Measure DC offset, Hum, DBs gain, Oscillation

[awright]

dB Measurement

Hi David:

Yes. I yield. You are correct. And I even emphasized my sloppiness by bringing up the fact that the decibel is basically a power measurement.

But there is rigorous correctness and there is practical, conventional usage. The rigorous usage is common and necessary in applications like telephony, cable distribution systems, RF engineering, and some audio distribution systems where impedance matching is critical to system performance and power levels are the critical parameters to be evaluated.

But there's a whole world of activity out there where power gain/loss and/or optimum power transfer is not the critical parameter and where the decibel is still a very useful tool, even when it is "incorrectly" applied. These are applications in which voltage (or current) are of primary interest and power level is not, such as in many measurement instruments.

In many applications, impedance is not known to any precision or is of no particular concern except that the user wants the input impedance of his measurement device to be high enough that it does not load the measurement point in an uncontrolled manner, thus changing the measured quantity, and he wants the output impedance of his measurement instrument to be low enough that it is not loaded down in an uncontrolled manner by whatever he is driving. Then, if he is using an amplifier that is called (incorrectly) a 60 dB amplifier because the output voltage is 1000 times greater than the input voltage, he can calculate the measured voltage by dividing the output voltage of his amplifier by 1000 without even knowing, except in the most general manner, the impedances involved. And if I want to design a "60 dB" gain brick, with the output voltage 1000 times larger than the input voltage, to have handy on my bench, I'm not really very interested in what the power gain is. Still very useful, even if not rigorously correct.

Getting the beginning electronics hobbyist to grasp the concept and use of the decibel for simple bench measurements is hard enough without forcing him to also deal with impedances, up front. You have correctly called attention to the importance of impedance in rigorous application of the decibel calculation. But a lot of very useful work can be done without worrying about exact impedances.

awright

 

[David Bridgen]

Re: dB Measurement

Getting the beginning electronics hobbyist to grasp the concept and use of the decibel for simple bench measurements is hard enough without forcing him to also deal with impedances, up front.

Getting a beginner, whether they be hobbyist or otherwise, to deal with impedances insofar as the use of decibel notation goes, is imperative right from the start. Too bad if they find it hard.

A poorly taught or understood concept is much harder to correct later on. The longer someone indulges in sloppy practice the more vigorously they will argue in its defence. The fact that the ubiquitous "everybody else" does it this way or that way is no defence at all. It simply propagates and encourages the misconception.