Download Synchronous demodulator

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.
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 Equipment Required:
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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.