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Ultra-Linear SSPA for the PCS and UMTS Markets

SM1720-48L Microwave Journal Article

Third generation (3G) wireless systems will need to produce more reliable RF signals than those of the present since data, rather than voice, will dominate the percentage of “bits” being transmitted.

Given that there are restrictions on how much power a transmitter can produce, increasing the output power of a given site is not an option. Therefore, a larger number of smaller cell sites will be needed to properly cover a given area. This presents a major challenge for the service providers since available land in many parts of the U.S. is difficult to obtain, and the “not in my back yard” (NIMBY) attitude of most Americans hinders progress. To overcome these obstacles, base stations need to be more spectrally efficient, and require more stealth to better conceal their presence. However, they need to continue to produce signals that do not include unwanted intermodulation distortion products (IMDs). The RF/Microwave high power amplifier (HPA) is typically the leading cause of this type of distortion and therefore dictates the linearity of the entire transmitter. A common method used to reduce IMDs is to “back-off” the output power of the HPA until the desired carrier to intermodulation (C/I3) is achieved. The relationship between IMDs, intercept point and carrier levels is expressed as follows:

Single Carrier Level + (1¼2 x C/I3)= Output Intercept Point (OIP3)

With a single carrier level of +35.5 dBm and a desired C/I3 of 55 dBc, an amplifier with an OIP3 of +63 dBm would be needed. Since the P1dB of an HPA is typically 10 dB below its OIP3, a P1dB of +53 dBm would be required. This performance is approximately equal to that of a conventional 200 Watt amplifier. If +12V class A transistors are used, such an amplifier would consume over 60 Amps as well as being very large and costly. During the past 20 years, linearization methods have been incorporated into amplifiers to overcome the IMD issues. Many present-day transmitters use feedforward amplifiers for high linearity requirements, however, these types of units are typically too large and costly to be used in small, remote locations.

To address this problem, Stealth Microwave™ has developed the SM1923-44L. The unit has the same performance as a backed-off 200 Watt unit while consuming only 8.2 Amps at +12V and measuring only 7.50 L x 3.97 W x 0.79" H in size. Although the P1dB of this amplifier is +44 dBm (25 Watts), the OIP3 is greater than +63 dBm!

The SM1923-44L uses the latest GaAs FET technology along with a built in predistortion linearizer which produces the resulting IMD performance. The predistorter uses phase and amplitude correction to reduce the IMDs created in the HPA which increases the theoretical intercept point. Additional circuitry allows the linear gain of the unit to only change ±0.15 dB over a temperature range of 0 to + 55º C.

The unit is specifically designed for the Personal Communications System (PCS) and the Universal Mobile Telecommunications System (UMTS) markets. Wideband Code Division Multiple Access (W-CDMA) is a sophisticated technology that is presently touted as a worldwide standard for future 3G systems. The SM1923-44L provides 6 dB more output power over non-linearized amplifiers of equal peak power, see Figure 1. That means four (4) non-linearized amplifiers would be needed to provide similar adjacent channel power ratio (ACPR) performance that one (1) SM1923-44L provides.

Figure 1

The SM1923-44L has several built in features, including a single DC supply of 12 Volts, over voltage protection, and thermal protection which will turn the unit off if the case temperature exceeds 80º C, and will automatically turn on once the temperature has fallen back to normal levels. Logic On/Off control is also standard, allowing the end user to turn the unit on or off with a TTL signal. Three optional features are available: Forward/Reverse Power Detection which allows the end user to monitor output power and the VSWR of the transmit antenna, an RF sample port for analog monitoring of the output spectrum, and a pulse control circuit which allows the user to operate the amplifier between “on” and “standby” states with speeds up to 10 ms and on/standby isolation > 65 dB.

For full specifications, see Table 1. The price of the amplifier with all of the built in features including the linearizer is approximately one-third the price of a conventional 200 Watt amplifier. Lead time in stock to eight weeks ARO.