Analog Signals

Analog signals are continuous voltages over time in contrast to digital signals that are a discrete sequence of voltages over time. Since analog signals are continuous, they can have an infinite number of different voltages that cover the natural variation of physical phenomenon. This introduction to analog signals covers its usage, waveform properties, bandwidth, signal attenuation, DC coupling, and AC coupling.

Usage

Many devices use analog signals, such as audio, video, radio communication devices, and the output of sensors. An analog output can be any sort of continuous time-varying quantity that conveys the data being collected or transmitted. Sensors can output an analog signal (voltage or current) that varies according to what is sensed. For example, sound from a microphone, light from a photocell, wind speed from an anemometer, force sensitive resistors, temperature from the thermoelectric properties of a material, and many more sensors output analog signals. Audio and video are briefly discussed in the following subsections as examples of analog signals.

Audio

The analog audio signal coming from a microphone or output to headphones is a mixture of frequencies in the human audible range of 20Hz to 20,000Hz that varies continuously over time.

The output of an electret microphone is usually in the millivolt range if not amplified. Breakout board microphones often come with a pre-amplifier built in so it is compatible with the range and resolution of an ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D) in a sound card for a computer and in microcontroller boards. A microphone takes in a mic level signal (millivolts), amplifies it, and outputs a line level signal (around 1 Volt). A line level is the specified strength of an audio signal used to transmit analog audio between devices. Consumer electronic audio devices (e.g., sound cards) often have a connector labeled Line In and/or Line Out, where Line Out provides an audio signal output and Line In receives a signal input.

Video

Analog video signals used for TV formats such as NTSCNational Television System Committee (North America), PALPhase Alternating Line (Europe), and SECAMSequential Color with Memory (Africa, Middle East, and other regions), involve a mixture of time-division multiplexing (sync and video information) and frequency-division multiplexing (luminance, chroma, and sound).

Multiplexing is the process of simultaneously transmitting two or more individual signals over a single communication channel, which is how only a 1 wire yellow RCARadio Corporation of America line (signal and ground) can make a whole color TV image appear on a screen full of thousands of pixels. The composite video coming out of an RCA jack is an analog signal usually ranging between 0 and 1.073V, where tiny changes in the signal have a huge effect on the color or location of the video (sound is transferred on separate white and red wires). Some SBCSingle Board Computers, like the Raspberry Pi 4B, have an onboard 3.5mm jack that outputs composite video. A typical signal waveform for one horizontal scan line (row) of an image in composite video is shown in the figure below.

Waveform Properties

A periodic analog signal shown in the figure below has properties that can be used to characterize its structure and behavior.

Amplitude:

The height of the signal from a reference level such as zero.

Period:

The amount of time (T) the signal completes one cycle.

Frequency:

The number of repetitions (cycles) that occur in a specified amount of time, expressed in cycles/second or Hertz (Hz). A signal's frequency is inversely related its period: f = 1/T.

Phase:

The amount of horizontal shift relative to time zero, usually expressed in degrees or radians. A phase shift of 360 degrees (or 2π radians) gives the same wave with no shift.

DC Offset:

The amount of vertical shift in the waveform is typically referred to as a DC offset, where the waveform has DC and AC components as shown in the figure below.

Analog signals that do not repeat themselves are called aperiodic. For example, measuring wind speed, random sounds from a microphone, or electrical noise. These aperiodic signals can be considered similar to a periodic signal with an infinite period, where its spectrum of different frequencies can be decomposed using the same rules as a periodic signal.

Bandwidth

For analog signals, bandwidth refers to the range of frequencies that can be transmitted by a particular system or medium. The term bandwidth has different meanings depending on the context. In signal processing it is the difference between the upper and lower frequencies in a continuous band of frequencies. In instrumentation, such as an oscilloscope, it is the range of frequencies above 0 Hz a device exhibits for a specified level of performance (i.e., a measure of the highest frequency that can be processed). This article focuses on the term bandwidth in the context of instrumentation for data acquisition.

Analog signals other than an ideal sine wave are composed of a spectrum of different frequencies with a highest amplitude fundamental frequency plus many harmonics that fall off in amplitude. Limiting this frequency content (or bandwidth) of the waveform affects the measurement in two ways.

1. The higher order harmonics are filtered out when the bandwidth is too low, distorting and attenuating the original waveform, making it become similar to a sine wave.
2. A signal with any sharp rise time will be distorted with an error on the amplitude.

Signal attenuation refers to the loss of a signal, defined as the ratio of the output voltage to the input voltage given below.

Attenuation is normally expressed in decibels using this equation below.

An attenuation of 3dB means that the output voltage is 0.707 of the input voltage. An attenuation of 6dB means that the output voltage is 0.5 of the input voltage. An attenuation of 12dB means that the output voltage is 0.25 of the input voltage. The bandwidth of a measuring device is typically defined (IEEE 1057) as the maximum frequency with at which the amplitude of a sine wave input is reduced by 3dB (approximately 30%) as shown in the figure below.

For analog applications with no steep edges or sharp transitions, a bandwidth 3x higher than the highest sine wave frequency is usually sufficient as it contains the fundamental, 2nd, and 3rd harmonics.

DC and AC Coupling

An analog signal is composed of DC and AC components, where the DC component is the constant offset and the AC is the time varying signal. Detection and measurement of analog signals can be a task that requires some precision involving the right kind of coupling circuits. There are two main types of coupling circuits.

DC Coupling:

Essentially connecting the input directly to the signal being measured or detected, allowing both the AC and DC components to pass through.

AC Coupling:

Removes the DC offset of a signal, allowing only the AC component to pass through. This is performed with a capacitor that has the ability to block the low frequency signals (DC component) and allow high frequency signals (AC component)through.

AC coupling is used when the DC component doesn't matter. Using AC coupling for signals where the magnitude of the DC component is much greater than the magnitude of AC allows you to see small changes in the signal that may not be noticeable on a large DC offset. For instance, if you want to measure the noise on a DC level from a power supply, then AC coupling can extract only the noise.

Conclusion

Understanding the properties and usage of analog signals can be helpful when interfacing analog devices for data acquisition. Analog signals need to be converted to digital data using an Analog-to-Digital Converter (ADCAnalog-to-Digital Converter (ADC, A/D, or A-to-D)) in order to be processed by a microcontroller or computer. ADCs and interfacing analog signals are covered in the articles below.

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