Generation of binary signals required
If it is modulated by a bi-polar bit stream according to the scheme illustrated in Figure 1 below, its polarity will be reversed every time the bit stream changes polarity. This, for a sinewave, is equivalent to a phase reversal shift. The multiplier output is a BPSK signal.
The information about the bit stream is contained in the changes of phase of the transmitted signal. A synchronous demodulator would be sensitive to these phase reversals. The upper trace is the binary message sequence. There is something special about the waveform of Figure 2. The wave shape is 'symmetrical' at each phase transition.
In addition, the message transitions have been timed to occur at a zero-crossing of the carrier. Whilst this is referred to as 'special', it is not uncommon in practice. It offers the advantage of simplifying the bit clock recovery from a received signal.
Once the carrier has been acquired then the bit clock can be derived by division. The basic BPSK generated by the simplified arrangement illustrated in Figure 1 will have a bandwidth in excess of that considered acceptable for efficient communications. If you can calculate the spectrum of the binary sequence then you know the bandwidth of the BPSK itself. Bandlimiting can be performed either at baseband or at carrier frequency. It will be performed at baseband in this experiment.
Translation back to baseband is achieved with a synchronous demodulator, as shown in Figure 3 below. The translation process does not reproduce the original binary sequence, but a bandlimited version of it. The original binary sequence can be regenerated with a detector. This requires information regarding the bit clock rate. If the bit rate is a sub-multiple of the carrier frequency then bit clock regeneration is simplified. You will see in the experiment that the sign of the phase of the demodulator carrier is important.
There are techniques available to overcome this. One such sends a training sequence, of known format, to enable the receiver to select the desired phase, following which the training sequence is replaced by the normal data until synchronism is lost! An alternative technique is to use differential encoding. The lowpass filter is included as a band limiter if required. Alternatively a bandpass filter could have been inserted at the output of the generator.
Being a linear system, the effect would be the same. Watch the phase transitions in the BPSK output signal as this phase is altered. Viewing of the phase reversals of the carrier is simplified because the carrier and binary clock frequencies are harmonically related. Examine the transitions as the phase between bit clock and carrier is altered. Vary the phase shift and examine its effect on the spectrum. Can you explain this? Figure 3 shows a synchronous demodulator for a BPSK signal in block diagram form.
This has been modelled in Figure 6 below. In the first part of the experiment the carrier and bit clocks will be stolen. Phase reversals of o can be introduced with the front panel toggle switch. The LPF following the demodulator multiplier is there to remove the components at double the carrier frequency. Its bandwidth can be set to about 12 kHz; although, for maximum signal-to-noise ratio if measuring bit error rates, for example , something lower would probably be preferred.
The BPSK will have been bandlimited by the lowpass filter in the transmitter, and so the received waveform is no longer rectangular in shape. But you can observe that the demodulator filter output is related to the transmitted sequence the NRZ-L code introduces only a level shift and amplitude scale.
Notice the effect upon the recovered sequence when the carrier phase is reversed at the demodulator. T2 Patch up the demodulator of Figure 6. The received signal will have come from the transmitter of Figure 5. Adjust the decision point. Confirm that the phase of the receiver carrier for the NRZ-L line code is still important. T5 Investigate a change of bandwidth of the transmitted signal.
This you might expect; but, under certain conditions, it can increase as the bandwidth is decreased! How could this be? See Tutorial Question Q6. Would this influence the spectrum of the BPSK signal? Q4 Does making the bit rate a sub-multiple of the carrier frequency have any influence on the spectrum of the BPSK signal? What determines its bandwidth? At first glance this seems unusual? What phase was it optimizing, and what was the magnitude of this phase? Could you measure it? In these applications the modulator requires a pair of multi-level analog signals derived from a single serial binary data stream.
These two modules will be examined in this experiment, independently of the quadrature amplitude modulator and demodulator with which they will later be associated. Their purpose will be better understood if you are first reminded of their role in a quadrature modulator and quadrature demodulator. As a reminder, a block diagram of a quadrature modulator is shown in Figure 1.
This configuration is common to many communications systems. To generate the two multi-level analog signals mentioned above the input serial binary data stream is segmented into frames or binary words of L bits each, in a serial-to-parallel converter.
From each of these words is generated a unique pair of analog voltages, one of which goes to the I-path, and the other to the Q-path, of the quadrature modulator. No bandlimiting is shown in Figure 1, but in practice this would be introduced either at the input to each multiplier possibly in the form of a pulse shaping filter , or at the output of the adder, or both. A quadrature demodulator is illustrated in Figure 2. It is the purpose of the demodulator to recover their individual messages, which are presented to the two inputs of the decoder.
If the DSBSC phasing at the transmitter is ideally in quadrature, then the single phase adjustment shown is sufficient to separate the messages of the two signals. The decoder has a bit clock input stolen in the experiment, else derived from the incoming signal in practice and knows beforehand the number of bit periods L in a frame. Each waveform is sampled once per frame, and a decision made as to which of the possible levels it represents.
This will give a unique pair of levels, which represents a binary word of L bits. This decoded word is output as a serial binary data stream. The phasor diagram is one of the many ways in which some of the properties of these bandpass signals can be illustrated.
The signal constellation diagram shows the location of the tips of these phasors on the complex plane. It is displayed when the two baseband multi-level signals I and Q are connected to the X and Y inputs of an oscilloscope in X-Y mode. These signals can come from the encoder output, or from the decoder input. The first of these shows the constellation under ideal conditions.
The second shows the constellation after the signal has passed through the channel. After this, for taking decision fo logic 1 or 0, comparators are used. Comparators are Op-Amps operated is differential mode. One of the input terminal is kept at reference voltage and signal is applied at the other terminal. There are two type of comparator Positive and Negative comparator.
If signal is applied to Non-inverting terminal then it is Positive comparator. Positive comparator gives high when signal level is greater than reference voltage. If signal is applied to inverting terminal then it is Negative comparator. Negative comparator gives high when signal level is less than reference voltage. The operation of comparator is simple.
It either works in Inverting Positive comparator or Non-Inverting mode Negative comparator with very high feed-back resistance means very high gain i. In our project we uses simple envelope detector followed by three-stage magnitude comparator and a level translator.
After the envelope detection signal is fed to three-stage magnitude comparator. Three-stage comparator is used for reliable signal detection and noise rejection. At the last stage a level translator is used to get output voltage in unipolar or bipolar mode. Help us to improve this topic. Your feedback is essential to us. Please suggest your remarks, ratings and corrections regarding the above section.
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