2<\/sup> ) = 5.4 % ( not 9 % ).
\nHowever dBm \/ dB uncertainty values must be converted to linear values and then averaged using the above method.
\nIn practice, this inconvenience can often be avoided by use of a “decibel math” rule of thumb as follows:<\/p>\nf the 2nd highest logarithmic value is less than 80% of the highest value, the uncertainty is approximated to the largest value.
\nIf the 2nd highest value is over 80% of the highest value, multiply the highest value by 1.4 for the combined figure<\/i>.<\/p>\n
In practical situations, one or two uncertainty figures tend to dominate the others. Here are some examples:
\nThree different uncertainty values of 0.3 dB, 0.2 dB, 0.1 dB, give a combined uncertainty of 0.35 dB. The error in ignoring the lower values is only 0.05 dB.
\nTwo similar uncertainty values of 0.3 and 0.29 dB, give a combined uncertainty of 0.39 dB. The approximated figure is 0.42 dB, out by only 0.03 dB.
\nFive different uncertainty values of 0.4, 0.2, 0.1, ,0.09, 0.05 dB give a combined uncertainty of 0.42 dB. This is within 0.02 dB of the approximated figure of 0.4 dB.<\/p>\n
dB Averaging<\/h2>\n
Within the telecommunications industry, a links attenuation is commonly calculated by averaging a bi-directional measurement. There are various reasons given for this, but the most important practical reasons are that this method eliminates meter calibration errors and minimises the effects of source drift. This is of course simple stuff, however the hidden flaw is that it is mathematically incorrect for logarithmic or dB units to be treated this way. The correct method is to first convert to linear, then average and finally convert back to dB. Further reading on this topic can be found in Kingfisher Application Note A14, Improving Attenuation Measurement Accuracy<\/p>\n
Electro – Optic Conversion:<\/h2>\n
Optical signals are usually converted to an electric current, which is then measured as a voltage across a resistor (in a trans-impedance amplifier).
\nThe electrical power dissipated by the resistor goes up with the square of the voltage.
\nSo the optical receiver power also goes up with the square of the optical signal.<\/p>\n
So when working in volts, the relationship is defined as: dB = 20 log (V1<\/sup>\/V2<\/sub>)<\/strong><\/p>\nWhere
\nV1 = measured voltage
\nV2 = reference voltage ( eg 1 mV )<\/p>\n","protected":false},"excerpt":{"rendered":"
An Introduction Most communication systems (human speech, sonar, microwave, radio, co-ax, fiber optics, twisted pair etc) are simply described in terms of: Transmitter power Transmission path degradation Receiver sensitivity (power) 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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":1028,"menu_order":0,"comment_status":"open","ping_status":"open","template":"","meta":[],"_links":{"self":[{"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/pages\/1037"}],"collection":[{"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mpc-oman.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1037"}],"version-history":[{"count":2,"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/pages\/1037\/revisions"}],"predecessor-version":[{"id":1260,"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/pages\/1037\/revisions\/1260"}],"up":[{"embeddable":true,"href":"https:\/\/mpc-oman.com\/index.php?rest_route=\/wp\/v2\/pages\/1028"}],"wp:attachment":[{"href":"https:\/\/mpc-oman.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1037"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
![\"How](\"http:\/\/www.kingfisherfiber.com\/Application-Notes\/images\/A01.gif\")
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<\/a><\/p>\n![\"Figure](\"http:\/\/www.kingfisherfiber.com\/Application-Notes\/Images\/A01_Fiber_Span_Loss.png\")
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