dB or dBm? What Are These and What Is the Difference between the Two?


This article explains the concepts of decibels (dB) and decibel-milliwatts (dBm), essential for measuring signal strength in technologies using lasers and LEDs. It breaks down how dB uses a reference level for quick comparisons, while dBm sets this reference precisely at 1 milliwatt to ensure accurate assessments. We’re also drawing on their critical role in maintaining equipment functionality and enhancing safety in technological environments, offering insights into their practical applications.

The concepts of decibels (dB) and decibel-milliwatts (dBm)l

Page Contents

Explaining Decibel (dB)

So, there is a signal that can be detected. To obtain a numerical representation of this signal (a sort of description of it), the concept of signal power was introduced. Power is measured in watts (W). Everything would be fine, but it's just not convenient! Why is that so? Because such description doesn’t make it clear if the power is sufficient, or perhaps it is low and things are going wrong? Therefore, inventive people have long been using the concept of "decibel".

What exactly is a decibel? A decibel is a ratio. A ratio of what to what? It is the ratio of the power of a measured signal to a reference power level. For ease of use in the formula that will be shown below, certain multipliers (10) and logarithms (the common logarithm with base 10) are used. Why this complexity? It is done to quickly understand if the measured signal is lower, higher, or perhaps equal to the reference signal after simple calculations.

Explaining Decibel where A is the measured signal and A0 is the reference signal

Introducing Decibel-Milliwatts (dBm)

In modern networking technologies, which are generally well-known, lasers and light-emitting diodes (LEDs) are used. They emit light into fiber and, at the other end, this light is received. The light can be measured using various devices that output results in specific units of measurement. Modern devices are capable of measuring power in watts (W), decibels (dB), and several other units, such as dBm, which are not yet entirely clear to us.

As for watts, it’s pretty straightforward: you can tell if the power is high or low, for example, 0.05 W. Decibels are more complicated: to understand them, you need to know the exact values of the light signal “before” and “after” it passes through the fiber, which isn’t always convenient (for example, traveling 80 km to make a measurement and then returning to make similar measurements on site). Thus, other units of measurement, such as dBm, have been introduced and used to simplify such measurements.

Calculating the gain in Decibels for Power Quantities

dBm is essentially the same as a decibel, but in this case, the reference signal used has a power of 1 milliwatt (mW). Now everything falls into place! This makes calculation more convenient: just take the logarithm of the signal power and multiply by 10. This way you can always be sure that the reference level from which all measurements are taken is constant and always equals 1 mW.

Explaining Decibel where A is the measured signal power in milliwatts

Let’s look into the following case. Let’s take a 1550/1310 20 km WDM SFP module. Let’s assume it’s fully operational and in good working order. We’ve measured its performance and found it to be -6 dBm. “Wow! That's a negative value! That must be bad!” might claim people who aren’t especially knowledgeable about technicalities. However, this assumption would be incorrect. Why? Because -6 dBm corresponds to about 0.25 mW.

For the calculation, we use the same formula, but this time we perform the calculations in reverse order:

10lg X = -6;
lg X = -6/10;
lg X = -0.6;
X = 10^(-0.6);
X = 0.25

Thus, the situation turns out to be far from as bad as it seemed! The negatives have disappeared.

Decibel Conversion

To spare time and effort, smart people made a small table. It may not be as accurate as a formula, but it helps you understand what's going on faster and easier.

dB to Times Conversion Table

dB 40 dB 20 dB 10 dB 6 dB 3 dB 1 dB 0 dB -1 dB -3 dB -6 dB -10 dB -20 dB -40 dB
Times 10,000 100 10 4 2 1.26 1 0.79 0.5 0.25 0.1 0.01 0.0001

The question remains: why such a low power? The explanation is that light can indeed be dangerous, and laser technology with a power of 500 mW can pose dangers, such as disrupting air traffic (this is true, and in many countries, the use of such “gizmos” is restricted by law). Additionally, optical fiber is very thin and can heat up. This can lead to burns or even the melting of the fiber.

Toolboom Team

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