Digital to analog conversion,psk,ask,fsk,qam

1. DATA
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Digital to Analog Conversion

2. Digital to Analog conversion
■ Digital to analog conversion is the process of changing one
of the characteristics of an analog signal based on the
information in digital data.
Topic Discuss in This section
1. Amplitude Shift Keying (ASK)
2. Frequency Shift Keying (FSK)
3. Phase Shift Keying (PSK)
4. Quadrature Amplitude Modulation (QAM)

3. Digital to Analog Conversion:
■ A sine wave is defined by three characteristics, which are
Amplitude, Frequency and Phase.
■ When we vary any one of these, we create a different version of that
signal wave. So, by changing one characteristic of a simple electric
signal, we can use it to represent digital data.

4. Types For Conversion:
■ Figure 5.2

5. Digital Data:
■ A digital data refers to information that has discrete values.
Analog Signals:
■ An analog signal is a continuously varying electromagnetic wave that may be
propagated over a variety of media.
Data Element:
■ A smallest piece of information that has to be changed, the bit. (1,0)
Signal Element:
■ A smallest unit of signal that is constant. OR A pulse of constant amplitude, frequency
and phase.

6. Bit Rate:
■ Bit Rate is the number of bits in one second.
■ Its unit is Bits Per Second (bps).
Baud Rate:
■ Is the number of signal element in one second
■ Its unit is signal elements per second (baud)..
“In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.”
S=Nx1/r bauds
Where S is baud rate, N is bit rate and r is the number of data bits per signal element.

7. 5.7
An analog signal carries 4 bits per signal element. If
1000 signal elements are sent per second, find the bit
rate.
Solution:
In this case, r = 4, S = 1000, and N is unknown. We can
find the value of N from
Example 5.1

8. Amplitude Shift Keying:
■ In amplitude shift keying, the amplitude of the carrier signal is varied to create signal
elements.
■ Both frequency and phase remain constant while the amplitude changes.
Types of ASK:
■ Binary Amplitude Shift Keying (BASK)
■ Multilevel Amplitude Shift Keying (MASK)

9. Binary ASK:
■ In BASK, the two binary values are represented by two different amplitudes of the carrier
frequency.
■ That is, one binary digit (1) is represented by the presence, at constant amplitude, of the carrier, the
other (0) by the absence of the carrier.
■ For LED (light-emitting diode) transmitters, one signal element is represented by a light pulse
while the other signal element is represented by the absence of light.

10. Binary ASK:
■ The line encoding will determine the values of the analog waveform to reflect the digital data
being carried.
■ The bandwidth B of ASK is proportional to the signal rate S.
B = (1+d)S
■ “d” is due to modulation and filtering, lies between 0 and 1.
■ The ASK technique is used to transmit digital data over optical fiber.
■ BASK is susceptible to sudden gain changes and is a rather inefficient modulation
technique.

11. Multilevel ASK:
■ The above discussion uses only two amplitude levels.
■ We can have multilevel ASK in which there are more than two levels.
■ We can use 4,8, 16, or more different amplitudes for the signal and modulate the data
using 2, 3, 4, or more bits at a time.
■ Although this is not implemented with pure ASK, it is implemented with QAM (as we
will see later).

12. Frequency Shift Keying:
■ In frequency shift keying, the frequency of the carrier signal is varied to represent data.
■ The frequency of the modulated signal is constant for the duration of one signal element, but
changes for the next signal element if the data element changes.
■ Amplitude and phase remain constant for all signal elements.

13. Binary FSK:
■ One way to think about binary FSK (or BFSK) is to consider two carrier frequencies.
■ In Figure, we have selected two carrier frequencies,f1 and f2.
■ We use the first carrier if the data element is 0; we use the second if the data element is 1.
■ However, note that this is an unrealistic example used only for demonstration purposes.
Normally the carrier frequencies are very high, and the difference between them is very small.

14. Bandwidth for BFSK:
■ If the difference between the two frequencies (f1 and f2) is 2f, then the required Bandwidth
B will be:
B = (1+d)S +2f

15. Multilevel FSK:
■ Similarly to ASK, FSK can use multiple bits per signal element.
■ We can use more than two frequencies.
■ For example, we can use four different frequencies f1,f2,f3 ,and f4 to send 2 bits at a time.
■ To send 3 bits at a time, we can use eight frequencies.
■ Bandwidth B for MFSK will be:
B= (I +d)S + (L - 1)2∆f => B =L * S
■ Where L stands for levels and S stands for Baud rate.

16. Phase Shift Keying:
■ The Shift Keying Technique in which we vary the phase of the signal to represent digital
data is called Phase Shift Keying.
■ In this type of shifting only the phase changes while the frequency and amplitude
remains constant.
■ We will discuss the following types of PSK:
1. Binary Phase Shift Keying ( BPSK )
2. Quadrature Phase Shift Keying ( QPSK )
■ The simplest scheme used for PSK is binary phase shift keying.

17. Binary Phase Shift Keying:
■ The simplest scheme uses two phases to represent the two binary digits 1 and 0, ( as shown
in Figure ) and is known as binary phase shift keying.
■ A phase shift of 180⁰ ( π ) is equivalent to flipping the sine wave or multiplying it by -1.
■ This leads to a convenient formulation and better understanding of the BPSK.
■ Its bandwidth is same as binary ASK. And no bandwidth is wasted for separating two
carrier signals.

18. Quadrature Phase Shift Keying:
■ To increase the bit rate, we can code 2 or more bits onto one signal element.
■ In QPSK, we parallelize the bit stream so that every two incoming bits are split up and PSK a
carrier frequency. One carrier frequency is phase shifted 90⁰ from the other – in quadrature.
■ The Phase Shift Keyed Signals are then added to produce one of 4 signal elements.
■ And that’s why it has four levels, L = 4.

19. Constellation Diagrams:
■ A constellation diagram helps us to define the amplitude and phase of a signal when we
are using two carriers, one in quadrature of the other.
■ The X-axis represents the in-phase carrier and the Y-axis represents quadrature carrier.

20. Quadrature Amplitude Modulation:
■ Quadrature Amplitude Modulation is a combination of ASK and PSK.
■ QAM can also be considered as the extension of QPSK.
■ QAM takes advantage of the fact that it is possible to send two different signals
simultaneously on the same carrier frequency, by using two copies of the carrier
frequency.

21. ■ Q#.1Why we need signal conversion?
■ Ans : Baseband data transmission
Passband data transmission

22. Thank You    