Schmitt trigger oscillator

Like the Wein-bridge oscillator, the Schmitt trigger only needs a power supply of 5 to 15V to begin its oscillation. This entails that the maximum lead that the clock pulse can drive is in the 1 mA range. The oscillation is controlled by the RC time constant and the hysteretic native to the Schmitt trigger. 

Advantages Low part count, moderate frequency stability Disadvantages Harmonic distortion not controlled 

File names hcoms2 (IsSpice), hc04ps (PSpice), michmic (Micro-Cap) 

The most important characteristics of a good model are the positive and negative hysteresis points. Unfortunately, manufactures of the Schmitt trigger give a loose specification as to what these points are. Table 8.5 shows data taken from Radiation Hardened Product Databook (Harris Semiconductor 1993) on the CD4093BMS NAND Schmitt trigger. 

Consider the CD4093B model contained in the IsSpice package. The circuit is shown in Fig. 8.57. The results are displayed in Fig. 8.58. 

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Figure 8.60 PSpice results of Schmitt trigger oscillator using a 7414.

Figure 8.62 Circuit board data from Schmitt trigger oscillator.

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