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Oulu Polytechnic / Institute of Technology Version 12.11.2003
Telecommunications Laboratory
Modulation Methods
1. Purpose of the exercise
Different modulation methods are used to transmit information from one place to another. In this exercise you will study some analog and digital modulation methods. The methods are implemented with the Leybold telecommunication education modules.
2. Equipments
FM/PM-modulator Leybold 73627
FM/PM-demodulator Leybold 73628
ASK/FSK/PSK Modulator Leybold 73633
Power supply Leybold 72686
Waveform generator HP 33120
Oscilloscope HP Infiniium
Adjustable DC voltage supply Mascot
Voltmeter Fluke
3. FM = Frequency modulation
3.1. Preparations
Study the theory of FM with your theory book and the Leybold’s material in the background file. Answer the following questions:
What are the carrier’s variables that are used in angle modulation methods?
How much is the carrier frequency of the FM modulator? What about it’s adjustment range?
How much is the maximum deviation of the FM modulator?
3.2. Measurements
3.2.1. Starting the FM modulation
Build the connection according to the next picture:
3.2.2. The characteristic curve of the modulator
Connect the input of the modulator into the ground (0 V). Let the modulator warm up for about 5 minutes. Set the frequency adjustment of the modulator to ultimate left position.
Measure the frequency of the FM modulator’s output signal with the oscilloscope. That is the smallest frequency of the VCO (Voltage Controlled Oscillator)
fmin :______
Measure next the upper limit of the output frequency by turning the frequency adjustment into the rightmost position.
fmax :______
The center frequency of the modulator is then counted from the following formula:
f0 = (fmin + fmax) / 2 =______
Tune the output frequency of the modulator to the center frequency with the aid of the oscilloscope.
Use then the DC voltage supply to feed the voltage (U1) to the input of the FM modulator. Measure the output frequency of the FM modulator with the oscilloscope. Find the characteristic curve of the modulator by changing the DC voltage from –10 V to +10V in 1 V steps and measuring the output frequency. Use the voltmeter to check that the input voltage is accurate. Register the results in the following table.
Table 1: The modulator constantof the FM modulator
U1 / V / fVCO / kHz / U1 / V / fVCO / kHz / U1 / V / fVCO / kHz
-10 / -3 / 4
-9 / -2 / 5
-8 / -1 / 6
-7 / 0 / 7
-6 / 1 / 8
-5 / 2 / 9
-4 / 3 / 10
Draw the characteristic curve by hand or with the computer. Count the sensitivity of the modulator from the slope of the graph.
The sensitivity of the modulator : kFM = DfVCO / DU1
=______Hz/V
Is the graph linear in the whole scale?
______
4. ASK and FSK (Amplitude and Frequency Shift Keying)
Information can be transmitted simply by connecting a suitable carrier on and off. This is called keying. In transmission technique the keying is understood as modulating a continuous carrier with a digital signal.
4.1. Preparations
Study the theory of digital modulation with your theory book and the Leybold’s material in the background file. Answer the following questions:
What is the frequency of the carrier signal in the ASK/FSK/PSK modulator when the ASK is used?
______
What are the two carrier frequencies of the FSK modulator?
______
4.2. Measurements
4.2.1. ASK
Build a connection according to the following picture.
Feed a rectangular signal from the waveform generator to the modulators first amplifier’s input. Set the frequency to 10 kHz and the amplitude to 4 V.
Connect the modulator’s input signal (amplifier’s output!) to channel 1 of the oscilloscope and the modulator’s output signal (ASK) to channel 2 of the oscilloscope. Study the display in time domain and save the picture into your measurement report.
Explain your findings:
Next you must change the place of the connecting plug so, that the signal goes through the filter. Study the two signals again and save the picture. What does the presence of the filter achieve?
The last task with the ASK is to feed the signal straight to modulator’s input without the filter and the amplifier. (Take the connection plug away.) Compare the signals with the signals of the first case. How does the amplifier affect to the amplitude of the modulated output signal?
4.2.2. FSK
Build the connection according to the following plan.
Feed a rectangular signal from the waveform generator to the modulators first amplifier’s input. Set the frequency to 10 kHz and the amplitude to 4 V.
Connect the modulator’s input signal (amplifier’s output!) to channel 1 of the oscilloscope and the modulator’s output signal (FSK) to channel 2 of the oscilloscope. Study the display in time domain and save the picture into your measurement report.
Explain your findings:
Next you must change the place of the connecting plug so, that the signal goes through the filter. Study the two signals again and save the picture. Does the effect of the filter show as clearly as in the ASK case?
Then feed the signal straight to modulator’s input without the filter and the amplifier. (Take the connecting plug away.) Compare the signals with the signals of the first FSK case. How does the amplitude of the input signal affect to the amplitude of the modulated output signal?
Consider a situation in a transmission channel, when noise will be summarized to the ASK signal and to the FSK signal. Which one will be more easily disturbed? Why?