Question

1. Explain what is meant by baseband signal and baseband transmission.

2. Explain one approach for PWM modulation and detection (demodulation). Please draw the required circuits and diagrams.

Answer 1

1.In telecommunications and signal processing, baseband is the band of frequencies from close to 0 hertz up to a higher cut-off frequency or maximum bandwidth. Baseband can be synonymous with lowpass or non-modulated, and is differentiated frompassband, bandpass, carrier-modulated, intermediate frequency, or radio frequency (RF).

A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated

There are two main categories of digital communication transmission methods: baseband and passband.

• In baseband transmission, line coding is utilized, resulting in a pulse train or pulse amplitude modulated(PAM) signal. This is typically used over non-filtered wires such as fiber optical cables and short-range copper links, for example: V.29 (EIA/TIA-232), V.35, IEEE 802.3, SONET/SDH.

• In passband transmission, digital modulation methods are employed so that only a limited frequency range is used in some bandpass filtered channel. Passband transmission is typically utilized in wireless communication and in bandpass filtered channels such as POTS lines. It also allows for frequency-division multiplexing. The digital bit stream is converted first into an equivalent baseband signal, and then to a RFsignal. On the receiver side a demodulator is used to detect the signal and reverse the modulation process. A combined equipment for modulation and demodulation is called a modem.

2.There are different methods for extracting the message signal from a PWM wave. One method which is very simple is discussed here. It is very easy to convert the PWM wave to a PPM wave, and for the PPM wave the demodulation circuitry is extremely simple. The idea is hence to convert the PWM wave to the PPM wave and then demodulate it. The following block diagram represents the implementation of a PWM de-modulator.