![]() Meters are available with resolution ranging from 0.1 to 0.001 dB / dBm, with cost differences to match:Ġ.001 dBm / dB resolution may be occasionally useful in carefully controlled laboratory conditions, however even in this situation, it's very difficult to make use of this much resolution.Ġ.1 dB resolution may not be enough. How much dBm / dB measurement resolution do I need? It is a universal unit that an engineer can apply to any communications link.It offers constant resolution for a given number of decimal places, which improves calculation confidence.It reduces large numbers to a convenient size (eg -70 dB = 1/10,000,000).The dBm decibels unit also has the following useful attributes: P2 is fixed (typically at 1 milliwatt) for dBm P2 = Reference power level (e.g in Watts) For example, an allowable measured margin of 3.12 dB in the above case would not be acceptable if the test uncertainty is larger, resulting in increased rework. Improved test accuracy results directly in allowing more variability in acceptance criteria. So test accuracy is important to reduce measurement uncertainty. In this case, due to measurement uncertainty a measured margin of between 3.12 to 4.88 is actually marginal, eg it could be good or bad, depending. The traditional RMS method shown here is more readily understood, and is quite adequate for most purposes. It is used in calibration laboratories and may result in slightly smaller uncertainty values. The most recent one is the Welch-Satterthwaite equation, however this is more complex and beyond this article. Note there are a few ways to "correctly" calculate uncertainty. So in this case, the actual system margin corrected for this measurement uncertainty is 4 - 0.88 = 3.12 dB Wrong! Uncertainty = 5 x 0.41 dB = 2.05 dB.Here is a summary of wrong and right maths results: Suppose the test accuracy is plus or minus 0.41 dB dB (eg 10%) for each of the 2 absolute power measurements, and each of the 3 loss measurements, then that's 5 lots of 0.41 dB uncertainty, This is where the maths gets a bit difficult, since dB uncertainties do not just add and, and in any case linear uncertainties are most easily added using an rms method. However, some allowance must be made for power & loss measuring uncertainties. In this example system, if the total transmission loss is 19 dB, then the spare system margin is 4 dB. Let's look at the allowable loss budget for a typical system: ![]() The reference level for optical systems is usually 1 mW, since the absolute transmitter power is often about this power level, making it a convenient stating point. The 'm' refers to the reference level used, in this case mW (milli Watts). How this makes calculations simple is shown in an example of a fiber optic transmission system:Ībsolute power levels in this example are expressed in dBm and generally refer to input and output power levels. The decibel unit allows these 3 system parameters to be easily calculated by addition and subtraction, rather than multiplication and division. The universal measurement system adopted for this purpose is the Decibel, which is a logarithmic unit. No-attenuation factors such as dispersion are often summarized as a simple "equivalent loss" penalty, so they can be treated in the same way, Path degradation may involve a combination of factors, such as attenuation and dispersion. Note that transmitter power and receiver sensitivity are absolute power levels (eg Watts or dBm), whereas the transmission path degradation is a relative value (eg % signal reduction or dB), which is generally independent of the actual power level involved. It is therefore quite natural that communications engineers should use a system of units and measurements that enables these three elements to be easily defined and calculated. Most communication systems (human speech, sonar, microwave, radio, co-ax, fiber optics, twisted pair etc) are simply described in terms of:
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