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Communication system lab Report 4 Group: Mohammad Ibrahim Amer ozeir Written By: Mohammad Ibrahim Instructor: Mrs Nahida Abdullah Due Date: march 27, 2011 Spring 2010/2011 DSB demodulation. Objectives: To know that AM is a special form of frequency mixing. Define the envelope curve demodulation and explain the role of each element in it. Define the synchronous demodulator and its principle and explain the role of each element that forms it. Define the carrier recovery and know how we perform it. Perform an experiment that will introduce to us the frequency response of the band-pass filter. Perform an experiment to investigate a synchronous demodulation with carrier recovery. Perform an experiment that explains to us the influence of the carrier’s phase. Define the asynchronous demodulation of DSB-AM without and with carrier and perform an experiment about it. Equipment Required: Pcs Com3lab board #70071 TX 433 transmitter Com3lab board#70072 RX 433 receiver Oscilloscope FFT module Carrier signal(frequency fc) Voltage controlled oscillator Function generator Procedure: Amplitude modulation (AM): For transmission purpose, the information (AF signal) is superimposed in RF signal. Where: AF signal: Wanted signal in low frequency range (signal before modulation or after demodulation) RF signal: RF frequency carrier signal, is modulated onto the wanted signal (AF signal) In the case of AM, the carrier amplitude is modulated as a function of the AF signal (single multiplication). The spectrum of the modulated signal then consists of the carrier frequency fc, which is frequently superfluous to needs, and the 2 sidebands forming a spectrum reflected on either side of the carrier. AM is thus simply a special form of frequency mixing. Envelope curve demodulator: The envelope curve of the amplitude-modulated RF signal UAM represents the actual modulation voltage, the AF wanted signal Um. A simple form of demodulation that is the recovery of the wanted signal from the modulated signal and is performed by rectifying the modulated signal via a diode and suppressing the high-frequency components using a low-pass filter at the diode output. Demodulation: recovery of the modulating signal (wanted signal) from the amplitude or frequency-modulated signal. It is assumed that the spectrum of the wanted signal lies between fL and fH and the carrier has a frequency of fc. What must then hold true for cut-off frequency FG of the low pass filter for the demodulator to operate correctly? Experiment 1: In the following experiment we will investigate envelope curve demodulation of an amplitude-modulated 10 KHZ carrier, which is transmitted from the TX433 (70071) transmitter board to the RX433 (70072) receiver board via line. A sine wave of frequency 500 HZ is used as a modulating signal. The carrier signal is also transmitted. First we Set up the frequency experiment circuit above, then we Activate the AM modulator on the TX433 (70071) board and use the function generator to set a DC-free sine wave of frequency f=500 HZ with Upp=3.8 V. Now we use the oscilloscope to determine the curve of the amplitude – modulated signal in front of a behind the rectifier and the output signal of the low pass filter. Question: Which signal is obtained at the output of the low-pass filter? Answer: the wanted signal, i.e. the 500 HZ sine wave. Then at the output of the low pass filter you recover the original wanted signal, only the amplitude is different. Synchronous demodulator: If you multiply the DSB-modulated signal Uam with the carrier again in the receiver, you obtain the original baseband as well as the sidebands either side of the frequency 2fc, twice that of the carrier. By using low-pass filtering you then recover the wanted signal Um from this. Essential for correct demodulation is having the correct phase, i.e. synchronous carrier feed. -DSB: abbreviation standing for double sideband0doublr-sideband amplitudemodulated signal. : Band pass filter with phase shifter for filtering the carrier out of the spectrum of the DSB-modulated signal. : Amplifier for the amplification of the recovered carrier (to reduce distortion we adjust the gain of the carrier signal). : Product detector it represents a multiplicative mixer stage for the two signal supplied. : Low pass filter used to filter the baseband. Experiment 2: In the subsequent experiment we shall investigate the synchronous demodulation of a DSB-modulated signal. A dc-free sine wave is used as the wanted signal. This signal have a frequency of f=1500 HZ and amplitude Upp=3.8 V. In the first part of the experiment the carrier will not be recovered from the modulated signal, but merely transmitted to the receiver via separate line. -Synchronous demodulation: in this form of demodulation the RF signal is returned to the sideband in the demodulator by multiplication with a frequency and phase-synchronous subcarrier. Now we use the oscilloscope to determine the curve of the amplitude – modulated signal on the receiver side in front of and behind the multiplier as well as the output signal of the low-pass filter. Finally we use the spectrum analyzer to determine the frequency spectra of the individual signals. Carrier recovery: In order to recover the carrier from the DSB-modulated, a band-pass filter is needed which filters out the upper and lower sideband. As the sidebands can lie in very close proximity to the carrier in broadband signals, this filter must have cutoff edges which are as steep as possible. A downstream phase-shifter then provides for carrier signal synchronization with the correct phase. Band-pass filter: filter which only allows signal components in a certain frequency range (pass-band) to pass and suppresses all other frequencies. Phase-shifter: unit which delays a signal’s phase by certain value. Frequency response of the band-pass filter: In the subsequent experiment the frequency response (amplitude characteristics) of the band-pass filter in the” AM demodulation “ field in the COM3LAB RX433 board are determined using the bode module. To do this, an analysis is performed of the frequency range from 1000 up to 25000 HZ. Set the following parameters: Fmin=1000 HZ, Fmax=2500 HZ, steps=50, Upp= 20v and determine the amplitude characteristics of the band-pass filter. Question: at which frequency does the amplitude characteristic of the band pass filter have its maximum? Answer: at approx.10 KHZ. The amplitude response shows the typical response of a band-pass filter. Experiment: In the subsequent experiments we will be investigating synchronous demodulation with carrier recovery. At first we will look at the carrier signal regained from the DSB-modulated signal in the band-pass/phase-shifter unit. Set up the experiment circuit above. Activate the AM modulator signal on the TX433 board and set the function generator to a DC-free sine wave with a frequency of f=1500HZ and Upp=2v. Now use the oscilloscope to determine the curve of the recovered carrier signal (output signal of the band-pass/phase-shifter unit) as well as the corresponding amplitude spectrum. Influence of the carrier’s phase: Besides the imperfect filtering of the carrier frequency (residuals of the lower and upper sideband) the band-pass filter also causes a phase-shift &0 between the original carrier fc (transmission carrier) and the recovered carrier fc’ (receiver carrier, auxiliary Faux).In the following experiment we will investigate the influence of this phase-shift on the demodulated signal. Set up the experiment circuit above. Activate the AM modulator signal on the TX433 board and set the function generator to a DC-free sine wave with a frequency of f=1500HZ and Upp=2v. Now simultaneously record the curve of the transmitter and receiver carrier on the oscilloscope. As result the phase shift &0 between transmitter and receiver carrier affects the amplitude of the demodulated signal. Taking negative frequencies into consideration for a precise understanding of asynchronous demodulation dealt with on the following pages it is necessary to consider not only the baseband of the signal but also the “reflected baseband” brought about by the in the 0 HZ axis . In DSB modulation there then arise lower and upper sidebands both in the positive and in the negative frequency ranges. Asynchronous demodulation. If the carrier Uc’ (t) used in the receiver for demodulation demonstrates a different frequency to that of the transmitter carrier Uc (t). Then we have asynchronous demodulation. In the case of DSB modulation e.g. of a sinusoidal wanted signal of the frequency f1 (a) the effect of this is that instead of the original spectral line at f1. Two spectral lines at f1 + ^fc and f1-^fc arise after demodulation (d). In this case ^fc is the difference between the two carrier frequencies. Asynchronous demodulation: AM demodulation in which a frequency-shift occurs between the original carrier and the auxiliary carrier. The result is that the wanted signal is not recovered error-free. Experiment: In the following experiment we shall be investigating the asynchronous demodulation of a DSB-modulated signal. To generate the receiver carrier, the function generator of t e RX433 board is used. As the wanted signal a sine wave of frequency 1500 HZ is selected. Set up the experiment circuit below. Activate theTX433 board and set the function generator to a DC-free sine wave of frequency f=1500 HZ and Upp=2v. On the frequency generator of the RX433 board set a DC-free sine wave of frequency f=10KHZ and Upp=5V as the auxiliary carrier (receiver carrier signal). Determine the time curve and the spectrum of the demodulated signal (output signal of the low-pass filter). Select a frequency range up to 5000HZ for the spectrum analyzer and X/div=1ms for the oscilloscope. Repeat the experiment for receiver carrier frequencies at 9900HZ and 10100HZ and compare the results. Now connect the demodulated signal to the L input of the audio stage, open the operating panel for the audio section, activate the loudspeaker and repeat the experiment. Asynchronous demodulation of DS-AM with carrier: If in the case of DSB modulation the transmitter carrier is also transmitted, an additional spectral line appears at ^fc in the spectrum of the asynchronous demodulated signal, which is caused by the carrier. In synchronous demodulation (^fc=0) this line precisely corresponds to the DC component of the demodulated signal (spectral line at f=0). Conclusion: - The AM is thus simply a special form of frequency mixing. - The envelope curve modulation is a simple form of demodulation to get Uam and it is performed in two part: 1- First we use a diode to recover the wanted signal(the diode pass the positive side of the signal Uam(max)) 2- Second we use a low pass filter to suppress the high frequency components and then we recover the original wanted signal. - The synchronous demodulator is second type of demodulation use a in first a band-pass filter to filter the carrier of the modulated signal then we use an amplifier for the amplification of the recovered carrier, then the product detector is used to represent a multiplicative stage and finally the low pass filter is used to get the original signal Uam. - In a frequency response of the band-pass filter we know that the amplitude response shows a typical response of a band pass filter. - In the synchronous demodulation with recovered carrier the band pass has very steep cut off edges, and the phase shift transmitter a receiver carrier affects the amplitude of the demodulated signal. - To understand the asynchronous demodulation it is necessary to consider the baseband and the reflected baseband. - By using the asynchronous demodulation the result that the wanted signal is not recovered error-free.