3B Module Analog Pulse Modulation

Generation and Demodulation of Pulse Modulation Techniques

1. Pulse Amplitude Modulation (PAM)

Definition:

PAM is the simplest form of pulse modulation where the signal is sampled at regular intervals. In PAM, each sample's amplitude is directly proportional to the amplitude of the modulating signal at that exact moment, allowing it to effectively convey information through varying pulse heights.

Generation:

A PAM signal is generated using two critical components: a sampler and a sampling signal (also referred to as a carrier pulse). The sampler takes the modulating signal and samples it based on the timing provided by the carrier pulse. The output is a pulse train where the amplitude of each pulse intricately reflects the amplitude of the modulating signal during the sampling instances. The sampling theorem, which states that a signal can be perfectly reconstructed from its samples if it is sampled at a rate greater than twice its highest frequency, plays a vital role in PAM.

• Figure 1 illustrates the PAM signal generation process and the associated frequency spectrum, depicting the relationship between the PAM signal, the message signal, and the sampling signal.

Demodulation:

The demodulation process of PAM is crucial for recovering the original modulating signal. This is achieved using a low-pass filter that eliminates high-frequency components and ripples, allowing the important amplitude information of the original modulating signal to be preserved. After filtering, the signal is amplified using an inverting amplifier to ensure the output amplitude closely mirrors that of the original signal.

• Figure 3 represents the PAM detector setup, while Figure 4 compares the modulated and demodulated PAM signals, highlighting the effectiveness of this technique for accurate signal recovery.

2. Pulse Width Modulation (PWM)

Definition:

PWM is a modulation technique where the width of the pulses in a pulse train is varied to convey the information encoded in the modulating signal. The pulse width directly relates to the amplitude of that modulating signal, enabling efficient information transfer.

Generation:

PWM is generated through the use of a comparator that compares two input signals: the modulating signal and a sawtooth wave operating at a specific carrier frequency. The comparator produces PWM signals based on the intersection of the modulating signal and the sawtooth wave. When the sawtooth signal rises above the modulating signal, a pulse is generated. The duration (or width) of this output pulse is directly proportional to the amplitude of the modulating signal.

• Figure 5 depicts the PWM generation process, while Figure 6 demonstrates the waveform characteristics of the resulting PWM signal in relation to the original modulating signal.

Demodulation:

For demodulation of PWM signals, a ramp signal is initiated at the positive edge of the PWM pulse and continues until the subsequent negative edge. The amplitude of this ramp signal varies in accordance with the pulse widths of the PWM signal, which reflect the amplitude of the original modulating signal. The ramp signal is then passed through a low-pass filter to recover the envelope of the message signal, ensuring the output resembles the original signal.

3. Pulse Position Modulation (PPM)

Definition:

PPM conveys information by varying the timing or position of each pulse in a pulse train, rather than their amplitude. This technique enhances resilience against noise and allows for more efficient bandwidth usage in data transmission.

Generation:

PPM is produced from PWM by first inverting the PWM signal to reverse the pulse polarities. Following this inversion, the signal is differentiated to generate both positive and negative spikes. A pulse generator is then triggered by these positive spikes, producing fixed-width pulses aligned with the PWM falling edges, thus yielding a PPM signal where the pulse position carries the encoded message information.

• Figure 7 illustrates the conversion of PWM into PPM, and Figure 8 displays the waveform of the resulting PPM in relation to the original modulating signal.

Demodulation:

To retrieve the envelope of the PPM signal, a ramp signal begins at the positive edge of a pulse and ceases at the next pulse's positive edge. The ramp's height is directly correlated with the time delay between the pulses, indirectly mirroring the amplitude of the modulating signal. This ramp signal is then filtered with a low-pass filter to produce the demodulated output that corresponds to the original message signal.

• Figure 8 also illustrates the modulation and demodulation waveforms for both PWM and PPM, emphasizing the transformation and recovery process in these modulation techniques.