
Noise generators
Noise generators are widely used for measure-
ments on receivers, frequency division multiplex
(FDM) line and radio circuits, and so on. Figure
9.22 shows a variety of these. In Figure 9.22a a
diode with a plain tungsten heater provides the
source of noise. The tungsten heater provides a
less copious supply of electrons than the usual rare
earth activated cathode, so that there is no cloud of
electrons surrounding it forming a virtual cathode,
as in a conventional valve. This temperature
limited mode of operation also avoids some
sources of additional noise associated with coated
cathodes. The number of electrons emitted pe