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#36: Hex Inverter Oscillator
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The delay is created by the alternate charging of capacitor C1 (through R1) and discharge of C1 (through R2). The output is a square wave of fixed frequency. Many different inverter IC’s are capable of this operation (SN54LS05, SN74LS04, CD4049). Make sure the propagation delay is small compared to the period of the oscillator frequency desired.
- Circuit tip: If you are attempting to breadboard this circuit, you may need bypass capacitors (0.1 uF capacitors from Vdd to the local IC ground) in order to minimize jitter and noise effects. These IC’s switch at a very fast rate, and can easily cause switching noise to appear on the outputs.
- Circuit tip: A minimum propagation delay is required for the start up of this circuit. Make sure the data sheet or SCD of this part has a minimum listed propagation delay.
The circuit schematic is shown in Figure 36-1
Figure 36-1: Inverter oscillator schematic
The IsSpice simulation results are shown in Figure 36-2. These results compare very well with the measured performance (Figure 36-3).
Figure 36-2: IsSpice simulation results of node 5 and output waveforms
Figure 36-3: Hardware resulting waveforms of Inverter oscillator schematic.
The circuit was also simulated in Pspice and Micro-Cap. The results of those simulations are shown in Figure 36-4 and Figure 36-5. In figure 36-5, interestingly, the Micro-Cap results showed an output frequency of roughly twice that of Pspice or IsSpice. The reason for this is the Micro-Cap model for the 4049 inverter was used. This model had a hysteretic input threshold. In actuality, the part has a linear region and is not hysteretic, however, the Harris data sheet for this part does show the threshold levels of the model are within the data sheet requirements. This illustrates the dependency of the frequency on the input threshold of the 4049 inverter.
Figure 36-4: Pspice resulting waveforms of Inverter oscillator schematic
Figure 36-5: Micro-cap resulting waveforms of Inverter oscillator schematic.
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Run Time Summary |
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IsSpice v 7.6 |
PsPice v 6.3 |
Micro-Cap V v2 |
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4.066 Sec |
2.94 Sec |
8.85 Sec |
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Advantages: Low parts count, good drive capability
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Disadvantages: The accuracy over life, temperature may be unpredictable |
Filenames: 4049osc (IsSpice) ps_4049 (Pspice) mc_4049 (Micro-Cap)
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