﻿ Everything you need to know about decibels

# Everything you need to know about decibels

2017-10-10
One dB (decibel) really makes a huge difference!

Why such terms decibels are used? When you compare cable performance and a manufacturer states that the attenuation (power loss) for a cable operating at 100MHz, with a length of 90 meters is 20dB, it means that the signal strength has dropped by a factor of 100. If you apply an input power of 10 watts, the output will be only be 0.10 watts! That is a 50 percent power loss for every 3dB of attenuation.

You therefore want to have low decibel values of attenuation as this means that less of the signal is lost on its way to the receiver. You also want higher crosstalk decibel values (ACR-F, NEXT, PS ACR-F) and return loss because that means less signal is measured on neighboring wires.

A decibel may be thought of as audible noise. When talking about sound, a decibel is not a unit of measurement, but is used to express a ratio of sound pressure. A decibel is however also commonly used when deﬁning attenuation, return loss and crosstalk.

As with sound, in electrical and communications transmission performance, the decibel is a ratio rather than a unit of measure. As digital and analog communication signals are electrical energy instead of sound pressure, the dB unit is the ratio of input power to output power.

The decibel value is independent of the actual input and output power (or voltage) and is thus considered a generic performance specification. Understanding what decibel numbers mean is important when comparing cabling media or performance measurements.

Decibels basics

The “bel” part of decibel was named after Alexander Graham Bell, the inventor of the telephone, and by the – also twisted pair, which is less known fact.

A decibel is a logarithmic ratio of power (or voltage) output to power (or voltage) input.

The decibel is therefore a convenient way to express power loss or gain, regardless of the actual values.

Note: For measurements such as attenuation, ACR-F, NEXT and return loss, the decibel value is negative because it represents a loss, but the negative sign is often omitted when the measurement is given, as it is assumed that the number indicates a loss.

Attenuation is described in decibels. If you for example measure two cables of identical length, the one’s attenuation could be 15dB, while the other could be 21dB.

As we know that a lower attenuation is better, the cable with a 15dB attenuation is better than the one with a 21dB attenuation. The small attenuation difference of 6dB actually means that the cable with 21dB attenuation actually has 50 percent more attenuation of voltage or current than the other one.

To fully understand performance speciﬁcations of cables, you need to know how a decibel is calculated.

Power and Decibels

When measuring power (watts), decibels are calculated as follows:

dB = 10 x log10 (P1/P2)

P1 indicates the output power, while P2 is the reference, or input power.
If we for example apply a reference power level (P2) of 1.0 watts and the measured power level (Pl) at the other end of the cable is 0.5 watts, 50% of the signal was lost due to attenuation. Plugging these values into the formula gives us a value of 3dB. This calculation means that:

Every 3dB of attenuation causes a 50% signal power loss through the cable. We want a low attenuation value, since a higher power level will then arrive at the destination.
Every 3dB of return loss translates into 50% less signal power being reﬂected back to the source. We want a high decibel value for return losses, since less power will then be returned to the origin.
Every 3dB of NEXT translates into 50% less signal power being allowed to couple to adjacent pairs. We want a high decibel value for crosstalk values, as higher values indicate that less power will be coupled to adjacent pairs.

An increase of 10dB results in a tenfold increase in the actual parameter measured.

The table below shows the logarithmic progression of decibels for power measurements.

Table 1: Logarithmic progression of decibels

 Decibel Value Actual increase in measured parameter 3 dB 2 10 dB 10 20 dB 100 30 dB 1,000 40 dB 10,000 50 dB 100,000 60 dB 1,000,000

Decibels and Voltage

Performance speciﬁcations and cable testers typically refer to voltage rather than power ratios. When referring to voltage (or current), decibels are calculated slightly differently than for power. The following formula is used:

dB = 20 x log10 (P1/P2)

P1 indicates the input voltage or current, and P2 is the reference (or output) voltage or current.

When we use a reference value of 1.0 volt for P2 and 0.5 volts for P1 (the measured input), you get a value of -6dB. The calculation means that:

Every 6dB of attenuation translates into 50% of the voltage being lost to attenuation. We want lower decibel attenuation values, as a higher voltage level will then arrive at the destination.
Every 6dB of return loss translates into 50% less voltage being reﬂected back to the source. We want higher decibel values for return loss, as less voltage will then be returned to the source.
Every 6dB of NEXT translates into 50% less voltage coupling to adjacent wire pairs. We want a high decibel value for crosstalk values, as higher values indicate that less power will be coupled to adjacent pairs.

Table 2 shows various decibel levels and the corresponding voltage and power ratios. Note that for the power ratio, a cable with an attenuation of 10 dB, only 10% of the signal transmitted will be received on the other side.

Table 2: Decibel levels with corresponding voltage and power ratios

 dB Voltage ratio Power ratio 1 1 1 -1 0.891 0.794 -2 0.794 0.631 -3 0.707 0.500 -4 0.631 0.398 -5 0.562 0.361 -6 0.500 0.250 -7 0.477 0.224 -8 0.398 0.158 -9 0.355 0.125 -10 0.316 0.100 -12 0.250 0.063 -15 0.178 0.031 -30 0.032 0.001 -40 0.010 0000 -50 0.003 0.000

Practical examples

Let’s now look at the speciﬁed channel performance for Category 5e versus the channel performance for Category 6 cable at 100MHz.

Media type Attenuation NEXT Return loss
Category 5e 24 30.1 10.0
Category 6 21.3 39.9 12.0

For the values to be compared properly, we’ll convert them to the actual percentage of loss. For this example, we’ll use voltage.

Use each decibel value and solve for the P1/P2 ratio using this formula:

Ratio = 1/ (Inverse log10 (dB / 20))

Existing standards allow a transmission to lose 99% of its signal to attenuation and still be received properly.

For an Ethernet application operating at an input voltage of 2.5V, the measured voltage at the receiver must be greater than 0.025V. In the Category 5e cable example, only 6.3% of the voltage is received at the destination, which calculates to about 0.16V.

You can also consider Copper LAN Extenders, like these, to extend your Ethernet distance to more than actual distance of Cat5E or Cat6 allows.

For Category 6 cable it calculates to 0.22V volts, which is still almost 10 times the minimum required voltage for the signal to be received.

You will be able to better compare the performance of any media by using these techniques for reversing the decibel calculation.
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