The method of assessment of the linearity that is proposed here requires the use of a small two tones tones generator easy to build and a spectrum analyzer or a low cost discarded selective voltmeter.
As the amplitude-frequency response and the group delay distortion, the linearity is an important characteristic of a video transmission chain. A bad amplifier linearity, or a lack of linearity in AM or FM modulators, discriminators,… is prejudicial to the quality of the pictures (unsaturated colors) and provoke bothersome shortcomings when one tries to transmit two audio sub-carriers.
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The method proposed here is known for a long time of the transmission specialists. It is used to qualify the SSB transmitters and was, no long ago, used to measure the performances of analog microwave links. It consists in injecting, in the chain or in the device to test, two sinusoidal signals with a relatively important level, and to observe the spectrum of the output signal. So, if all was OK, one would recover in the output the two original sinusoidal signals, and nothing else….. However such an input signal is especially aggressive and a bad linearity, even very low, will create detectable intermodulation products. Indeed, when a device is not perfectly linear, it also behaves as a mixer, and signals at different frequencies are created.
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