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Chapter 5

Analog Transmission

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5151 DIGITALTOANALOG CONVERSIONDIGITALTOANALOG CONVERSION

Digital-to-analog Digital-to-analog conversionconversion isis thethe processprocess ofof changing changing oneone ofof thethe characteristicscharacteristics ofof anan analoganalog signal based on the information in digital data. signal based on the information in digital data.  (^) Aspects of Digital-to-Analog Conversion  (^) Amplitude Shift Keying  (^) Frequency Shift Keying  (^) Phase Shift Keying  (^) Quadrature Amplitude Modulation Topics discussed in this section: Topics discussed in this section:

Figure 5.1 Digital-to-analog conversion

Figure 5.2 Types of digital-to-analog conversion

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.

Example 5. An analog signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many data elements are carried by each signal element? How many signal elements do we need? Solution In this example, S = 1000, N = 8000, and r and L are unknown. We find first the value of r and then the value of L.

Bandwidth of ASK

 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

Figure 5.3 Binary amplitude shift keying

Example 5. We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 1? Solution The middle of the bandwidth is located at 250 kHz. This means that our carrier frequency can be at f c = 250 kHz. We can use the formula for bandwidth to find the bit rate (with d = 1 and r = 1).

Example 5. In data communications, we normally use full-duplex links with communication in both directions. We need to divide the bandwidth into two with two carrier frequencies, as shown in Figure 5.5. The figure shows the positions of two carrier frequencies and the bandwidths. The available bandwidth for each direction is now 50 kHz, which leaves us with a data rate of 25 kbps in each direction.

Frequency Shift Keying

 The digital data stream changes the frequency of the carrier signal, fc.  For example, a “1” could be represented by f 1 =fc +f, and a “0” could be represented by f 2 =fc-f.

Figure 5.6 Binary frequency shift keying

Example 5. We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What should be the carrier frequency and the bit rate if we modulated our data by using FSK with d = 1? Solution This problem is similar to Example 5.3, but we are modulating by using FSK. The midpoint of the band is at 250 kHz. We choose 2Δf to be 50 kHz; this means

Coherent and Non

Coherent

 In a non-coherent FSK scheme, when we change from one frequency to the other, we do not adhere to the current phase of the signal.  In coherent FSK, the switch from one frequency signal to the other only occurs at the same phase in the signal.