What is Attenuators

Chapter 1.7 Attenuators

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 16CHAPTER 1. AMPLIFIERS AND ACTIVE DEVICESAn adaptation of the dBm scale for audio signal strength is used in studio recording andbroadcast engineering for standardizing volume levels, and is called the VUscale. VU metersare frequently seen on electronic recording instruments to indicate whether or not the recordedsignal exceeds the maximum signal level limit of the device, where significant distortion willoccur. This “volume indicator” scale is calibrated in according to the dBm scale, but does notdirectly indicate dBm for any signal other than steady sine-wave tones. The proper unit ofmeasurement for a VU meter is volume units.When relatively large signals are dealt with, and an absolute dB scale would be useful forrepresenting signal level, specialized decibel scales are sometimes used with reference pointsgreater than the 1 mW used in dBm. Such is the case for the dBWscale, with a referencepoint of 0 dBW established at 1 Watt. Another absolute measure of power called the dBkscalereferences 0 dBk at 1 kW, or 1000 Watts.• REVIEW:• The unit of the bel or decibel may also be used to represent an absolute measurement ofpower rather than just a relative gain or loss. For sound power measurements, 0 dB isdefined as a standardized reference point of power equal to 1 picowatt per square meter.Another dB scale suited for sound intensity measurements is normalized to the samephysiological effects as a 1000 Hz tone, and is called the dBAscale. In this system, 0dBA is defined as any frequency sound having the same physiological equivalence as a 1picowatt-per-square-meter tone at 1000 Hz.• An electrical dB scale with an absolute reference point has been made for use in telecom-munications systems. Called the dBmscale, its reference point of 0 dBm is defined as 1milliwatt of AC signal power dissipated by a 600 Ω load.• A VUmeter reads audio signal level according to the dBm for sine-wave signals. Becauseits response to signals other than steady sine waves is not the same as true dBm, its unitof measurement is volume units.• dB scales with greater absolute reference points than the dBm scale have been inventedfor high-power signals. The dBWscale has its reference point of 0 dBW defined as 1 Wattof power. The dBkscale sets 1 kW (1000 Watts) as the zero-point reference.1.7AttenuatorsAttenuators are passive devices. It is convenient to discuss them along with decibels. Attenu-ators weaken or attenuatethe high level output of a signal generator, for example, to providea lower level signal for something like the antenna input of a sensitive radio receiver. (Fig-ure 26,1.10) The attenuator could be built into the signal generator, or be a stand-alone device.It could provide a fixed or adjustable amount of attenuation. An attenuator section can alsoprovide isolation between a source and a troublesome load.In the case of a stand-alone attenuator, it must be placed in series between the signalsource and the load by breaking open the signal path as shown in Figure 26,1.10. In addition,it must match both the source impedance ZI and the load impedance ZO, while providing aspecified amount of attenuation. In this section we will only consider the special, and most