Note: The following technical article was current at the time it was published. However, due to changing technologies and standards updates, some of the information contained in this article may no longer be accurate or up to date.
What's in a dB?
The decibel (dB) is considered the baseline by which all telecommunications designers look to compare cabling system performance. But, what is a dB? And, what performance advantage does a margin of a few decibels really offer? The answer can be found by looking at the origins of the terminology.
First used for measuring the intensity of sound, the decibel was named after Alexander Graham Bell. A decibel is a convenient way for engineers to describe the input to output ratios of either power or voltages as shown in Figure 1. The benefits of specifying performance in units of decibels include:
- Decibels can be used to describe performance independently of an application's operating voltage or power - therefore, it is a "generic" performance specification.
- The decibel is calculated on a logarithmic scale that allows performance specification across a wide range of voltage/power.
- Decibels may be added and subtracted (versus multiplying and dividing their corresponding ratios), thereby facilitating calculations and graphical solutions.
- Transmission performance is most commonly specified in units of dB.
The simplest way to examine how the decibel function operates is to assume a reference voltage of 1. Substituting 1 volt into the decibel function and solving for the corresponding decibel that relates to half of the signal strength (0.5 volts) demonstrates that an improvement in performance by 6 dB results in a reduction in the signal strength by one-half.
This means that:
- A 6 dB crosstalk requirement would translate to 50% of the signal voltage being allowed to couple onto adjacent pairs
- A 6 dB return loss requirement would translate to 50% of the signal voltage being reflected back on a transmission line
- A 6 dB attenuation requirement would translate to 50% of the signal voltage being lost along a transmission line
As these equations demonstrate:
- Better crosstalk loss (NEXT, FEXT, and ELFEXT) and return loss performance is specified by a larger performance limit (in decibels) because less signal voltage is coupled or reflected
- Better attenuation performance is specified by a smaller performance limit (in decibels) because less signal voltage is lost or attenuated
The second column of Table 1 lists the equivalent voltage loss per 1 volt that a sampling of decibel values correlate to. Note that every 6 dB results in a 50% change in signal voltage. Actual voltage drops can be calculated by multiplying output signal voltage by the equivalent decibel loss on a per volt basis. For example, if an Ethernet application operates at 2.5 volts, then a 40 dB NEXT loss value is equivalent to: (0.01) (2.5 V) = .025 V of NEXT loss |
A Note about Power
Telecommunications decibel limits are typically cross-referenced back to voltage ratios. However, power ratios can be specified just as easily. Substituting 1 volt into the decibel function and solving for the corresponding decibel that relates to half of the signal power (0.5 watts) demonstrates that an improvement in performance by 3 dB results in a reduction in power by one-half.
This means that:
- A 3 dB crosstalk requirement would translate to 50% of the signal power being allowed to couple onto adjacent pairs!
- A 3 dB return loss requirement would translate to 50% of the signal power being reflected back on a transmission line!
- A 3 dB attenuation requirement would translate to 50% of the signal power being lost along a transmission line!
The third column of Table 1 lists the equivalent power loss or drop per 1 watt that a sampling of decibel values correlates to.
Converting decibels to simpler units
Converting a specified decibel limit to an actual percentage of voltage magnitude that is coupled, lost, or reflected in a given application can simplify comparison of actual performance benefits delivered by different category-rated components. The example that follows will focus on connecting hardware performance, however, this comparison method is applicable to all component and cabling system claims. Table 2 provides the '568-A category 5, SP-4195-A category 5e, and proposed TIA PN-2948 category 6 requirements for connecting hardware at 100 MHz.
Table 2
TIA Specified Limits at 100 MHz in decibels
Parameter |
'568-A category 5 |
SP-4195-A category 5e |
PN-2948 category 6 |
Attenuation |
0.4 dB |
0.4 dB |
0.2 dB |
* Category 5 FEXT loss requirements have not been specified for "legacy" category 5 connecting hardware. New category 5 designs shall comply with category 5e FEXT loss requirements.
Table 3 depicts the same category 5, category 5e, and category 6 connecting hardware attenuation, return loss, far-end (FEXT) crosstalk, and near-end (NEXT crosstalk) limits converted to percentage of voltage magnitude coupled, lost, or reflected.
Table 3
TIA Specified Limits at 100 MHz in decibels Converted to Percentage of Voltage Magnitude Coupled, Lost, or Reflected
Parameter |
'568-A category 5 |
SP-4195-A category 5e |
PN-2948 category 6 |
Attenuation |
4.5 % lost |
4.5 % lost |
2.3 % lost |
It is interesting to see how the decibel function can prove misleading when comparing performance data. For example, an 8 dB improvement in connecting hardware return loss performance (from category 5 to category 6) results in a 60% drop in reflected signal voltage, while a 14 dB improvement in connecting hardware NEXT performance (from category 5 to category 6) results an 80% drop in coupled signal voltage.
What do decibel values correlate to in terms of performance headroom claims?
Performance claims in decibels may also be converted to a voltage percentage lost, coupled, or reflected for any cabling product or system. An example of such a conversion using the typical and worst case performance claims for the Siemon category 6 MAX™ 6 outlet follows. Refer to Table 4 for guaranteed worst case and typical Siemon category 6 MAX™ 6 outlet performance at 100 MHz. These values will be used as a baseline for all decibel to voltage percentage conversions.
Table 4
Siemon MAX™ 6 Guaranteed Performance at 100 MHz in decibels
Parameter |
MAX™ 6 Worst Case Performance |
MAX™ 6 Typical Performance |
Attenuation |
0.06 dB |
0.05 dB |
Table 5 depicts the same MAX™ 6 outlet attenuation, return loss, far-end (FEXT) crosstalk, and near-end (NEXT crosstalk) limits at 100 MHz converted to percentage of voltage magnitude coupled, lost, or reflected.
Table 5
Siemon MAX™ 6 Guaranteed Performance at 100 MHz in decibels Converted to Percentage of Voltage Magnitude Coupled, Lost, or Reflected
Parameter |
MAX™ 6 Worst Case Performance |
MAX™ 6 Typical Performance |
Attenuation |
0.7 % lost |
0.6 % lost |
One of the advantages of comparing data on a percentage voltage basis is that signal performance margin is independent of the normalizing tendency of the decibel function. For example, the Siemon MAX™ 6 outlet worst case return loss performance of 28.9 dB calculates to a 6.9 dB margin over the TIA performance limits, but this performance actually delivers more than a 50% drop in reflected signal voltage. The charts in Figure 2 depict the comparative voltage percentages for all category connecting hardware attenuation, return loss, far-end (FEXT) crosstalk, and near-end (NEXT crosstalk) limits versus typical and worst case MAX™ 6 outlet values.
Understanding the decibel function can provide additional insight into performance claims. Note that, in addition to reviewing specific frequency points of interest, an important aspect of complete component qualification includes knowledge of worst case performance specifications and margins guaranteed over the full category frequency range of interest. Furthermore, always be sure to look for linear frequency response in addition to worst case values when analyzing performance data.
Percent Signal Voltage Lost (Connecting Hardware Attenuation) at 100 MHz
Percent Signal Signal Reflected (Connecting Hardware Return Loss) at 100 MHz
Percent Signal Voltage Coupled (Connecting Hardware NEXT Loss) at 100 MHz
Percent Signal Voltage Coupled (Connecting Hardware FEXT Loss) at 100 MHz
Figure 2 - Comparative Voltage Percentages - Various Categories and the MAX™ 6 outlet
Conclusion:
Converting decibel claims and margins to percentage voltage loss, coupled, or reflected can provide meaningful insight into the magnitude of a manufacturer's performance claims.
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