TABLE OF CONTENTS So far we have stated that individual elements will affect the overall system performance without defining either the element parameters or desired performance. It should be possible to communicate intelligibly over an FDM link. This requires some minimum bandwidth determined by the channel filter usually 300 to 3400 Hz (submarine cable systems may use 200 to 3050 Hz) and also minimal impairment from noise, distortion or interference.
Interference may be caused by crosstalk from adjacent channels or signaling tones in the system. Distortion will result from overloading of amplifiers, mixers, and so on. Noise can be either thermal or intermodulation noise. With a large number of channels in a system, all behaving as independent signal sources, we can invoke the central limit theorem and treat the composite signal as Gaussian noise. Any nonlinear component in the system will produce intermodulation distortion and spread this noise to all channels.
For a given FDM system, there will be an optimum loading level where the effects of intermodulation and thermal noise are balanced. In the interests of economy, this will be a fine balance, so it is important that the correct loading level is attained to meet the system performance objectives.
An individual voiceband will be shifted up and down in frequency and pass through many repeater amplifiers as it travels through the network. The gain of each of these stages must be accurately determined so that correct loading is achieved at all points in...
