH3 now displays the generator signal with a higher resolution. 5. Markers: The FSH3 has markers for reading off signal levels and frequencies. Markers are always positioned on the trace. Both the level and frequency at their current positions are displayed on the screen. Press the marker key. The marker is activated and is automatically positioned on the trace maximum. A vertical line on the measurement diagram indicates the marker frequency. A short blue horizontal line on the trace indicates the level. 6. Display of 2nd harmonic: To measure the 2 nd harmonic voltage, set the Start and Stop Frequency as follows: Press the FREQ key. Select START Enter 5 using the numeric keypad and confirm the entry with the MHz key. Press the STOP softkey. Enter 25 using the numeric keypad and confirm the entry with the MHz key. Page 6 of 10

7 Note: Note down the frequency of the 2 nd harmonic with the help of MARKER. To display clearly, adjust both START FREQ and STOP FREQ. Adjusting Ref Level (AMPT->Ref Level) does not change the amplitudes as long as harmonics are shown. 8. Similarly, find the 3 rd, and 4 th harmonics of the 10 MHz signal. Note down the frequency, Power in dbm and Power in mw. Use the following table to report your readings for the signal Fundamental 2nd Harmonic 3rd Harmonic 4th Harmonic Frequency dbm Power(mW) Volts (calculated) Note: To get the power in watts, press the AMPT key, press the UNIT soft key, select the unit you want from the submenu, and confirm by pressing ENTER key. The reference unit is set. To calculate volts, power in watt is used rather than in mill watts R=50 Ω (Spectrum Analyzer Impedance) Use V=sqrt(Power*R) to calculate the voltage. II. Measuring the fundamental and harmonics of a square signal of different frequency Using the function generator, produce a 1.0Vpp, 100 khz square wave with no DC components. Input this signal to the spectrum analyzer. Measure and record the amplitude of the fundamental harmonic and the next three harmonics in dbm and there corresponding frequencies. Convert the measurements to volts. Report your results on the table provided and sketch your readings. Page 7 of 10

8 Fundamental 2nd Harmonic 3rd Harmonic 4th Harmonic Frequency dbm Power(µW) Volts (calculated) Question1:What was the difference in terms of the harmonic frequencies? III. Measuring the fundamental and harmonics of a triangular signal Using the function generator, produce a 0.5Vpp, 100 khz triangular wave with no DC components. Input this signal to the spectrum analyzer. Measure and record the amplitude of the fundamental harmonic and the next three harmonics in dbm and there corresponding frequencies. Convert the measurements to volts. Report your results on the table provided and sketch your readings. Frequency dbm Power(µW) Volts (calculated) Fundamental 2nd Harmonic 3rd Harmonic 4th Harmonic IV. Measuring the fundamental and harmonics of a sinusoidal signal Adjust the function generator to output a 0.5Vpp, 1 MHz, sine wave with no DC components. Input this signal to the spectrum analyzer. Measure and record the amplitude of the fundamental harmonic and the next three harmonics in dbm and there corresponding frequencies. Convert the measurements to volts. Report your results on the table provided and sketch your readings. Page 8 of 10

9 Fundamental 2nd Harmonic 3rd Harmonic 4th Harmonic Frequency dbm Power(µW) Volts (calculated) Question2:Why only fundamental component is strong for a sinewave? Report Requirements: Individual lab report Attached with the cover page (available on the website) This lab report should contain the following: 1. Statement of the Problem: Define the problem and goals of the experiment. 2. Results: Fill out the tables and sketch your readings (not screen shots), either dbm vs. frequencies or mw vs. frequencies. 3. Answer questions 1 and Exercises: Attached 5. Conclusions: Give comments to the lab. Explain discrepancies between theory and practical results of experiments, if any. Exercises: 1. Suppose you set the function generator to output the signal below: x(t) = 1.5 sin (2000 π t) The fundamental frequency of the signal is: a) 1000 Hz b) 2000 Hz Page 9 of 10

10 c) 1000 π Hz d) 2000 π Hz 2. Suppose you set your function generator to input a 1.0 Vpp, 100kHz Square wave, no DC components signal into a spectrum analyzer. At what frequency is the 2 nd harmonic: a) 200 khz b) 300 khz c) 200 π khz d) 300 π khz 3. Convert the following into mw (power milliwatts): 0 dbm 4. Convert the following into dbm: 10 mw Page 10 of 10