# Analog Signal Transmission, Processing, and Modulation

October 1, 2019

The classic serve in tennis requires a continuous, fluid motion.  We can think of a serve as similar to the action of ocean waves flowing to shore.  The motion begins with holding the ball loosely before a consistent toss and then transitions to the simple backswing.  This rhythmic, semi-circular motion transfers energy from body to arm and allows players to generate the racquet speed necessary to power the ball without any wasted effort.

Think of analog signal transmission in the same way. Everything begins through the fundamental relationship between voltage, current, and resistance that made up the first chapter of our studies about electronics. A continuously varying current or voltage conveys signals, video information, audio information, images, or data. As we know from viewing a sine wave, the amplitude, voltage, current, phase, or frequency vary smoothly and continuously over time.

All this occurs because of electric and magnetic fields. A voltage cutting across the impedance of an antenna creates an electric field that remains proportional to the rate of change of voltage. Electric current passing through the impedance of an antenna creates a magnetic field. With a steady voltage or current, the fields do not change. Varying the voltage and current causes the fields to propagate.

Because a changing electric field generates a magnetic field and a changing magnetic field generates an electric field, electromagnetic radiation (EMR) occurs. Analog signal transmission produces and controls the EMR for the purpose of transmitting and receiving information between two points that do not have a direct electrical or line-of-sight connection.

## Amplitude, Frequency, and Phase Modulation

Analog signal transmission became popular through the transmission of radio signals and occurs through modulation—or the ability to intentionally vary a signal. Sinusoidal voltages represent a time-varying quantity. The signal amplitude varies with time. However, the transmission of radio frequency (RF) signals depends on variations in frequency over time and the quantity of signals located within a frequency range.

During the early 1900’s, researchers found that they could vary the amplitude of a high-frequency sinusoid called a carrier wave proportionally according to the amplitude of continuously varying information or baseband signal. At that time, amplitude modulation became a common method for transmitting radio signals.

If the input signal for an amplitude modulated radio signal has a low amplitude, the carrier wave amplitude also decreases. Frequency modulation improves on the transmission quality of amplitude modulation by modifying the instantaneous frequency of the carrier wave according to the amplitude of the input signal. The instantaneous frequency equals the frequency of a signal at any given moment.  Because frequency occurs as a function of time, it does not have a constant value.

Working through analog signals can be particularly challenging for any design

Phase modulation provides another method for analog signal transmission and works well for higher data rates. With this method, modulating the phase of a carrier signal to follow the amplitude of the input signal allows the transmission of data. The signal carrying the data encodes as variations in the instantaneous phase of a carrier wave.

## Demodulating the Signal

Changing the amplitude, frequency, or phase of an analog signal adds audio, video, or data information to the signal through modulation. As a result, different types of demodulation circuits exist. While the amplitude variation peaks of an AM waveform represent the original information signal, the waveform contains the carrier frequency, the sum frequency, and the difference frequency with the modulating intelligence held in the difference between the frequencies.

Using an antenna, the transmitter circuit sends the signal to a receiver antenna for demodulation—or the removal of the information at a demodulation circuit. For amplitude modulation, circuits may use a series detector diode, a shunt detector diode, a common emitter transistor circuit, or a common base transistor circuit to rectify the signal and extract information or filters to separate the information from the carrier signal.

Demodulating an FM signal requires a different approach to recover the original information signal because the variation of the instantaneous frequency of the carrier—above or below the center frequency—carries the information. Usually housed within one IC package, one type of demodulation circuit uses a Voltage Controlled Oscillator (VCO) to generate a frequency that matches the original carrier frequency and a phase comparator to compare the phase of the generated frequency with the received FM signal.  Then, a low-pass filter removes the unneeded portion of the phase comparator output and leaves a signal that matches the original information signal.

No matter the waveform, ensure your analog signal design can work with it.

Phase modulated signals require a more complex approach because the amount and rate of phase shift in a carrier wave contains the information. In brief, the demodulator circuit converts the phase modulated signal into two outputs. Then, the circuit sums, modifies, and amplifies the outputs to generate a baseband signal that has the same amplitude across the frequency range.

## Analog Signals and Noise

Analog signal generation depends on electromagnetic radiation to broadcast and receive signals. However, the dependence on EMR also introduces noise into the circuit. In addition, analog signal transmissions remain vulnerable to signal degradation.

Components associated with the analog signal can generate internal noise while man-made devices and natural phenomena can also develop noise. Both types of noise can limit the capability of a demodulation circuit to identify and accurate duplicate the original information. Electronic circuits must have proper grounding and trace designs to reduce susceptibility to noise interference.

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