Product Feature
A Compact High Power SSPA for Satellite Communication Applications
Advanced Microwave Technologies Inc.,
ADVANTECH
Dorval, QC, Canada
Today's emerging satellite broadband applications require high power amplifiers. In recent years, significant advances have been made in solid-state technology that have resulted in solid-state power amplifiers (SSPA) becoming a truly viable alternative to the traveling-wave tube amplifier (TWTA) for microwave high power amplifier applications.
SSPAs offer a number of distinct advantages over their TWTA counterparts. One main advantage is their superior linearity performance compared to TWTA devices. To meet the same intermodulation performance requirement, the output power back-off of a TWTA is typically 4 dB more than a SSPA.
Another main advantage of SSPAs is the built-in redundancy. High power SSPAs combine a number of transistors in parallel to achieve the high output power. This technique introduces a feature whereby the output power will gradually reduce in the unlikely event of a failure in any of the parallel transistors. Therefore, there is an inherent element of built-in redundancy. The degree of built-in redundancy is related to the number of parallel devices in the output stage. Tube products have no such inherent redundancy. If the tube fails, a great deal of loss is introduced and link failure is inevitable. Therefore, the classic 1:1 configuration is not the only choice for high reliability when using SSPAs.Other advantages to choosing SSPAs include more reliability, longer service life, smaller overall size, no wear and tear during hot standby, and 50 percent lower electricity consumption.
High Power SSPA Design
Solid-state devices offer low power, and the power output from a single solid-state device decreases rapidly with increasing frequency. Currently the maximum output power of a single power FET for C-band use is still far below 100 W. In many satellite applications, amplifier power levels are required that far exceed the capability of any single device or amplifier. It is therefore desirable to extend the output power by utilizing combining techniques to take advantage of the many desirable features of SSPAs, such as small size and weight, reliability and superior linearity performance.
A simple combining system comprised of two transistors can be used as an example to easily understand how to increase the output power of an amplifier. The output power from a power combiner having no resistive losses is given by
P = 0.5[P1 +P2 +2(P1 xP2 )0.5 x cos(q)]
where
P = output power from the combiner in watts
P1 = output power from transistor 1 in watts
P2 = output power from transistor 2 in watts
q = phase difference between the outputs from two transistors
Clearly, in this case, the output power and phase of transistors 1 and 2 must be kept at the same level in order to achieve the maximum output power. Consequently, it is a major challenge for RF designers to efficiently combine a number of transistors in parallel.
|
Table 1 | |
|
Frequency range (GHz) |
5.850 to 6.425 |
|
Saturated output power (nominal) (dBm) |
+60 |
|
Output power (P1dB) (dBm) |
+59 |
|
Gain at rated power (Gmax = Gmin +5 dB (dB) |
70 min. |
|
Gain flatness over 600 MHz (dB) |
±1 max. |
|
Gain variation (0° to 50°C) (dB) |
±1.5 |
|
Variable output attenuation (dB) |
20 min. |
|
Noise figure (dB) |
8 at max. gain |
|
Spurious at rated power (dBc) |
-65 max. |
|
Harmonics at rated power (dBc) |
-60 max. |
|
Two-tone intermodulation (dBc) |
-36 max. at 7 dB back-off |
|
Operating voltage (VAC) |
180 to 264 |
|
Power consumption (nominal) (W) |
4800 |
1000 Watts at C-band
Using a revolutionary combining technique, the first compact 1000 W C-band SSPA for satellite up-link applications has been developed. The model ARMA-C1000 high power rack-mount SSPA is a compact, 6-rack unit amplifier with exceptional linearity and operating efficiency. The use of the high efficiency solid-state power modules and conservative thermal designs contribute to the trouble free operation of the amplifier. Table 1 provides a summary of the amplifier's significant performance characteristics. Figure 1 shows a block diagram of the complete 1000 W C-band rack-mount SSPA, which includes the RF amplifier module, RF output arm, monitor and control system, and power supply system.
RF Amplifier Module
The RF amplifier module includes circuitry for amplification, temperature compensation and gain adjustment. A conservative design approach, combined with proprietary techniques for the design and assembly, results in a product of exceptional stability, linearity, performance and reliability.
The gain adjustment has a typical range of 20 dB. The amplifier module provides its status and accepts commands via the monitor and control (MAC) system. The current drawn by each transistor in the amplifier is continuously monitored and an alarm is activated when the current in any of the transistors falls outside of a preset window. Activation of the alarm does not depend on the presence or absence of an RF signal.
RF Output Arm
The RF output arm connects the output of the amplifier module to the output waveguide flange of the amplifier rack-mount assembly. The RF output arm contains the output isolator and all the couplers for the RF output sample port, the RF incident power detector and the RF reflected power detector.
Power Supply
The power supply system consists of two 5" x 5" x 14" switching power modules, each with an output of 12 V at 250 A. The total output power of power supply subsystem is 6000 W max. The power supply has self-contained forced air cooling and essential protection features for over-current, over-voltage and over-temperature conditions. The module design of external power supplies gives operators great flexibility and redundancy of operation and maintenance.
Monitor and Control System
The MAC module contains a microprocessor that monitors all key operating parameters and status of the amplifier (output power, baseplate temperature, power consumption and switch position). For gain adjustment, the 12-bit DAC provides a 0.1 dB step increment across the full 20 dB range of the attenuator. Through a menu driven interface, all key operating parameters and status can be verified locally via an alpha-numeric display or remotely via the RS232 or RS422/485 serial interface. The built-in test facilities (output RF detector and reflected power detector), when combined with other monitored parameters, provide an effective means for troubleshooting. The calibrated input and output sample ports provide convenient means to locally verify the operation during service.
Safety Features
A thermal shutdown feature is incorporated into the system to protect the amplifier from permanent damage. The thermal shutdown feature operates at a baseplate temperature of 80°C and is self-healing. When the baseplate temperature drops to below 60°C, the amplifier re-starts automatically. A thermal shutdown is always preceded by a thermal alarm, which activates at a baseplate temperature of approximately 70°C.
The isolator in the output arm of the SSPA is designed to withstand high reflected RF power. The amplifier is shutdown when the reflected power is greater than 25 percent of the rated output power.
Redundant Operation
The 1000 W C-band SSPA may be configured to operate in 1:1 or 1:2 redundancy mode. No extra controller is required for redundancy operation, as the built-in controller in each amplifier provides this function.
Application
The ARMA-C series amplifiers are designed for satellite up-link applications in ground station terminals. With the addition of the appropriate waveguide and switch kit, these amplifiers can be easily converted for operation in a redundant configuration with full remote monitor and control capability via the serial interface. Additionally, with an external phase combining system, up to four high power SSPA units may be combined to increase the amplifier output to 3200 W.
Advanced Microwave Technologies Inc.,
ADVANTECH,
Dorval, QC, Canada (514) 420-0045.
Circle No. 303