High Linearity SSPAs for 2.3 to 2.7 GHz Applications
An introduction to small, high efficiency, ultra-linear solid-state power amplifiers
The demand for high speed data transmission is on the rise. Thanks to the Worldwide Web, data transmission has become the fastest growing segment of the telecommunications market. Fiber optics and high speed copper, DSL and T1 transmission have satisfied many of the urban areas, but suburban and rural America continue to crawl at 28 and/or 56 kbps. Since cable TV companies have been slow to capture the growing market, a large window of opportunity has been opened.
Many wireless operators are currently performing trials in wireless local loop and fixed point broadband access using new techniques such as orthogonal frequency division multiplexing (OFDM). Several frequency bands have become popular for these trials, including wireless communication services (WCS) at 2.305 to 2.360 GHz, industrial, scientific and medical (ISM) at 2.385 GHz, and multichannel multipoint distribution systems (MMDS) at 2.500 to 2.686 GHz.
These trials demand ultra-linear RF subsystems with very low intermodulation distortion (IMD) products. Since the high power amplifier is typically the leading cause of IMD products, it usually dictates the linearity of the entire transmitter. Another hurdle system providers face is rough environmental conditions since the subsystems are typically placed on telephone poles and/or rooftops to maximize RF coverage. With this type of deployment, a large, feedforward basestation amplifier cannot be used since its power consumption and cooling requirements are too difficult to satisfy under the aforementioned conditions. A small, high efficiency, ultra-linear amplifier is the only solution.
To address this market, the company's existing 2.3 to 2.7 GHz high power amplifier line has been completely redesigned. By incorporating the latest GaAs technology, the amplifiers deliver similar performance to their predecessors, while consuming 20 to 40 percent less DC power. The new amplifier's exterior dimensions of 7.5" * 4.0" * 0.8" remain unchanged. The units operate in frequency bands between 2.3 and 2.7 GHz, and range in output power (P1dB) from 12 to 25 W. Power levels to 100 W (SM2527-50 & 50L) are also available. The line-up also consists of models incorporating Stealth's proprietary predistortion linearization, which improves the output third-order intercept point (OIP3) by 6 to 7 dB. Table 1 lists key specifications for two of the amplifier models, the SM2325-41L (2.3 to 2.5 GHz) and SM2527-44L (2.5 to 2.7 GHz).
The pre-distortion circuitry uses amplitude and phase correction to reduce the IMDs created in the amplifier. For the SM2325-41L, IMD levels are 60 dBc, at an average output power of 35 dBm, vs. 40 dBc for a non-linearized unit, as shown in Figure 1.
Similar performance can be produced in the 2.5 to 2.7 GHz band with the model SM2527-41L amplifier. If more power is required, 20 W non-linearized (SM2527-43) and 25 W linearized (SM2527-44L) versions are also available. Figure 2 shows the model SM2527-44L amplifier's linear gain (S21 ) and Figure 3 displays its input SWR (S11 ).
All units have several built-in features, including a single DC power supply, over/reverse voltage protection and thermal protection, which automatically turns the unit off if the baseplate temperature exceeds +70° C and will reset once the temperature returns to operating conditions. Logic on/off control is also standard, allowing the user to turn the unit on and off via a TTL signal. Optional features include forward/reverse power detection, an RF sample port for analog monitoring and, for the first time, a TTL pin for high speed switching at rates up to 10 µs. Additional information on this new line of amplifiers may be obtained from the company's Web site at www.ssbtech.com/stealth.htm.
Stealth Microwave, a division
of SSB Technologies Inc.,
Trenton, NJ (609) 538-8586
or (888) 772-7791.
Circle No. 301