If you're at all familiar with copper cable, you've probably heard about crosstalk--the phenomenon by which a signal transmitted on one pair or one channel creates an undesired affect on another pair or channel.

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Crosstalk causes interference on an affected pair of conductors or overall cable creates errors or prevents data transmission. For example, have you ever heard someone else's conversation while on the ? This is caused by interference between adjacent wires.

Many Ethernet cable testers can measure crosstalk.  But are you aware of the difference between the near end and far end crosstalk parameters you need to test for in balanced copper network cabling systems? As part of our Cable Testing 101 series, we thought it would be prudent to take a closer look.

At the Near End

Near end crosstalk (NEXT), is a performance parameter measured within a single link/channel. It measures the signal coupled from one pair to another. The pair causing the interference is referred to as the "disturbing pair" while the pair impacted by the crosstalk is the "disturbed pair." 

NEXT is expressed in decibels (dB), and it varies with the frequency of the transmission since higher frequencies create more interference. The higher the dB value, the less crosstalk is received by the disturbed link/channel. For example, a Category 5e cable characterized to 100 MHz might have a NEXT value of 45.8 dB at 20 MHz and a NEXT value of 35.3 dB at 100 MHz, indicating better NEXT performance at the lower frequency.

The measurement is called "near end" because it measures crosstalk at the same end of the link/channel where the signal is sourced

Pair twists are what help to cancel out NEXT--different twist rates on each pair prevent the pairs from picking up signals from adjacent pairs. That's why it's important to maintain pair twists as close to termination as possible. Twist rates are also optimized for crosstalk performance and isolation has improved with each category of cable. That's why a Category 6 cable characterized to 250 MHz has a NEXT value of 44.3 dB at 100 MHz compared to 35.3 dB for Category 5e at the same frequency.

Power sum near end crosstalk, abbreviated at PSNEXT, is simply a calculation that sums the NEXT measurement of all adjacent pairs. NEXT measures the crosstalk on each pair of a four-pair cable as affected by the other three pairs individually. PSNEXT is simply the sum of the crosstalk of all three adjacent pairs, and it's important because it tells us what the impact is on a pair when used in a network where all four pairs are transmitting signals (i.e., BASE-T).

And last but not least, near end crosstalk testing includes PSACRN--power sum attenuation to crosstalk ratio, near end (previously called PSACR but renamed to distinguish it from PSACRF explained below). Calculated using PSNEXT and insertion loss values (see our previous 101 Series blog on insertion loss), it tell us the difference between each pair's attenuation and the combined crosstalk received from the other three pairs. The purpose is to ensure that received signals are strong enough in relation to the noise in the cable. The higher the PSACRN value, the better the performance.

At the Far End

Far end crosstalk, abbreviated FEXT, is also measured within a channel. It's a lot like NEXT but as its name infers, it is measured at the far end of the channel. However, by itself FEXT doesn't tell us much since signals are attenuated over distance.

To provide a more significant result, the attenuation (insertion loss) is removed from the FEXT result and referred to as equal level far end crosstalk (ELFEXT). In recent years, TIA renamed this parameter attenuation to crosstalk ratio, far-end, or ACRF for short.

Like NEXT, ACRF measurements are summed for each of the three disturbing pairs giving power sum ACRF (PSACRF). This PSACRF parameter used to be referred to as power sum ELFEXT (PSELFEXT) before the TIA renamed the parameter.

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So what about alien crosstalk? Once we move to the higher frequencies of Category 6A to support 10GBASE-T applications, we now have to worry about crosstalk between cables, not just within. In an upcoming 101 Series blog we will delve into the alien crosstalk parameters (warning: there are multiple).

Best Electrical Testers and Meters for Voltage Diagnosis

Last updated April 21,

Electrical testers and meters are diagnostic tools for testing different wire connect types. These tools determine if the wires are hot. Testers and meters also verify whether an electrical device is functioning correctly. And they indicate whether a receptacle is wired properly. This guide details the types of electrical testers and meters so you can select the right tool for you.

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Table of Contents

1. Choosing the Best Electrical Tester

2. Selecting the Best Multimeter

Choosing the Best Electrical Tester

Voltage testers and receptacle analyzers detect the presence of electrical voltage in cable, wires, circuit breakers, light fixtures, outlets and switches. Testers and analyzers indicate if the power is off at an outlet. They also let you know whether a device is connected properly.

• A non-contact voltage tester works well for all types of electrical work. They provide a reading without the need to touch the wires. Non-contact voltage testers emit a sound or illuminate when voltage is detected. However, they do not provide a numerical reading of any current voltage.
• Electronic voltage testers measure alternating current (AC) and direct current (DC) and give a numerical reading of the voltage present. Some electronic voltage testers also measure continuity.
• Receptacle analyzers plug into an outlet to determine whether that outlet is working properly. Some analyzers test both GFCI (ground fault circuit interrupter) outlets and standard outlets. Different lights illuminate to signal if the outlet is accurately grounded and wired. The analyzer also identifies common wiring faults if the outlet is not wired correctly.
• Circuit breaker finders identify the circuit breaker controlling an outlet. They consist of two parts: a transmitter and a receiver. The transmitter plugs into the outlet and the receiver passes over the circuit breakers in the breaker panel. When the receiver passes over the correct breaker, it emits a sound or lights up to signal which circuit breaker to turn off before working on an outlet. Some circuit breaker finders include a light socket adapter for finding the circuit breaker that controls a light fixture.
• Continuity testers determine if a light switch is malfunctioning. They measure whether an electrical circuit is open or closed. A light switch circuit is closed when the light is on. It is open when the light is off. Closed circuits have continuity and conduct electricity while open circuits do not have continuity because they cannot conduct electricity.
• Plug-In Testers have LCD displays for voltage and circuit information, especially more modern ones.

It's also a good idea to check the voltage range that the device is designed to test for. The higher the voltage range, the more versatile the tool.

Safety Tip: Be sure to turn off power to a circuit before performing a continuity test.

Selecting the Best Multimeter

Of the several meter types, a multimeter is the most budget friendly. It performs several functions in one unit. Multimeters come with black and red lead probes attached to the ends of wires that can be held against battery terminals or inserted into outlets for testing.

One multimeter can do the work of three tools. It can be a voltmeter that measures voltage or an ammeter that measures current, an ohmmeter that measures resistance or an advanced multimeter model to measure continuity, temperature and more.

• Digital multimeters display measurements as a number and offer the most reliable readings.
• Analog multimeters display measurements across a bar.
• Auto-ranging meters do not require you to manually set the expected voltage range.
• Professional multimeters can have backlit displays, work lights, high safety ratings and magnetic mounts.
• Clamp meters primarily measure current. Some clamp meters measure voltage, temperature and more. Clamp meters make it easier to measure the current in an electrical conductor. A wire can be threaded through the clamp opening without having to disconnect the wire.

Safety Tip: Before use, check your voltage testers' safety compliance ratings.

Understanding all the different types of testers and meters makes choosing the right one simpler. You can use an electrical tester to troubleshoot an outlet. Digital multimeters make readings easier to see under low light. Continuity testers work best for lights. There’s a tester or meter for every situation or application.

Need a meter or other electrical tool? The Home Depot Mobile App makes it easy to compare different types of testers and meters.

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