Lab-Report Analogue Communications. Frequency Modulation

Transcription

1 Lab-Report Analogue Communications Frequency Modulation Name: Dirk Becker Course: BEng Group: A Student No.: Date: 0/Mar/999

2 . Contents. CONENS. INRODUCION 3 3. FREQUENCY MODULAION SYSEMS 3 4. LAB PROCEDURE 4 a) Lab equipment 4 b) Carrier requency oscillator 5 c) Determination o L and C 6 d) Carrier requency variation with varicap diode 7 i. Frequency vs. Varicap voltage 7 ii. Amplitude vs. Varicap voltage 8 5. MODULAOR OPERAION 9 6. CONCLUSION 9 7. RELAED INERNE SIES (HISORY OF FM) 9

3 . Introduction Frequency modulation was irst introduced by Edwin Howard Armstrong in the early 930s. It took several years beore the advantages o FM were ully discovered. Frequency modulation needs a large bandwidth compared with Amplitude Modulation when good results should be obtained. Frequency modulation is widely used since the late 940s. he dierence between Amplitude modulation is, that instead o varying the amplitude o a carrier the carrier requency itsel is shited and carries the inormation. he amplitude o the carrier is held constant. So at the receiver the waves can be ampliied very much, clipped and any distortion in amplitude will not cause hearable distortions he SNR o FM is much better than the one o AM. 3. Frequency Modulation Systems A simple requency modulation system is shown in the ollowing igure. he let part o the circuit has got a ixed resonant requency 0. By applying the right part o the circuit it is possible to change this requency 0 by either changing the value o C or the varicap diode by applying a suitable voltage to it. he change o a requency according to an applied signal is known as requency modulation. he Lab should give an introduction to these requency modulation systems. 3

4 4. Lab procedure a) Lab equipment he ollowing igure shows a diagram o the used Lab equipment. Oscilloscope Unaohm AC Millivoltmeter hurlby 30V DC Supply osc CRO (Modulation and Carrier generation circuit) osc Hz (modulation requency) 0-30V Kikusui Frequency Counter Unaohm Function Generator he CRO unit generates the carries requency and contains a built in varicap diode and a calibrated capacitor with which the carrier requency can be either manually or electronically be changed by applying a voltage to the varicap. A hurlby 30V DC power supply and a Unaohm requency generator can generate an appropriate voltage thereore. he carrier requency could be checked in it s requency and amplitude by means o an oscilloscope, a counter and an AC Millivoltmeter. he attenuator o the counter had to be enabled because otherwise the counter is a load or the CRO and the output voltage and requency could not be read out exactly. As gate time 0.s was used, in order to readout the measured requencies properly. Counter are very exact electronic measuring instruments. hey have usually errors in the area o <0-5 rom the measured value. 4

5 b) Carrier requency oscillator First part o the Lab was to change the calibrated capacitor and to obtain a plot o the carrier (resonant) requency vs. the parallel capacitance. C/pF /khz 6 38, ,6 5 88, , , 40 45, , , Variation o the Capacitance in the LCR circuit oscillation requency / khz capacitance / pf he variance o the capacitance C results in a nearly linear variance o the resonance requency. 5

6 c) Determination o L and C With the dierent measured values o C and o it is possible to determine the values o C and L C C C L' (C L'(C C 35pF L' (C + C ) 0 + C L' (C (C C + C L' 53.4µ H C) + C + C 0 C 0 0 ) ) ) 0 (C + C ) + C 50pF (085kHz) (347kHz) 0 L' L' 0 (C + C ) 0 (C + C ) 0 0pF (347kHz) (085kHz) (35pF + 50pF)x(085kHz) 0 he error o this calculated values depends on the precision o the measured values o 0 and the precision o the calibrated capacitor. 6

7 d) Carrier requency variation with varicap diode Varicap diodes are semiconductors, voltage dependent, variable capacitors. heir mode o operation depends on the capacitance that exists at the p-n junction when the element is reverse biased. Under reverse bias conditions there is a region o uncovered charge on either side o the junction which leads to a capacitance. he Varicap was used instead o the calibrated capacitor. A plot o the voltage applied to the Varicap vs. requency and amplitude was obtained. i. Frequency vs. Varicap voltage Vd/V /khz 0 05,6 0,5 4,3 0,33,5 5,73 30,46,5 34, , , ,35 8 9,6 0 0,07 5 8,67 0 8, ,8 30 4,65 requency over varicap diode voltage requency / khz varicap diode voltage Vd / V 7

8 ii. Amplitude vs. Varicap voltage Vd Vo 0 0,95 0,5 0,95,075,5,5,5,5,35 3,4 4,5 6,58 8,6 0,6 5,6 0,6 5,6 30,6 ouput voltage over varicap diode voltage,8,6,4, ouput voltage Vo / V 0,8 0,6 0,4 0, varicap diode voltage Vd / V he output voltage is not at a stable level when only small voltages (low requencies) are applied to the varicap. his results rom the inner structure o the CRO. 8

9 5. Modulator operation When a signal with appropriate amplitude and requency is applied to the varicap the CRO circuit works as a requency modulator. he modulation requency was a Hz sinewave with 8.8V oset and an amplitude o 5. volts. he resulting modulated requency deviated between ~80KHz and 7kHz. When the oset voltage was to low, the CRO generated amplitude modulation, because the CRO was not able to supply a stable output voltage below V D 5V. With the low modulating requency o only Hz the modulating output could be obtained very well on the oscilloscope. 6. Conclusion he value o C must be in the area o C, because otherwise the carrier requency can not be varied very much. With the values o C and C the value o L is predetermined by the equation or the resonant requency. he capacitance o the varicap must be in the same area than the capacitance o the calibrated capacitor C, to obtain the same resulting requencies (ca. 00kHz-400kHz). High quality FM modulators must have a high linearity to prevent distortions. Also the amplitude o the output must be held constant, because no amplitude modulation must occur. 7. Related Internet Sites (History o FM) Dirk Becker, dirk.becker@gmx.de, 9