Data Transmission Techniques: Modulation and Digital Line Coding

Analog Data to Analog Signals

Techniques used for converting analog data to analog signals include:

• AM (Amplitude Modulation)
• FM (Frequency Modulation)
• PM (Phase Modulation)

Key components involve the Carrier and the Modulator, which generate the resulting AM, PM, and FM signals.

Digital Data to Analog Signals

These techniques involve shifting characteristics of the carrier signal to represent digital data:

Amplitude Shift Keying (ASK)

ASK modifies the amplitude of the carrier signal. Binary values are represented by two different amplitudes of the carrier. Note that using zero amplitude is sensitive to sudden changes in gain. ASK is often used in fiber optics.

Frequency Shift Keying (FSK)

FSK affects the frequency of the carrier. Data is represented by different frequencies near the carrier frequency. FSK is less sensitive than ASK and is used in transmission through modems and high-frequency radio transmission.

Phase Shift Keying (PSK)

PSK affects the phase of the carrier signal. The carrier signal is shifted to represent data:

• 0: Phase 0°
• 1: Phase 180°

QPSK (Quadrature PSK) is a variation where each element represents more than one bit.

The number of signal units (symbols) transmitted per second is denominated the Baud rate.

Digital Data to Digital Signals (Line Coding)

The digital signal can have several different states:

• Two states: Binary
• Three states: Ternary
• Four states: Quaternary

Signal Polarity Types

Unipolar

1 is represented by positive voltage (V+), and 0 is represented by the absence of voltage (zero voltage).

Polar

1 is represented by V+ and 0 is represented by V- (or vice versa).

Bipolar (Pseudoternary)

0 is represented by zero voltage, and 1 alternates between V+ and V-.

The method of representation used to transmit digital data via a digital signal is called the electrical code, line code, or baseband code.

Baseband refers to the set of signals that do not undergo any modulation process relative to their origin.

When designing line codes, it is important to ensure a zero DC component to allow the signal to pass through inductive elements (like transformers) and to avoid long sequences of consecutive zeros to maintain synchronization.

Systems often use AMI (Alternate Mark Inversion) and HDB3 (High-Density Bipolar 3-Zero) codes specifically to avoid long sequences of consecutive zeros.

In a typical PCM/TDM system (such as the E1 standard), 30 channels are used for voice/data. Time slot 16 is often reserved for signaling. Synchronization information is sent using 4 bits per channel.

Work Unit 2: Data Conversion Exercises

1. Total Harmonic Distortion (THD) Calculation

   The coefficient of harmonic distortion of a nonlinear system is estimated using the formula:

   THD = sqrt(A₂² + A₃² + A₄²) / A₁ × 100

2. 4-Bit A/D Converter Voltage Intervals

   Determine the binary input and output voltage intervals for a 4-bit Analog-to-Digital (A/D) converter with a reference voltage (V_ref) of 3V.

   The step size (Quantization Level) is calculated as:

   Step Size = V_ref / (2ⁿ - 1) = 3 / (2⁴ - 1) = 3 / 15 = 0.2 V

   Intervals:

   • 0000: 0 V to 0.2 V
   • 0001: 0.2 V to 0.4 V
   • ... continuing until reaching 3 V.

3. 3-Bit D/A Sawtooth Signal Generation

   Form a sawtooth signal using a 3-bit Digital-to-Analog (D/A) converter with a factor K = 10 mV, target frequency F = 1 kHz, and maximum voltage V_max = 70 mV.

   1. Table with Binary Values and Output Voltage (V_S)

      The output voltage is calculated as V_S = K × Decimal Value.

      • 000: V_S = 10 mV × 0 = 0 mV
      • 001: V_S = 10 mV × 1 = 10 mV
      • ... continuing until reaching 70 mV.

   2. Signal Graph

      Draw the resulting signal, showing the voltage steps (10 mV increments) and correlating them with the binary input values.

   3. Pulse Conversion Frequency

      Calculate the pulse conversion frequency (F_pulses) required for the signal at 1 kHz:

      F_pulses = 1 / 0.0001 s = 10,000 Hz = 10 kHz