Friday, February 3, 2023

The choice is linear or square law - Large signal or Small signal

 You need to know enough to decide between a linear or square law signal detection. The diode response curve is divided into 2 sections. The "knee" separates the 2 parts. Simply stated the knee is the point on the curve with a slope of 45 degrees. 

Below the knee the signal response is "square law". Above the knee the signal is linear. There is no way to have linear without passing through the square law region. This is another case where we "swamp" the small signal and ignore it. The signal response is based on the signal level we cannot just decide to use linear detection in a crystal set because the signal is to low.

While this circuit would be a reasonable representation for a high level detector it just is not what happens in a crystal set. Linear detection can be represented as rectification but a crystal set does not have the voltage required to turn the diode on.

This is the rest of that circuit. The amplifiers produce the signal required to turn the diode on.

Plate detection uses the non-linear section of the curve to produce higher gain on the peaks and "shifts the zero reference point from the curves mid point to develop the signal.

Now superimpose the signal over the knee and the positive half will be much higher than the negative. The signal is not rectified it is "distorted" because the positive signal accesses a higher part of the curve. This would be more like plate detection than rectification.

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