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SM1720-48L Microwave Journal Article

Ultra Linear Amplifier for PCS Systems

Today’s digital PCS systems require high linearity power amplifiers. Conventional analog systems could get away with lower linearity because analog communications could still be accomplished with noise in the transmission. But today’s digital systems do not work well with noise or distortion. When the bit error rate (BER) is greater than 10-7, a subscriber using the system will experience gaps in communication or even dropped calls.
The infrastructure for today’s PCS systems require amplifiers that can provide high linear output power. Stealth Microwave has developed an ultra-linear power amplifier that offers a solution that is compact, cost effective and power-efficient, and is ideally suited for systems using CDMA or TDMA standards.

Ultra-High Linearity Amplifier
The SM1722-41L is an affordable, high-quality, solid state GaAs amplifier designed for the Personal Communications System (PCS) market. Its ultra-high linearity makes it ideally suited for digital systems. A built-in linearizer increases the SM1722-41L’s output third order intercept point (OIP3) by dB.

The Stealth Microwave amplifier can be used in systems that require error-free digital data transmission for fax, video or voice transmission. Popular uses of Stealth’s model are in repeater stations which - when used properly in conjunction with base stations - save initial costs for setup, save power and money over the long run while running the network, and can fill in the gaps in a service provider’s network.

The amplifier is temperature compensated so it can be used in systems without the need for air conditioning or large cooling fans. It’s also smaller, measuring 6.7 x 4.0 x 2.0", which saves the PCS service provider real estate in their base station or repeater system.

Stealth’s unit also includes an integral heatsink and is available in modular form (standard), rack mount, or as a stand-alone laboratory amplifier. Additional features include over and reverse voltage protection with auto reset, and logic on/off control, a single DC supply, and an optional output harmonic filter.

The SM1722-41L’s DC input voltage requirement is +12 volts and DC input current is 5.5 amps. In order to achieve the same linearity, a non-linearized 100W amplifier would be required. This type of amplifier typically uses 400W of DC power, which is six times that of the SM1722-41L.

Stealth’s amplifier accomplishes its efficiency through linearization technology. The correction circuit reduces intermodulation distortion which improves linearity, allowing a higher output power for a given intermod.

If a non-linearized 100W amplifier was used in a multicarrier environment, the PCS provider would have to back the amplifier off to decrease the intermodulation distortion and provide less output power. If this type of amplifier is backed down to 10W, for example, it would still require 400W of DC power. The majority of this power is wasted as heat. Therefore, the SM1722-41L is more efficient in terms of total power usage, cost, and size.

SM1722-41L vs. Feed-Forward Systems
To compensate for distortion without backing off, some people have turned to feed-forward systems which also provide excellent linearity. Feed-forward techniques are a more common method used to improve linearity in high-power amplifiers.

Although feed-forward amplifiers generally provide better intermod performance when compared to predistortion, the circuitry is typically more complex, large, and more expensive.

Conclusion
The NIMBY syndrome (Not in My Back Yard) has fueled a recent public outcry against building new digital PCS and cellular sites in many communities around America. As a result, many service providers have had to rethink how they expect to build their infrastructure. The use of repeaters and smaller, less intrusive base stations is now the focus, and with this new need are requirements for smaller, more efficient components.

A linearized system, like Stealth Microwave’s SM1722-41L, provides ultra-high linearity with less cost and also saves the PCS service provider operating expenses because it requires less DC power than conventional systems. Because of it’s much smaller size it is ideal for smaller repeaters and base stations that are less obtrusive. So the fallout of the NIMBY syndrome can be averted, while still providing exceptional service in the infrastructure for PCS systems.