A transponder is a broadband RF channel used to amplify one or more carriers on the downlink side of a geostationary communications satellite. It is part of the microwave repeater and antenna system that is housed onboard the operating satellite. Examples of these satellites include AMC 4 and Telstar 5, located at 101 and 97 degrees west longitude, respectively. These satellites and most of their cohorts in the geostationary orbit have bent-pipe repeaters using C and Ku bands; a bent pipe repeater is simply one that receives all signals in the uplink beam, block translates them to the downlink band, and separates them into individual transponders of a fixed bandwidth. Figure 1 shows the basic concept. Each transponder is amplified by either a traveling wave tube amplifier (TWTA) or a solid state power amplifier (SSPA). Satellites of this type are very popular for transmitting TV channels to broadcast stations, cable TV systems, and directly to the home. Other applications include very small aperture terminal (VSAT) data communications networks, international high bit rate pipes, and rural telephony. Integration of these information types is becoming popular as satellite transponders can deliver data rates in the range of 50 to 150 Mbps. Achieving these high data rates requires careful consideration of the design and performance of the repeater.
Figure 1. Elements of a basic Satellite link showing ground stations and the satellite transponder. The nature and location of the various system impairments are also shown.
The most significant impairments to digital transmission come about in the filtering, which constrains bandwidth and introduces delay distortion, and the power amplification, which produces AM/AM and AM/PM conversion. These effects will be discussed in detail later in this article. For maximum power output with the highest efficiency (e.g., to minimize solar panel DC supply), this amplifier should be operated at its saturation point. However, many services are sensitive and susceptible to AM/AM and AM/PM conversion, for which backoff is necessary. With such an operating point, intermodulation distortion can be held to an acceptable level; however, backoff also reduces downlink power.
The transponder itself is simply a repeater. It takes in the signal from the uplink at a frequency f1, amplifies it and sends it back on a second frequency f2. Figure 2 shows a typical frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz, and the downlink frequency is at 4 GHz. The 24 channels are separated by 40 MHz and have a 36 MHz useful bandwidth. The guard band of 4 MHz assures that the transponders do not interact with each other.
Figure 2. Basic 24 channel C-band transponder frequency plan