FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MARA Program: EPO 422
STUDENT KIT TM LAB MODULE Sem : 2
Ownership : Center of Communication Engineering Studies (CoCES) FKE D.id :
Date Issued : 2009
COMMUNICATION ENGINEERING LABORATORY II COURSE CODE: EPO 422 FM TRANSMITTER
Authors Prepared by :
1.Norbaiti Binti Sidik 2.Kamarulzaman Bin Md.Nor
Date : Dec 2009
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Date :
nd
Date :
1 Revision :
2 Revision : Endorsement by Center of Studies Chair :
Signature :
Date :
FM TRANSMITTER
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REQUIREMENTS:
MODULE OUTCOMES: Upon completion of this experiment, students should have the: MO1 : Ability to observe and measure the Crystal Amplifier Oscillator Measurements, Buffer Tunning Measurements, Frequency Deviation of a transmitted signal and RF Amplifier Measurements. MO2 : Understandings of the major sections of an FM radio with a digital display.
INTRODUCTION FM Transmitter The intelligence to be transmitted may be imposed as changes in the frequency being transmitted. This type of modulation is called frequency modulation (FM) and has certain inherent advantages over AM transmission. In FM, intelligence is conveyed by varying the frequency of a constant amplitude RF carrier. The modulating signal varies the carrier frequency to develop the transmitted frequency. The amount of carrier frequency deviation depends on the amplitude of the modulating signal. The louder the sound, the greater the audio amplitude and, therefore, the more the transmitted frequency deviates from its resting carrier frequency. The rate at which the modulated frequency varies from the carrier frequency,- that is, the number of excursions above and below the carrier frequency - depends on the frequency (tone) of the modulating signal. A high frequency tone causes rapid variations of the carrier as compared to a low frequency tone. The amplitude of the transmitted FM wave remains virtually constant. A very important characteristic of FM when compared to AM is that FM is comparatively noise-free. Noise interference, for the most part, is due to sources that are external to the receiver. Noise randomly adds to the amplitude of the RF wave. AM systems are limited in their effectiveness to suppress or eliminate this interference because the noise amplitude variations are demodulated with the intelligence. In an FM system, amplitude variations are clipped off; only changes in frequency are demodulated as the intelligence. This ability of an FM system to reduce noise interference also provides for very good co channel interference rejection. At the receiver, the desired signal need be only twice as large as the undesired signal to suppress the interference completely. Fig. 2-20 illustrates a block diagram of a typical FM transmitter. FM modulation and frequency multipliers were covered in this study. The heart of the FM transmitter is the modulated oscillator. An applied audio signal causes the frequency of this oscillator to change (deviate).
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The audio signal containing an intelligence signal is usually very weak and is amplified by the audio signal amplifier before driving the reactance modulator. The modulator converts the audio signal into variations in reactance. This reactance change is applied to the oscillator, causing the frequency of the oscillator to change. If the audio signal is a sine wave, the frequency out of the modulated oscillator will swing the same amount above and below the center frequency of the oscillator. How far it swings depends on the amplitude of the modulating signal. And how rapid it swings depends on the frequency of the modulating signal. In a commercial FM broadcast system, the Federal Communication Commission (FCC) has established that the maximum deviation shall be limited to plus or minus 75 kHz. An additional ±25 kHz is provided as a guard between stations. This is designated arbitrarily as 100% modulation. In a military FM broadcast system, the frequency deviation allowed may be reduced to plus or minus 40 kHz. In this case, plus or minus 40 kHz is 100% modulation.
Fig. 2-20 Block diagram of an FM transmitter Modulation Assume that Fig. 2-20 represents an FM transmitter system having a plus or minus 75 kHz frequency deviation. Assume that the output from the modulated oscillator is 5 MHz with a frequency deviation of 4.1 kHz. Further, assume that the transmitter has been assigned an output frequency of 90 MHz. The most common method of getting to 90 MHz from the 5 MHz oscillator would be to use two frequency triplers and a doubler. The total multiplying effect by which the oscillator frequency is increased is 18 times (18 x 5 = 90MHz). The deviation of the oscillator, to produce ±75 kHz, is determined by dividing 75 kHz by 18. This yields 4.167 kHz, which would be 100% modulation of the oscillator. The multiplier multiples both the carrier frequency (5 MHz) and the deviation (4.1 kHz). A frequency multiplier is nothing more than an amplifier, usually operated Class C, with the output tuned to a harmonic of the input. Operating these stages class C eliminates any amplitude variations that may be caused by the modulator. The driver is an intermediate amplifier used to develop the power necessary to drive the RF output power amplifier. The power amplifiers are usually heavy current devices and they require a large input signal. The power amplifier increases the power of the RF signal so that the antenna can radiate it. The antenna produces an electromagnetic wave, which is directly 3
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proportional to the amplifier current. An important fact to remember is that in FM a wide bandwidth is necessary. Whether operated with a plus or minus 75 kHz or plus or minus 40 kHz deviation, the driver, power amplifier and the antenna must have a bandwidth wide enough to pass all of the desired sidebands. Experimental FM Transmitter The laboratory experimental module KL-93063 consists of a three-stage, crystal-controlled transmitter. Fig. 2-21 shows the circuit diagram, which consists of the following sections. a. Crystal oscillator, Q1 b. Varactor (capacitive) modulator, D1 c. Buffer-doubler, Q2 d. RF amplifier, Q3 e. Audio amplifier, 1C1 -Q4 f. FM deviation detector, 1C2-Q5 The crystal-controlled oscillator operates at a frequency of 3.579545 MHz, which can be shifted (warped) by means of an electronically controlled capacitor called a varactor or varicap. Changing of the dc voltage on the varicap (D1) causes a change in its capacity which is in series with the crystal, and this capacity change will cause the oscillator frequency to shift. The frequency can be remotely controlled since only dc is required on the varactor. However, if an audio voltage is also applied to the dc level at TP1 the varicap's capacity will shift at an audio rate and the crystal will become reactance (capacitive) modulated. The level of audio voltage determines the amount of deviation; the typical crystal can be warped by a voltage over a range of approximately 200 to 600 Hz (narrow-band FIVI). Transistor Q2 is wired into a buffer circuit. This circuit is designed to act as a straight-through amplifier tuned to the crystal frequency, a doubler or a tripler. As a doubler, the RF output is twice the crystal frequency (approximately 7.150 MHz). As a tripler, the RF output is 10.7386 MHz. The buffer output feeds RF amplifier Q3. This amplifier, like the buffer, is capable of operating as a doubler or a tripler. Capacitors C4 and C7 provide the tuning of the two RF tank circuits. The buffer is inductively coupled to the second RF amplifier by transformer T1. The modulator consists of an audio amplifier (ICl-Q4), which can be used to modulate the oscillator. An audio signal inserted into the input jack will provide narrow band FM transmission. This can be received by an FM receiver, such as KL-93064. During FM modulation, the frequency deviation of the carrier is multiplied by the buffer andlor RF output amplifier. If the buffer stage is used as a tripler and the output amplifier does not multiply, a 400 Hz frequency change at the crystal will appear as 1200 Hz at the antenna output of Q3. In this transmitter, it is intended that the RF output to the antenna be in the range of 10.7 MHz, a frequency which can be detected by the frequency-deviation meter or an FM receiver. Since FM receivers operate with an intermediate IF frequency of 10.7 MHz, the transmitter will be heard by an FM receiver placed near by. If the FM receiver is tuned across the dial, the transmitter will be heard at 10.7 MHz intervals. Although the transmitter was designed to provide an RF output at 10.74 MHz, it could just as easily produce an RF output in the 7 MHz range. When the oscillator is modulated at this frequency, the frequency deviation is 800 Hz (2 x 400 Hz).
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The circuit of the deviation meter consists of a crystal-controlled reference oscillator Q5, (3.579545 MHz) and a tachometer 1C2. The tachometer registers the deviation of the transmitter's crystal oscillator from its established frequency. When the frequency-adjust control VR1 is moved, the differential signal voltage at the tachometer output indicates the amount of frequency deviation present. The frequency of the tachometer's reference oscillator is adjustable by capacitor C28. This capacitor is adjusted so that when VR1 is at midrange (approximately 7.5V at TP1), the deviation meter reads zero. The frequency of the reference oscillator is adjusted to 3.579545 MHz. The reference oscillator and the transmitter's RF output at the antenna both feed the tachometer through a diode and differentiator circuit. When VIR1 is rotated in either direction, the voltage across R20, a 100K resistor in the tachometer circuit is a series of pulses between +0.5 to 1 V. As VIR1 is moved the number of pulses will increase or decrease. The output of the tachometer at TP9 audio output is a series of square waves whose frequency is directly related to the differential frequency between the reference oscillator and the RF carrier of the transmitter. The square-wave pulses are rectified by diodes D3 and D4, and the meter sees a current which varies linearly with frequency. The reference oscillator and the tachometer serve as a sensitive replacement for a discriminator detector. This circuitry is required to convert a frequency change deviation into a voltage or current change. The linearity of the meter scale is determined by components C37 and R22. The amount of deviation is shown on a deviation meter.
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FM TRANSMITTER
Crystal Oscillator 3.58 MHz
Doubler /Tripler
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RF Amplifier
Frequency Adjust (D1)
Audio amplifier
Reference Oscillator
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Frequency Tachometer to show frequency deviation
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EXPERIMENTAL WORK AND EXERCISES: TITLE: FM TRANSMITTER OJECTIVES:
Upon completion of this experiment, students should be able to: 1. Observe and measure the Crystal Amplifier Oscillator Measurements, Buffer Tunning Measurements, Frequency Deviation of a transmitted signal and RF Amplifier Measurements. 2. Understand the major sections of an FM radio with a digital display. EQUIPMENT REQUIRED •
FM Transmitter Module KL-93063
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DC Supply Module Cl-18001
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Dual-Trace Oscilloscope Hint: with FFT function of spectrum analyzer
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Function Generator
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Frequency counter (7 or 8 digits)
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Digital Multimeter (DMM)
Crystal Oscillator Measurements 1 Connect an oscilloscope and a counter to TP2 and record the frequency when the FREQUENCY ADJUST VR1 is set to midrange (approximately 7V at TP1). Fo = _____________MHz. What type of wave shape is observed?________________ Hint: it is easier to observe by FFT function of the oscilloscope of spectrum analyzer.
2. Rotate the FREQUENCY ADJUST knob (VR1) first fully clockwise and then fully counterclockwise. Record the frequency meter change in hertz. Fmax = ___________MHz, fmin = ____________MHz. Record the total frequency change. The frequency change is____________Hz.
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3. Plot a curve of the frequency change versus the dc voltage to D1 (TP1). Use increments of 1.0 V. Record the frequency at TP2.
DCV 12
FREQ. ______
DCV 7
FREQ. ______
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6
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10
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5
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9
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4
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8
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3
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Voltage vs frequency
4. What is the maximum obtainable deviation of the oscillator? fd =_____________Hz. Buffer Amplifier Tuning Measurements 5. Set the scope to FFT mode (1.25Mhz) and connect it to TP4. Vary VR1 at midrange (6V). Tuned C4 and T1 through entire range until maximum db Record and save the FFT waveform of the buffer double and triple frequencies. Deviation Measurements In the following measurement, the module's deviation meter will measure the amount of deviation of the transmitter. Since the transmitter is reactance-modulated, the output frequency indicated on the deviation meter will be compared with the input voltage change. The output of the tachometer is a constant voltage whose frequency varies directly with a frequency shift of the carrier oscillator. This output voltage can be used to provide automatic frequency control (AFC) or automatic fine tuning (AFT). Either of these may be used in a FM receiver. This varicap voltage can also provide for frequency selection in a radio or a television set.
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Note: You need to do the calibration first. 1.Set MOD/OSC switch to MOD position. 2. Adjust the screw on the tachometer so that the output is 0KHz. 3. Set MOD/OSC switch to OSC pos. 4. Set VR1 to 6V. 5. Adj, C28 until you see the tachometer stop at 1KHz.( use 1KHz as starting point,so any value read from the tachometer need to subtract 1KHZ. 6.Connect the counter to TP6 and adjust C4 and C7 for a 10.7 MHz RF output at the antenna output. _______________________ 7.Set VR1 to its midrange. Using this setting as the starting point, rotate VR1 between -max and +max. Record the meter frequency versus the dc modulation voltage at TP1. Complete the table. TP1 (V) 8 9 10 11 12
Meter Deviation (Hz)
TP1 (V) 7 6 5 4 3
Meter Deviation (Hz)
As previously observed, a change in the voltage of VR1 represents a shift in hertz of the oscillator. In view of this fact, the voltage output of the tachometer is directly affected by a shift in frequency. The total change in frequency deviation is: Compare your results the reading from Tachmeter and Frquency Counter.
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REVIEW QUESTIONS
1. In an FM transmitter, the output of the modulated oscillator is coupled to the: ___________a. driver
___________ c. power amplifier
___________b. reactance
___________ d. frequency doubler
2.Frequency-modulated systems are relatively noise free because noise primarily produces amplitude variations in the RF stage. (True or False) 3.An oscillator with a center frequency of 6 MHz is deviated 3.5 kHz. The signal is coupled to two doublers and a tripler. What is the output frequency and amount of frequency deviation? a. Output frequency___________ b. Frequency deviation_________
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What is the function of the driver?
a. A stage, operated class C, whose output is tuned to a harmonic of its input. b. A stage that controls antenna rotation. c. An intermediate amplifier used to develop the power to drive the power amplifier. d. An amplifier used to develop the power to be radiated by the antenna.
5. The function of frequency multipliers is to: a. Increase the frequency deviation only. b. Increase both the frequency of the carrier and the amount of deviation. c.
Increase the frequency level of the carrier, but not change the amount of deviation.
d. Increase the carrier frequency 2.5 times.
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The purpose of the reactance modulator is to.
a. Convert changes in audio into changes in reactance. b. Convert changes in reactance into audio. c. Convert frequency.,changes into reactances. d. Convert frequency changes into audio.
7. The amount of frequency deviation is determined by the audio amplitude . (True or False)________ 8. The two forms of FM modulation are__________and 9. The commercial bandwidth of FM is____________kHz. 10. Commercial FM broadcasting operates in the frequency range of _________ to ________ MHz.
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PRE-LAB WORK: NONE
CONCLUSION: Conclude in detail the findings and the outcomes of this experiment. This should include the summary of knowledge gained, comments and discussion of the results, errors and their possible sources and how this experiment can be improved.
REFERENCES: References :
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LAB REPORT MARK AND FEEDBACK SHEET GROUP:
ASSESSOR:
STUDENTS’ NAME:
STUDENTS’ ID NO:
GROUP:
PROGRAMME CODE:
EXPERIMENT DATE:
LAB INSTRUCTOR:
PRE-LAB WORK (0%)
NONE
RESULTS (40%)
DISCUSSIONS (40%)
CONCLUSION (20%)
TOTAL MARKS (100%)
COMMENTS
Signed (Assessor):
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Date: ……………………………
