Title : Frequency Modulation - Demodulation Demodulation & Phase Modulation Modulation Objectives : 1. To understand the concept of frequency modulation (FM) and phase modulation (PM). 2. To learn the working mechanism of DL 2501 and DL 2513C boards. Introduction : Frequency and Phase Modulation
Information can be transmitted by adequately varying (modulating) one or more parameters of a carrier signal. These parameters are amplitude, frequency, and phase. When the frequency of a carrier signal is varied in a system with respect to the modulating signal variations, frequency modulation (FM) is obtained. Similarly, by varying the phase of carrier signal, so-called phase modulation (PM) is obtained. FM and PM are not independent, since the frequency cannot be varied independently from the signal phase, and vice versa. The selection of the frequency or phase modulation for a given application in the communication field is almost fully depends on the reception methods that are required to be used. In fact, the generation and the processing of phase or frequency modulated signals is essentially similar, while the reception methods required for information retrieval are significantly different.Both FM and PM systems offer the advantage of high noise immunity. This is especially true for pulsed noises, provided that the receivers are designed to be insensitive to amplitude variations.
Apparatus : 1. 2. 3. 4.
1 DL 2501 board 1 DL 2513C board 1 DL 2555AL power supply module 1 function generator
Procedures : Study of the Varicap FM Modulator
1. The board is connected to the power supply. The current’s polarity is aware. 2. The output of the audio generator is connected to the input of the modulator using a short cable. The oscilloscope is connected with the probe ground connected to the lower terminal of the secondary winding of L1. The probe must be connected to upper terminal. 3. It is suggested that the same lower terminal of the secondary winding of L1 is grounded, to provide a better protection against noises, therefore obtaining a sharper display on the oscilloscope. 4. The amplitude of the connected audio signal is set to a minimum and the power supply is switched on. The oscilloscope is adjusted until a sharp display is obtained. 5. The amplitude of the frequency generator is gradually increased to obtain a clear display of the frequency modulated signal. The shadowed area on the oscilloscope is composed of a bundle of different sine wave which is all synchronized from the left end of the display. 6. The bundle amplitude corresponds to the frequency deviation of the modulator. The latter, on its turn, depends on the amplitude of the connected modulating signal. 7. It can be immediately checked that the varying the amplitude of the modulating signal the span of the bundle is proportionally varied. 8. At high modulation levels a small amplitude modulation normally occurs, superposed to the high frequency modulation. 9. The maximization criterion of the measurable signal is at the expense of modulation linearity. A square wave of about 500 Hz with an amplitude variable between 0 and about 8 V peak to peak is connected to the “AF IN” in order to measure the modulator linearity. 10. The reason of using a rectangular waveform instead of a sine wave in this experiment is that sine waves corresponding to the landing levels of the modulating signal can be better displayed on the oscilloscope screen. Therefore the frequency measurement can be made easier. 11. The external square wave generator and the oscilloscope are connected to the output of the modulator. A signal of amplitude variable is connected with 500 mV steps from 0 to 8V peak-to-peak. The output frequency deviation is measured. 12. The frequency deviation can be measured by adjusting the time scale of the oscilloscope so that at least 3 full cycles are displayed on the oscilloscope. The bindle span in correspondence of the third period is detected and divided by 3. 13. The difference is therefore obtained between the maximum and minimum frequencies and the frequency was calculated. The reason why it is recommended to measure the frequency deviation using 3 or more cycles is higher accuracy solution can be obtained. Study of Phase Modulator
1. The tuned load L1 is excited by the current C3. To this load the current is quadrature, of controllable intensity, and can be subtracted (or added, as a function of the references). 2. The current in quadrature is exactly equal to the current that could be subtracted (or added) to a reactive component of value variable according to the modulating signal, in parallel connected to L1.
3. The tuned load appears therefore the “detune” at the rate of the modulating signal. The operating point of the circuit moves along the “bell” shaped curve of the tuned circuit and the variable phase shifts results for the voltage signal. 4. The proposed exercise consists of functional checking of the circuit. As carrier generators, the signal generated by the varicap modulator oscillator can be used. 5. The output of the transformer L1 and the varicap modulator was connected to the ground (lower terminal) and to the input marked “RF IN” (upper terminal), respectively. 6. The ground of the oscilloscope probes was connected with the probe for channel 1 to the input terminal (RF IN) and the probe for channel 2 to the output terminal (RF OUT). 7. The ground of the oscilloscope probes was connected to the same ground terminal of the phase modulator circuit. 8. It is suggested to use short and rational connections for the measurements to be accurately performed. 9. The output of the low frequency audio generator must be connected to the input labelled AF IN of the phase modulator. 10. The equipment synchronize the oscilloscope was switched on using the RF input signal of the modulator. 11. By varying the amplitude of the modulating signal, the corresponding phase variation on the output signal of the modulator can be measured. 12. In the reactance modulator, very often an amplitude modulation of the signal occurs being the DRAIN tuned circuit “detuned” by the out of phase currents appearing as generated by reactive components in parallel connected to it. 13. As a function of the quality factor Q of the oscillating circuit a corresponding percent phase variation occurs at a parity of modulating signal, and also a corresponding amplitude modulation. 14. The phase modulator of DL 2501 is sized and adjusted to generate phase variations of great amount, neglecting linearity features. This is to provide significant and easily measurable deviations. The linearity characteristic of the modulator is measured by connecting a DC signal to the AF IN input rather than the modulating signal from the frequency generator. 15. By varying the DC signal between -6 and +6V, the consequent phase variation of the output signal can be detected. 16. The same band measurements are used as performed in the varicap modulator. 17. The measurement has to be repeated for different amplitude values of the input signal and at a parity of amplitude for different values of modulating frequency. Study of Elementary FM Demodulator
1. The elementary FM demodulator on board DL 2501 consists of an amplifier-limiter whose function is to limit the amplitude of the frequency modulated signals connected to the input. The set of C1 and R1 is a derivative circuit that generates across R1 constant area pulses for each transition of the input FM signal of the amplifier. 2. The positive peaks of this derivative signal are sent through the diode D1 to the low-pass filter whose function is essentially to make an average of the pulsed signal. A slowly variable signal is obtained whose level depends on the rate at which the equal area pulses occur on the input side. 3. The detector of DL2501 board must operate in a frequency range between 500 kHz and 750 kHz and has to be able to provide an audio si gnal of adequate amplitude for relatively small variations of the input FM signal frequency.
4. The functional study of the elementary demodulator is performed by connecting its input to the output of the varicap modulator and the signal from the audio generator on board DL 2501 as the modulating signal. 5. The oscilloscope probe is connected between the output terminals of the low-pass filter of the demodulator and ground. The demodulated signal can be displayed. 6. When the amplitude levels of the modulating signals are low, the waveform is reconstructed with fidelity. When the amplitude is increased, deformations and linearity losses occur, due to linearity loss in the modulator and output filter of the demodulator, because of excessive frequency variation bringing the operating point outside the linear zone of the filter falling edge.
Results : Signal/
Volt/div
Time/div
Voltage,
Period, T
Frequency, f
Wavelength, λ
Parameter
(V/div)
(ms/div)
V (V)
(ms)
(Hz)
(m)
VM
0.20
1.000
5.2
8.000
125.00
2.4(10 )
VFM
0.05
0.001
0.7
0.008
1.25(10 )
2400.0
0.50
0.005
11.0
0.004
2.50(10 5)
1200.0
0.05
0.500
1.0
4.000
250.00
1.2(10 6)
Signal through Demodulator (before diode) Signal through Demodulator (after diode) Table 1
Signal through Modulating Signal Generator (Vm )
Signal through Varactor Reactance Modulator (VFM )
Signal through Demodulator
Signal through Demodulator (after
(before diode)
diode)
Signal through Phase Modulator for channel 1 (VC)
Signal through Phase Modulator channel add, (VPM)
Signal through Phase Modulator for channel 2, (VM)
Discussion (Questions): Study of the Varicap FM Modulator
a) The frequency deviation for FM. ∆f = 125 Hz – 120Hz =5 Hz The modulation index for FM can be calculated by using formula. M = ∆f / f m=5 / 125=0.04 where : f frequency deviation f m modulating frequency b) Obtain the numerical results in tabular form and plot a graph. For the graph, the horizontal axis is the peak-to-peak voltages of the modulating signal, while the vertical axis is the frequency deviations measured on the modulated signal. The graph that we should obtain is somewhat like below, but as we doesn’t varies the value, the graph is likely to be constant
The basic principle behind FM is that the amplitude of an analog baseband signal can be represented by a slightly different frequency of the carrier. The modulation index affects the modulated sinusoid in that the larger the modulation index, the greater the instantaneous frequency can be from the carrier. The instantaneous frequency range of the modulated signal is much smaller with a smaller FM deviation. c) Comment the results. The most interesting result is a flattening of the modulation characteristics of the circuit in correspondence of the higher amplitude values of the modulating signal. The most interesting result is a flattening of the modulation characteristics of the circuit in correspondence of the higher amplitude values of the modulating si gnal.
From the result that generated by the machine, signal through modulating signal generator, Vm has more high in frequency but lower in the amplitude compared to the signal through reactance modulator, V FM that has the result in vice versa.
Study of Phase Modulator. a) Differences between FM and PM
FM
PM
Amplitude of modulated wave kept fixed, but frequency is varied by modulating signal. The spectrum is more complex, not easy to analyses. In digital oscillator, FM was added to the frequency before the phase integration. Modulation index,
Amplitude modulated wave kept fix, but phase was shifted by the modulation signal. The spectrum is clear and easy to analyses. In digital oscillator, PM was added to the phase after the phase integration [1]. Modulation index, ,
b) Function of varicap modulator Varicap is actually known as varactor diode. It is a diode that has variable capacitance which is a function of the voltage that is impressed on its terminals. The varactor diode are operated reversed-biased, and therefore no current flows.
Study of Elementary FM Demodulator 1. What is the significance of having low-pass filter compared to high-pass filter? A low pass filter only allows signals with low frequencies to go through and a high pass filter only allows high frequency signals to go through. A high-pass filter allows for easy passage of high-frequency signals from source to load, and difficult passage of low-frequency signals.
2. What is the contribution of the FM demodulator to the aircraft system? FM signals can only travel as far as the horizon, which has the advantage of reducing interference, and coverage is therefore more stable than with AM. It is used in aircraft system as to communicate with air traffic tower since it is more stable and less interference. Therefore, the information can be transmitted and processed quickly. Conclusions :
As a conclusion, the following objectives are achieved: 1. Students are able to understand the concepts of frequency modulation (FM) and phase modulation (PM). 2. Students are to learn the working mechanism of DL 2501 and DL 2513C boards.
References : 1. FM Demodulation/Detection. Retrieved May 25, 2015, from Radio -Electronics.com.: http://www.radio-electronics.com/info/rf-technology-design/fm-reception/fmdemodulation-detection-overview.php 2. What is FM: Frequency Modulation Tutorial, Poole, I., (n.d.).Retrieved 1st May 2015 from http://www.radio-electronics.com/info/rf-technology-design/fm-f requency-modulation/whatis-fmtutorial.php 3. What is PM, Phase Modulation, Poole, I., (n.d.). Retrieved 1stMay 2015 from http://www.radio-electronics.com/info/rf-technology-design/pm- phase-modulation/what is pmtutorial.php 4. Varactor Diode Modulator. Retrieved May 25, 2015, from DAEnotes: http://www.daenotes.com/electronics/communication-system/varactor-diode-modulator 5. Filters. Retrieved May 25, 2015, from DAFX: Digital Audio Effects: http://www.music.mcgill.ca/~ich/classes/FiltersChap2.pdf
