Satellite Communications Payload and System

Chapter 5

Low-Noise Amplifier and Frequency Converter

5.1 Introduction

This chapter is about the low-noise amplifier (LNA) and the frequency converter, two units in the front end of the payload. We discuss their architecture, redundancy scheme, and environmental considerations and give an example of their communications-related specification parameters. We also explain some of the parameters along the way.

The LNA sets, or to a large extent sets, the noise figure of the payload (Section 7.12.1). Its essential feature is that it has a low noise figure and high gain. Any resistive losses before the LNA add one-to-one to the payload noise figure in dB, so the LNA must be placed as close as possible to the antenna terminal.

The frequency converter changes the carrier frequency of the signal. Uplink and downlink frequencies are always different. Most commonly for nonprocessing payloads, there is only one frequency conversion, a downconversion (D/C). However, if the uplink and downlink frequencies are higher than Ku-band, there may be a downconversion to an IF at which the signal is filtered, followed by an upconversion (U/C). For regenerative payloads, the D/C would be to baseband, followed by an U/C. The D/C is almost always located right next to the LNA to reduce the losses on the following transmission line to the IMUXes, since lower frequencies have less loss in waveguide and coaxial cable.

The rest of this chapter is organized as follows:

• Section 5.2: LNAs and frequency converters ...