High Power Amplifiers are used to amplify a signal before it is transmitted to the satellite. There are various mechanical means to amplify a signal before uplinking it to the satellite. Which type of amplifier you chose depends on a number of factors. Linearity, efficiency, reliability and cost are major considerations in an uplink design.
The three types of amplifiers are:
- Klystron Power Amplifiers (KPA)
- Traveling Wave Tube Amplifiers (TWTA)
- Solid State Power Amplifiers (SSPA)
KLYSTRON POWER AMPLIFIERS (KPAs)
The Klystron tube is perhaps the oldest technology and the most widely used.
Advantages
- High power output
- Easier to repair
- Cheaper dollar per watt operational cost
- Linear power consumption
- Longer life (up to 8 years!!)
Disadvantages
- Bigger and heavier than TWTAs or SSPAs
- Small frequency range (40-80 Mhz range)
TRAVELLING WAVE TUBE AMPLIFIERS (TWTAs)
TWTAs are built using foot-long vacuum tubes to do their amplifying. Power is applied to the tube generating temperatures in the plasma ranges. A radio frequency is then emitted into the tube and the radio wave is amplified as it passes through the tube's heated interior. Pre-distortion linearizers are often used with TWTAs to increase the amplification ability of the device. These devices have the advantage of being able to amplify a wide range of frequencies (about 500 Mhz range) allowing them to handle an entire satellite from one antenna (dish).
Advantages
- Simultaneously amplify a wider range of frequencies.
- Smaller device
Disadvantages
- Harder to repair
- More expensive to operate than KPAs or SSPAs
- Shorter Lifespan (4-6 years)
SOLID STATE POWER AMPLIFIERS (SSPAs)
SSPAs perform signal amplification via solid state electronics rather than a vacuum or Klystron tube. Typically, the SSPA uses a frequency-combination method of amplifying the signal. The SSPA's use quartz based oscillator clocks and signal mixers combined in series to step up the power. Because these systems work in series, when any given oscillator/amplifier component fails, the signal weakens rather than failing completely as occurs with TWTA and KPA's.
Advantages
- Physically much smaller than KPAs and TWTAs
- Builds amplification in stages
- Device typically experiences only partial failures which allows the system to continue functioning, but at lower output power.
Disadvantages
- Higher power consumption
- Failures introduce instabilities in the transmission.
- Non-linear power consumption over amplification range.