Abstract
Attenuators, or pads as they are often known as, are networks that simulate a lossy transmission line, so that the signal at the output is smaller than at the input but not changed in any other way. Like a transmission line, they are designed to have a specific characteristic impedance, commonly 50 Ω and like a good transmission line their frequency characteristic is flat. Unlike a length of lossy line though they provide no delay and the path length through an attenuator is ideally zero. A variable attenuator is useful for many measurement applications. Continuously variable attenuators using resistive elements have been designed and produced but are expensive, because three resistors have to be varied simultaneously, with non-linear laws. Continuously, variable attenuators working on a rather different principle are readily available at microwave frequencies. Piston attenuators, working on the waveguide beyond cut-off principle are also available for use at V/UHF. Alternatively, attenuators adjustable in 1 dB steps are modestly priced and very useful. Fixed pads are useful for providing some isolation between stages, albeit at the expense of a power loss. In particular, the use of a pad reduces the return loss of a poorly matched load seen by a source, or vice versa. A phase equalizer has no attenuation at any frequency, for which it is also alternatively known as an all-pass filter (APF), and it is used to provide a phase shift that is dependent upon frequency. A typical application is in a digital phase modulation system where an LC or (more usually) active RC low-pass filter is used at baseband prior to the modulation stage, to limit the bandwidth of transmitted signal. An APF can also be used to correct the phase distortion introduced by the baseband filter.
Published Version
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