A Linearized MMDS Solid-state Amplifier

A Linearized MMDS Solid-state Amplifier

Stealth Microwave
a division of SSB Technologies Inc.
Union, NJ

Due to advances in digital compression technology for cable television systems, renewed interest in the 2.5 to 2.7 GHz frequency range has increased in the US and abroad. Multichannel multipoint distribution systems (MMDS) that operate in this band are upgrading from analog to digital television service. The new formating allows several television channels to be transmitted vs. a single analog channel. These multichannel configurations require ultra-linear RF subsystems to avoid generating unwanted intermodulation distortion (IMD) products. Typically, the RF/microwave high power amplifier (HPA) is the leading cause of this type of distortion and, therefore, dictates the linearity of the entire transmitter.

A common method used to minimize IMDs is to back off the output power of the HPA until the desired carrier-to-intermodulation product ratio (C/I) is achieved. The relationship between IMDs, intercept point and carrier levels is given as

where

OIP₃ = output third-order intercept point

With a single carrier level of 32.5 dBm and a desired IMD level of ñ55 dBc, an amplifier with an intercept point of 60 dBm would be needed. Since the P1dB of an HPA is typically 10 dB below the OIP₃ , an HPA of 50 dBm (100 W) would be required. If transistors that operate in the 2.6 GHz range are used, such an amplifier would consume approximately 30 A, be quite large in size and have a high price.

The model SM2527-41L linearized MMDS solid-state amplifier has been developed to offer performance similar to a backed-off 100 W HPA but consumes only 5.5 A at 12 V DC in a housing that measures 7.5" x 4" x 2". Although the P1dB of this amplifier is 41 dBm (12 W), the intercept point measures greater than 60 dBm, resulting in similar IMD performance to the 100 W amplifier. Figure 1 shows the amplifierís typical output spectrum with multiple single carriers at 35 dBm average output power.

Fig. 1: The amplifier's output spectrum at 35 dBm average power.

The SM2527-41L HPA uses the latest GaAs FET technology along with a built-in predistortion linearizer that produces the enhanced IMD performance. The predistorter uses phase and amplitude correction to reduce the IMDs created in the HPA. In turn, this predistortion pushes the theoretical intercept point higher. Additional circuitry allows the linear gain of the unit to change only ±0.15 dB over a temperature range of 0° to 55°C, as shown in Figure 2 .

Fig. 2: Gain variation vs. temperature.

The unit is designed specifically for the MMDS market. Many MMDS transmitters use feedforward techniques that are suitable for base station applications. However, for remote locations, size and power consumption are critical, making the SM2527-41L HPA a better solution.

The amplifier has several built-in features, including a single 12 V DC supply, overvoltage and reverse voltage protection, an integrated output isolator and thermal protection that resets automatically. Logic on/off control is also standard, allowing the end user to turn the unit on or off with a TTL signal. Two optional features are available, including forward/reverse power detection that allows the end user to monitor output power and the SWR of the transmit antenna, and an RF sample port for analog monitoring of the output spectrum. Table 1 lists the amplifierís key specifications.

The price of the amplifier with all of the built-in features, including the linearizer, is approximately one-third the cost of a standard 100 W amplifier. Delivery is stock to 10 weeks (ARO).

Stealth Microwave, a division of SSB Technologies Inc.,
Union, NJ (908) 687-3332.