In this lab work, you will study the various on-line codes for representing the binary information for its transmission on a communication channel. Two sets of codes are studied. The first provides a baseband signal. This one can be sent as it is. It can also modulate a carrier. The various digital modulations with carrier are also available (second way of transmission) and will be studied in principle and technique, especially at the demodulation level.
The MODICOM 5 system consists of two electronic boards. The MODICOM 5/1 board includes the conditioning of data (on-line coding) into baseband signals and, if necessary, carrier modulation and thus corresponds to the transmission part. The MODICOM 5/2 board performs the reverse operations and therefore corresponds to the reception part (see the block diagrams on the boards concerned).
The data conditioning part in baseband signals (information-to-signal coding) allows for working with the following on-line codes:
The carrier modulation part performs the following modulation types:
The boards MODICOM 5/1 and MODICOM 5/2 are inter-connectable upstream with the MODICOM 3/1 board for the transmitter part (MODICOM 5/1) and downstream with the MODICOM 3/2 board for the receiver part (MODICOM 5/2). The MODICOM 5/1 and 5/2 boards have signal generators with a carrier at 1.44 MHz for one, and a carrier at 960 KHz for the other. Moreover, the latter also has a version in quadrature (Q) relative to the other (I).
More precisely, the MODICOM 5/1 board comprises the following elements:
In addition, it is possible to simultaneously output a group of two digits (usable for QPSK modulation). Two modulation blocks with carrier and a block of summation make it possible to carry out the various types of digital modulation. On each modulation block, the gain and offsets of the carrier and the modulating signal are adjustable.
Similarly, the MODICOM 5/2 board (Receiver) includes the following elements:
IMPORTANT NOTE.– In all experiments, you will only power the boards (±12 volts; 5 volts) when the wiring has been fully completed and checked.
When using boards MODICOM 3/1 and 3/2, these will be used in the following modes and configurations:
From the MODICOM 3/1 board and continuous analog input for it, set the signal amplitude so that the binary word transmitted in serial form by the channel is the following:
for example. However, you can modify the information transmitted at your convenience.
The clock regeneration specific circuit will be used for the biphase code. What does it produce as a regenerated clock signal?
It will then be possible to transmit sinusoidal analog signals over the whole system (MODICOM 3/1; 5/1; 5/2; 3/2) using any of these codes.
We will study successively the digital amplitude modulation and demodulation (ASK), the digital frequency modulation and demodulation (FSK), the digital phase modulation and demodulation (PSK).
In analysis mode, the information sent will be taken equal to “0 1 0 0 0 1 1” for example.
An amplitude modulation will be performed using a modulation block and a 1.44 MHz carrier. The mode of the MODICOM 5/1 board will be set to position 1 and the SYNC CODE GENERATOR of the MODICOM 3/1 board to “OFF”. Perform amplitude demodulation as follows:
The SYNC CODE GENERATOR of the MODICOM 3/2 board will be set to “ON” and the clock regeneration block will be used with an appropriate setting of “PULSE GENERATOR DELAY ADJUST”.
You will adjust the gain and the offsets of the modulating signal and the carrier.
Examine the signals obtained at the output of each of the blocks of the demodulation system (the comparator threshold must be set correctly). Comments.
Change the amplitude of the transmitted signal (MODICOM 3/1) and check that the reconstructed signal is correct by setting the SYNC CODE GENERATOR of the MODICOM 3/1 board to “ON”.
Perform a digital frequency modulation according to the block diagram of Figure 6.2.
In reception, the signal is decoded by means of a phase-locked loop, a low-pass filter and a comparator, according to the block diagram of Figure 6.3.
The two boards MODICOM 3/1 and MODICOM 3/2 will be used to generate the information to be transmitted with modulation and to reconstruct the signal on reception. Set the SYNC GENERATOR of the MODICOM 3/1 board to “OFF” and the SYNC CODE DETECTOR of the MODICOM 3/2 board to “ON”.
Perform a digital phase modulation according to the block diagram of Figure 6.4.
In reception, the signal is decoded in two steps.
A demodulator composed of four blocks performs a PSK demodulation (see Figure 6.5).
In a second step a low-pass filter and a comparator provide a logical signal.
Give your general conclusions on PSK modulation.
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