8.2 Digital to Analog Conversion

Digital to analog conversion is a modulation process which involves changing a characteristic of an analog signal to represent digital data.

The analog signal is manipulated to represent two distinct values representing bit 1 and bit 0.

The base analog signal that is modulated is called the carrier signal.

There are three types of digital to analog modulation techniques, as shown in Figure 8.2:

		Amplitude Shift Keying (ASK),
		Frequency Shift Keying (FSK) and
		Phase Shift Keying (PSK).

Figure 8.2: Types of Digital to Analog Modulation

8.2 Digital to Analog Conversion

Amplitude Shift Keying (ASK)

In ASK, amplitude is the signal characteristic that is varied in the carrier signal to represent bit 1 and bit 0.

The frequency and phase of the signal remain unchanged.

As shown in Figure 8.3, the peak amplitude of the carrier signal has two levels, one level to represent bit 1 and the other level to represent bit 0.

On-Off Keying (OOK) is a popular ASK in which one of the bit values is represented by no voltage. This reduces the amount of energy required to transmit data.

The disadvantage of the ASK modulation is its dependence on the amplitude of the signal to represent data bits making it more vulnerable to errors due to noise interference affecting the signal's amplitude.

8.2 Digital to Analog Conversion

Frequency Shift Keying (FSK)

In FSK, the frequency of the carrier signal is varied to represent bit 1 and bit 0.

The frequency remains constant in each bit duration.

As illustrated in Figure 8.4, the signal has two frequencies, one frequency to represent bit 1 and the other frequency to represent bit 0.

FSK is more reliable than ASK with less error rates due to noise interference as signal amplitude is irrelevant in the modulation processes.

Thus, voltage spikes won't affect the way the receiver interpret the received signal.

8.2 Digital to Analog Conversion

Phase Shift Keying (PSK)

In PSK, the phase of the carrier is varied to represent bit 1 and bit 0.

Signal amplitude and frequency are unchanged during the modulation process.

The phase of the signal in the duration of each bit remains constant.

As illustrated in Figure 8.5, if the signal is started with bit 0 represented by a phase of 0 degrees, bit 1 can be represented by a 180 degree phase shift.

This method is called 2-PSK as two phases are used to represent bit 1 and bit 0.

4-PSK can also be used where the signal has 4 different phases.

Each phase shift in 4-PSK represents two bits.