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Open Mic - Make HFC network improvements a reality

Tue, 07/03/2012 - 11:40am
Kellie Chong, director of product line at RFMD

Welcome new GaN-based hybrid amplifiers.

RFMD's Kellie ChongHigh-data-rate and dense content-delivery-capable wireline networks are faced with formidable requirements as demands for higher data rates and more digital content increase.

Demands on operator networks include delivering more digital and mixed signal content while using less energy; delivering passive optical network (PON) competitive, instantaneous and sustained data rates of ≥ 1 Gbps in the downstream, without network bandwidth expansion through four, eight and higher-number channel bonding; moving to symmetrical ≥ 1 Gbps data rates; increasing all-digital signal delivery efficiency through increasing digital modulation complexity; expanding existing 750 MHz-capable plant bandwidth to 1 GHz or higher without the replacement of installed plant diplex filters, taps, coaxial cable and connectors; and decreasing plant headend and hub energy consumption while simultaneously increasing all digital unicast content delivery capability.

Relatively new to the cable TV market, GaN-based hybrid amplifiers installed in HFC networks can provide up to twice the delivered RF power at lower distortion levels – with higher composite carrier-to-noise (CCN) – and at 20 percent or lower power dissipation than any of today’s best available alternatives.

GaN amplifiers power both new and upgraded legacy operator networks equally well. A GaN-enabled HFC active node or line amplifier can accommodate a wide range of installed amplifier spacings, allowing operators to use existing architectures while immediately saving operating expenses and future-proofing their plants.

What makes GaN good for operator networks today and a better alternative than anything else available? Technically speaking, GaN’s advantages start with its wider bandgap material properties. In today’s process technologies, only diamond possesses a wider bandgap than GaN – 5.45 electron volts (eV) for diamond versus 3.45 eV for GaN – for material useful as the basis for linear amplification.

A material’s bandgap determines how electrons (and “holes”) behave within an amplifier’s physical structure and what it takes to get these electrons (and “holes”) “excited” in the sense that they are willing to give up their energy. When a bunch of electrons all move together in the same direction, they produce an electric current.

Electrons in the atom of a semiconductor material such as GaN can be thought of as being in various states – including their energy level, momentum and spin – with different probabilities of being in a given state. Two electrons can’t be in the same state at the same time – that is, at least one variable must differ. Some particular states are possible, and some are forbidden by the laws of quantum mechanics. Sets of possible states form regions that are called “bands.” Sets of states that are not possible form regions between those bands, and these are called “bandgaps.”

The higher-energy gap of GaN amplifiers gives them the ability to operate at higher temperatures, withstand higher operating and transient voltages, and provide improved distortion performance at lower direct current (DC) power dissipation versus amplifiers built using lower-energy gap materials (such as silicon at 32 percent of GaN’s energy gap level and GaAs at 41 percent of GaN’s energy gap level).

Today’s best amplifiers constructed using lower-energy gap materials are reaching their limits of operating frequency, breakdown voltage and power density. GaN-based amplifiers have only begun to define performance boundaries.

Looping back to the operator’s network challenges, operating from 20 percent or less DC power expended, GaN amplifiers deliver twice the RF signal level at equal or lower distortion than today’s best silicon or GaA-based active nodes and line amplifiers. Therefore, a node plus zero HFC network using GaN amplifiers can operate at greater distances from the active node to subscriber tap while using legacy coaxial cable, connectors and passives.

In fact, GaN-equipped active nodes and line amplifiers allow use of up to +18 -dB positive tilt (from 54 to 1002 MHz) in active nodes and line amplifiers, without increasing the number of, or spacing between, actives. This is possible because GaN amplifiers deliver both higher RF power output and lower distortion simultaneously at 1 GHz and higher frequencies without sacrificing reliability.

Operators can now consider upgrading their 750 MHz-capable plants to 1 GHz and higher operating frequencies by replacing legacy active nodes and line amplifiers, one-for-one, with GaN-equipped actives. This type of upgrade can be accomplished without changing existing line amplifier spacing and while using installed coaxial cable, connectors and taps.

Email: kchong@rfmd.com

Next month, Patrick Knorr, general manager of regional accounts at Synacor, will write about the usage billing essentials needed for success.

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