III. Gm-C Filtering - Epublications - Université de Limoges

I.2.b.ii High linearity
Although components such as amplifiers are considered as linear elements for small
signals, they actually have non-linear characteristics. When dealing with large signals,
distortion phenomena like compression or intermodulation occur and may strongly degrade
the wanted signal. The theory about distortions and its effects are discussed more extensively
in APPENDIX B. Figure 22 shows the degradation of a since wave due to second order
distortion (a) and to third and fifth order distortion (b).
Figure 22. Second order (a) and Third and Fifth order (b) distortion of a sine wave
References [I.5 and I.6], written by Charles Rhodes, highlight how crucial the linearity
of the TV receivers is. It demonstrates that nearly all interferences are due to third order nonlinearity
of the receiver when overloaded. Indeed, the receiver is then subject to multiple
distortion products which may fall in the desired channel, as described in Figure 23. Thus,
like noise, intermodulation products falling can mask and degrade the wanted signal.
To describe the degradation of the signal by noise as well as non-linearities, a factor
similar to the SNR is often used. It is called SNIR, standing for Signal to Noise plus
Interference Ratio.
Figure 23. Intermodulation products interfering with a desired signal
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