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Master 1er semestre

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MODULATION & DEMODULATION

rien 1. Objectives

Modulation and demodulation are used in many kinds of data transmission, both analogue and digital. The choice of one type of modulation is based on bandwidth and signal-to-noise ratio.

The base-band signal is seldom send directly in the transmission channel. It can be processed several times, going through encoding and/or modulation. By example, in radio broadcasting, the LF audio signal is modulating an IF carrier (Intermediate Frequency) of fixed frequency, the result is then shifted at the RF (Radio Frequency) of the emitter. The frequency range for RF transmission is very broad, going from 100 kHz to 30 GHz.

At the receiver side, the received RF signal is shifted back at the IF, where it is demodulated to recover the base-band signal.

Frequency shifting of a signal is made by multiplying it with a sine carrier at a fixed frequency equal to the desired frequency offset. This is done by a balanced mixer. A filter then removes unwanted spectral components.

The following figures show the principal of AM radio transmitter and receiver with dual frequency shift.


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In this lab work, we will study: - AM (Amplitude Modulation) modulation and demodulation - SSB (Single Side Band) modulation and demodulation - FM (Frequency Modulation) modulation and demodulation

2. Devices to study

- One type of Local Oscillator using LC elements - AM modulation with carrier using a balanced mixer - AM demodulation by envelope detector - AM demodulation by inverse modulation with a synchronous carrier - SSB modulation (AM modulation without carrier and side band filtering) - SSB demodulation by inverse modulation - FM modulation of an oscillator using a varicap - FM demodulation with PLL - Le FM demodulation with quadrature detector

Convention:

U1 : modulating base-band signal Up : carrier without modulation U2 : modulated signal U3 : recovered base-band signal after demodulation


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3. Bibliography

[1] Traité d’électricité Vol.XVIII: système de télécommunications, Prof. P.-G. Fontoillet, PPR.

[2] Circuits et systèmes électroniques, Prof. M. Kayal, EPFL. [3] Data sheet on the intranet.

4. The oscillator

The oscillator is a key element in all radio circuits. It is used to generate the carrier signals, and is often called Local Oscillator (LO).

4.1 Principle schematics

4.2 Data C = 330 pF L = 330 µH

4.3 Theoretical forecasts

4.3.1 Calculate the theoretical frequency of oscillation.

4.4 Measurements

4.4.1 To avoid to disturb the circuit with the parasitic capacitance of the measurement cables, always use 10x scope probe. With the spectrum analyser, use a passive 10x probe connected to a 1 MΩ input, if available, or an active probe connected to a standard 50 Ω input.

4.4.2 Observe the generated carrier, measure its frequency. Comment.

In all these experiments, the purity of the local oscillator is not critical, hence the use of an oscilator with an output signal more "rounded trapezoidal" than sinusoidal. In the AM experiment, the accuracy and the stability of the carrier's frequency are of little importance. In the SSB experiment, the accuracy and the stability of both the transmitter's and receiver's oscillators are critical, hence the use of external lab generators.


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5. Amplitude modulation AM

AM is often made by using a balanced mixer to multiply the base-band signal with the carrier. A component at carrier frequency is obtained by unbalancing the mixer or by adding a DC component to the base-band signal.


The mixer used is a Gilbert cell. The carrier is applied to the dual upper differential pair wich are non-linear. That is why the purity of the carrier is of little importance. The base-band signal is applied to the lower degenerated differential pair wich has a linear transfer curve.

5.2 Data

f1am = 300 Hz to 3400 Hz

fpam ≅ 455 kHz (not critical)


5.3.1 Explain shortly how this circuit works.

5.3.2 Sketch signals U1am, Upam and U2am.

5.3.3 Sketch the spectrum of these signals.

5.4 Measurements


5.4.2 Observe the following signals and their spectrum: U1am, Upam and U2am.

5.4.3 Measure the modulation factor.


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6 AM demodulation by envelope detection (A)


This demodulator is quite simple, it is a peak detector with a rectifying diode, a storage capacitor and a resistor for slow discharge. A low output impedance amplifier allows the fast charge of C. The low-pass filter removes the ripple at high frequency.

6.2 Data

U’2ama ≈ 9 VDC + 4 Vp-p fpama ≈ 455 kHz


6.3.1 Explain how this circuit works. Sketch signals U’2ama and U3ama. Explain the usefulness of the large DC voltage (compared to the HF amplitude) at the input of the peak detector.

6.3.2 Calculate R and C to obtain a ripple on U3ama of less than 5%.

6.3.3 How are chosen R' and C' ?

6.3.4 This demodulator works only for modulation factor lower than 1. Explain why.

6.3.4 How are related the signal-to-noise ratios before and after demodulation ?

6.4. Measurements


6.4.2 Observe following signals and their spectrum U2am, U’2ama, U3ama and U’3ama.

6.4.3 Measure the ripple on signals U3ama and U’3ama.

6.4.4 Observe the effect of overmodulation (modulation factor > 1).

6.4.5 Test the transmission of a real audio signal.


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7. AM synchronous demodulation (B)


An recovered carrier U'pam of constant amplitude and phase-synchronous with the received carrier is obtained by amplifying and limiting the modulated signal. The demodulation is then obtained by inverse modulation.

7.2 Data

U’2amb ≈ 1.3 Vp-p fpamb ≈ 455 kHz U’pamb ≈ 1.4 Vp-p


7.3.1 If the recovered carrier has a phase lag of 30° on the received carrier, sketch the signals U’2amb, U’pamb, U3amb and U’3amb.

7.3.2 What is the cause of this phase lag ? What is the effect of this phase lag on the amplitude of the demodulated signal ?

7.4 Measurements


7.4.2 Observe the followings signals and their spectrum: U1am, U2am, U3amb and U’3amb.

7.4.3 Observe the effect of an overmodulation (m > 1). Propose a solution to demodulate such a signal.


7.4.5 Compare these two methods of AM demodulation.


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8. SSB modulation by amplitude modulation without carrier and

side band filtering


SSB modulation is made by multiplying the carrier by the base-band signal in a balanced modulator. If the base-band signal has no DC component and the mixer is perfectly balanced, the spectrum of the modulated signal has no component at the carrier frequency but only two symmetrical side bands, from witch, one is selected by a sharp band-pass filter, the other one being attenuated as much as possible.

Because of the accuracy and stability requirements on the carrier frequency, a lab generator will be used to provide it.

8.2 Data

f1ssb = 300 Hz à 2700 Hz

fpssb ≅ 455 kHz precise value depends of the filter's performances. Because of the accuracy and stability requirements on the frequency of the local oscillator, a lab generator will be used.

Upssb ≅ 0.1 VRMS


8.3.1 Give the pro and con of the SSB modulation versus the others.

8.3.2 Explain how this circuit works.

8.3.3 Sketch the signals U1ssb, Upssb and U2am-p.

8.3.4 Sketch the spectrum of the signals U1ssb, Upssb, U2am-p and U'2ssb.

8.3.5 Explain the severe requirements on the filter.


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8.4 Measurements


8.4.2 With the filter not yet connected, measure the signal U2am-p and its spectrum, and adjust the pot BALANCE MOD to minimize the component at the carrier frequency

8.4.3 Using the spectrum analyser and its tracking generator as a network analyser, measure the frequency response of the filter. Quartz filters are very sensitive to source and load impedances. Be careful and use 2 kΩ source and load resistances.

8.4.4 Connect U2am-p to the input of the filter, and its output to to U’2ssb, adequate source and load impedances are given by the preceding and the following stages. With a base-band signal U1ssb at the minimum frequency (300Hz), observe the spectrum of U’2ssb and adjust the carrier's frequency to have the best possible attenuation of the lower side band without attenuating the upper side band of more than 1 dB. Comment.

9. SSB demodulation by inverse modulation


Inverse modulation is made by multiplying the modulated signal with a local carrier at the same frequency than the one used to generate the SSB signal. The base-band signal is then recovered by a low-pass filter. Remember that the oscillators used at the emitter and at the receiver are not synchronous and thus can never be exactly at the same frequency, and are not phase locked.

Because of the accuracy and stability requirements on the frequency of the local oscillator, a lab generator will be used.


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9.2 Data

f’pssb = fpssb

U'pssb ≅ 0.1 VRMS


9.3.1 Design the RC low-pass filter at the output of the mixer, knowing it as an internal 2.7 kΩ output resistance.

9.3.2 Sketch spectrum of signals U'2ssb et U3ssb.


9.3.4 What is the result on the recovered base-band signal of the difference between the transmitted frequency and the local oscillator in the receiver ?

9.4 Measurements


9.4.2 Adjust f’pssb as equal as possible to fpssb.

9.4.3 Observe the signal U’3ssb and its spectrum when U1ssb is a sinus at 300 Hz, than at 1 kHz. Comment the distortions and their causes.

9.4.4 Test the transmission of a real audio signal. Test the effect of a slight difference between emitter and receiver local oscillator frequencies


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10. Frequency modulation (FM)


Frequency modulation is made by controlling directly the frequency of an oscillator (VCO) with the base-band signal. In a LC oscillator, the frequency control is obtained by making a part of C with a varicap witch junction capacitance is function of the inverse voltage. For small variations the oscillations frequency offset is proportional to the voltage of the base-band signal.

10.2 Data

Varicap: BB204

L = 1 µH fpfm = 10.7 MHz

f1fm = 50 Hz à 15 kHz ∆fpfm = ± 75 kHz


10.3.1 Calculate the modulation factor and the maximum bandwidth B2fm covered by signal U2fm.

10.3.2 Using the data sheets of VARICAP BB204 with an inverse DC voltage of 4 VDC estimate the amplitude of U1fm and the capacitor needed to obtain the desired centre frequency and the frequency variation.

10.4 Measurements


10.4.2 With zero base-band signal, adjust C to have oscillation at 10.7 MHz.

10.4.3 With a sinus base-band signal at 800 Hz, measure the spectrum of signal U2fm and adjust the amplitude of the modulating signal to have the desired frequency span ∆fpfm and bandwidth B2fm.


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10.4.4 Why is centre frequency drifting when the amplitude of the modulating signal is increased ? Readjust C to have a centre frequency at 10.7 MHz.

10.4.5 Look at the spectrum of signal U2fm as function of the frequency of the base-band signal.

11. FM demodulation with PLL

If locked on the FM received signal, a PLL with linear VCO makes the VCO input voltage proportional to the frequency, and thus to the base-band signal.

11.1 Principle schematics rien

If locked on the FM received signal, a PLL with linear VCO makes the VCO input voltage proportional to the frequency, and thus to the base-band signal.

11.2 Data

fpfm = 10.7 MHz ∆fpfm = ± 75 kHz


11.3.1 Using the XR215 data sheet, verify that the PLL is correctly calculated.


11.4 Measurements


11.4.2 Measure the free running oscillating frequency of the VCO when zero signal is applied to the PLL input.


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11.4.3 When the FM signal is applied at the demodulator's input by connecting U2fm to

U2fma, verify that the PLL is locked.

11.4.4 Observe the demodulated signal and its spectrum. Comment.


12. FM demodulation with quadrature detector(B)


FM demodulation is obtained by multiplying the FM signal with a phase shifted image of itself. If the phase shift is a linear function of frequency, the result of the multiplication contains the recoverd base-band signal. The frequency dependant phase shifter is an external LC resonant circuit.

12.2 Data

fpfm = 10.7 MHz ∆fpfm = ± 75 kHz

L1 = 10 µH L2 = 2.2 µH Rp = 3.9 kΩ


12.3.1 One possible phase shifter is as follows :

Calculate the transfer function : H(jω) = U2(jω)U1(jω)


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12.3.2 Squelch Bode diagrams, phase and magnitude, around resonant frequency

12.3.3 Calculate the capacitor value needed.

12.3.4 Estimate the quality factor of the resonant circuit. How is it chosen ?

12.4 Measurements


12.4.2 Use the calculated C and adjust the inductances to maximise demodulated signal and minimise distortion.

12.4.2 Measure signal U3fmb and its spectrum. Comment.

12.4.3 Compare the two FM demodulators: PLL versus quadrature.

modulation & demodulation 1. objectives - … · epfl - leg laboratoires d'électronique...

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