Description
Part 1: Amplitude Modulation (AM)
Objectives: To understand the basic principles of amplitude
modulation and demodulation
Preparations
For this lab, the following Simulink blocks will be used.
I. DSB-LC AM System
Build the double‐sideband (DSB) large‐carrier (LC) AM system as illustrated.
I. DSB-LC AM System
Parameter setup:
Sine Wave 1: Sine type: Time based | Amplitude: 1 | Frequency (rad/sec): 1000*pi
Sine Wave 2: Sine type: Time based | Amplitude: 3 | Frequency (rad/sec): 600*pi
Sine Wave 3: Sine type: Time based | Amplitude: 1.5 | Frequency (rad/sec): 1200*pi
Constant (DC Bias): Constant value: 6
Carrier (Mod): Sine type: Time based | Amplitude: 1 | Frequency (rad/sec): 30000*pi
| Phase (rad): pi/2
AWGN Channel: Initial seed: randseed | Mode: Variance from mask | Variance: 0.01
Analog Filter Design: Design method: Butterworth | Filter type: Bandpass | Filter
order: 8 | Lower passband edge frequency (rad/s): 14300*2*pi | Upper passband edge
frequency (rad/s): 15800*2*pi
Rate Transition: Output port sample time: 1/50000
Math Function: Function: square
FIR Decimation: FIR filter coefficients: [1 1] | Decimation factor: 2
Discrete FIR Filter Coefficients: firpm(20, [0 0.03 0.1 1], [1 1 0 0])
Sqrt Function: signedSqrt
I. DSB-LC AM System
1. Observe the output on Spectrum (Source). What are the fundamental and
harmonic components of the source signal?
2. Observe the outputs on Scope (Source) and Scope (Mod). Explain the relationship
between the amplitude of the AM signal and that of the source signal.
3. Observe the outputs on Spectrum (Source) and Spectrum (Mod). Explain the
relationship between the spectrum of the AM signal and that of the source signal.
Comment on the transmission bandwidth of AM signals.
4. Compare the outputs on Spectrum (Rx) and Spectrum (BPF). Comment on what
information is needed to filter out the noise without distorting the desired signal.
Explain how and why the SNR at the output of Analog Filter Design (BPF) changes
compared with the SNR at the input of Analog Filter Design (BPF)
?
5. Observe the outputs on Spectrum (ED) and Spectrum (BPF). Explain the principle
of double‐sideband large carrier (DSB‐LC) AM demodulation.
6. Change the value of Constant (DC Bias), and observe how it affects the signal
recovered from Envelope Detector in comparison with the source signal. Explain
what is a feasible DC bias in relation to the amplitude of the source signal so that
successful demodulation can be guaranteed.
II. DSB-SC AM System
Build the DSB suppressed‐carrier (SC) AM system as illustrated.
II. DSB-SC AM System
Parameter setup:
Sine Wave 1: Sine type: Time based | Amplitude: 1 | Frequency (rad/sec): 1000*pi
Sine Wave 2: Sine type: Time based | Amplitude: 3 | Frequency (rad/sec): 600*pi
Sine Wave 3: Sine type: Time based | Amplitude: 1.5 | Frequency (rad/sec): 1200*pi
Carrier (Mod): Sine type: Time based | Amplitude: 1 | Frequency (rad/sec): 30000*pi | Phase (rad): pi/2
Carrier (Demod): Sine type: Time based | Amplitude: 1 | Frequency (rad/sec): 30000*pi | Phase (rad): pi/2
AWGN Channel: Initial seed: randseed | Mode: Variance from mask | Variance: 0.01
Analog Filter Design (BPF): Design method: Butterworth | Filter type: Bandpass | Filter order: 8 | Lower passband
edge frequency (rad/s): 14300*2*pi | Upper passband edge frequency (rad/s): 15800*2*pi
Analog Filter Design (LPF): Design method: Butterworth | Filter type: Lowpass | Filter order: 8 | Passband edge
frequency (rad/s): 800*2*pi
Rate Transition: Output port sample time: 1/50000
7. Repeat Steps 2‐3 for DSB‐SC AM.
8. Explain the differences between DSB‐SC and DSB‐LC in terms of modulation. Explain
how such differences affect the transmit power efficiency and the demodulation
process at the receiver.
Part 2: Frequency Modulation (FM)
Objectives: Understand the basic principles of
frequency modulation and demodulation
Preparations
For this lab, the following Simulink blocks will be used.
FM System
Build the FM system as illustrated.
FM System
Parameter setup:
Sine Wave (Modulating Signal): Sine type: Sample based | Samples per period:
1000 | Sample time: 1e‐6
Constant (Carrier Frequency): Constant value: 9000
Gain (Sensitivity Factor) Gain: 3000
Discrete‐Time Integrator Sample time: 1e‐6
Gain Gain: 2*pi
Trigonometric Function: Function: cos
Digital Filter Design: Response Type: Lowpass | Design Method: FIR → Window
| Filter Order: Specify order → 30 | Options: Scale Passband, Window → Kaiser |
Frequency Specifications: Units → Hz, Fs → 1e6, Fc → 1000
FM System
1. From the block configuration and the parameter setup, describe in mathematical
terms the process of FM modulation and demodulation.
2. Observe the output on Scope (Mod). Explain how the FM signal is related to the
modulating signal in the time domain. Use a sum signal of two sine waves as a
modulating signal, and compare the output with the sum of those when each of the
sine wave is used as a modulating signal separately. Comment on the linearity of FM
modulation in comparison with AM modulation.
3. Vary Gain (Sensitivity Factor) from small (i.e., the modulation index ߚ less than 1
radian) to large (i.e., the modulation index ߚ greater than 1 radian). Comment on the
sensitivity of the carrier to the modulating signal in terms of amplitude and
frequency variation.
4. Vary the modulating frequency and the amplitude of Sine Wave (Modulating
Signal). Observe the variations of the spectrum on Spectrum (Mod). Explain how
the transmit power changes accordingly and why. Comment on the difference
s
between an FM signal and an AM signal in terms of transmit power in relation to the
modulating signal.
Part 3: Power/Bandwidth trade
‐off in FM
Objectives: To understand the possibility of
Power/Bandwidth trade‐off in FM systems.
Preparations
For this lab, the following Simulink blocks will be used.
FM Transmission and Reception System
Build the FM transmission and reception system as illustrated.
FM Transmission and Reception System
Build the FM demodulator subsystem as illustrated.
FM Transmission and Reception System
Parameter setup (Overall):
Sine Wave: Sine type: Sample based | Samples per second: 1000 | Sample time:
1e‐5
Discrete‐Time Integrator: Sample time: 1e‐5
RMS: Running RMS
Random Source (Noise): Source Type: Gaussian | Variance: 0.01 | Sample time:
1e‐5
Analog Filter Design: Filter order: 8 | Lower passband edge frequency (rad/s):
8000*2*pi | Upper passband edge frequency (rad/s): 10000*2*pi
Saturation: Upper limit: 1 | Lower limit: ‐1
Rate Transition: Output port sample time: 1e‐5
Trigonometric Function: Function: cos
Parameter setup (FM Demodulator):
Saturation: Upper limit: 0.5 | Lower limit: ‐0.5
Analog Filter Design: Filter order: 8 | Passband edge frequency (rad/s):
200*2*pi
Discrete‐Time Integrator: Sample time: 1e‐5
FM SNR
1. Denote the modulating frequency as ݂. Vary Gain (Sensitivity Factor) ∆݂ as ∆݂ ൌ
0.1݂, 0.5݂, ݂, … , 10݂. Observe the output on Spectrum (Mod) and comment on
how the number of significant sideband pairs (i.e., power above ‐10 dBm) varies
with ∆݂. Explain how and why the power of the carrier component varies in the
frequency domain.
2. For each ∆݂ ൌ 0.1݂, 0.5݂, ݂, … , 10݂ , determine the actual transmission
bandwidth. Record the amount of power in percentage contained in the bandwidth
as estimated by Carson rule.
3. For each ∆݂ ൌ 0.1݂, 0.5݂, ݂, 5݂, 10݂, plot the SNR curves in one figure, where
the ݕ‐axis is labeled with pre‐detection SNRs and the ݔ‐axis is labeled with post‐
detection SNRs. Comment on how the SNRs vary in response to ∆݂.