nce of the products of cathodes of valves and semiconductors and it appears to be caused intermodulation. If a number of signals are passed through a non-li- by fluctuations in the conductivity of the medium. Because of the near device the result will be intermodulation products that are advances in cleaning and passivation techniques, which are em- spurious frequency components. These components may be inside ployed during component manufacturing process, a good device or outside the frequency band of interest for the device. will exhibit negligible flicker noise above 1kHz. The non-linearities cause each signal to combine with the other signals in the set, to produce a series of second order sum and difference products, third order products etc. Noise units and measurements For most analogue systems with many multiplexed channels, the addition of the very large combination of signals results in an output 10.9.1 Terms of measure noise spectrum which is approximately flat with a frequency across a narrow band of about 4kHz. The measurement of noise is an effort to characterise a complex Intermodulation noise differs from thermal noise since it is a signal. The noise measurement in telephone channels is further function of the signal power at the point of non-linearity. complicated by the subjective nature of the annoyance caused to users, rather than the absolute magnitude of the noise power. Anoise 10.8.7 Crosstalk measuring set must therefore measure the subjective interference effect as a function of the frequency as well as magnitude and give Crosstalk is the unwanted coupling between signal paths. There are the same readings for different types of noise which cause equal essentially three causes of crosstalk: interference. The value of 10 12 watts or -90dBm has been chosen arbitrarily Electrical coupling between transmission media such as be- as the noise reference value. The markings on a noise meter are in tween wire pairs on a VF cable system. or a capacitance decibels and measurements are expressed in dB