Schmitt triggers are one of those fundamental circuits students will build in their electronics classes, but they don’t always use later. Some circuits are important for both analog design and for specialty logic, including Schmitt triggers. One use of Schmitt triggers that does not get significant attention is for clock generation, and the method has been used in some microcontrollers. This involves forming a Schmitt trigger oscillator circuit to generate square waves with fast edges.
Just like any circuit with fast edges, a Schmitt trigger circuit can exhibit some underdamped oscillation, better known as ringing. It can do this natively without being measured directly, although the oscillation may be noticed elsewhere in the system. However, if you are probing your oscillator directly, the probe can affect both the frequency of the intended square pulses, as well as the ringing observed in the circuit.
Because measurements should produce the least impact on the system they are measuring, we’ll examine the factors in circuit design and in measurement that can produce undesirable oscillatory behavior in Schmitt trigger oscillators.
Schmitt Trigger Oscillations
The basic Schmitt trigger oscillator circuit is shown in the diagram below. When initially excited, the circuit will cause the input capacitor to charge, and the output will rail to the V+ voltage when the capacitor voltage exceeds the high threshold. Then, the capacitor will begin discharging, and the output will drop to the V- voltage. This repeats and effectively converts an exponential wave into a square wave.
In this circuit diagram, the feedback resistor and the input capacitor determine the repetition frequency. Of course, in this circuit, there will be parasitics that influence its operation and cause the real electrical behavior to deviate from the theoretical behavior. This will happen both in actual operation and when taking measurements of the circuit with an oscilloscope probe.
Oscillations in Operation
Oscillation could be seen at the output of a Schmitt trigger oscillator (or any oscillator) due to excess inductance on the output from the oscillator circuit. In effect, the feedback loop and the input start to act like an RLC circuit that is excited by fast edges. If there is enough inductance and low capacitance on the input power pins, this will produce an underdamped oscillation that appears as ringing on the oscillator output.
To suppress these oscillations, there are some simple high-speed design techniques can help you remove these oscillations:
- Use a ground plane below the circuit to remove inductance
- Use route power connections over ground
- Use SMD caps directly on the power pins
Oscillations in Measurement
During measurement and evaluation of a Schmitt trigger oscillator, it’s possible that the probe you are using will alter the circuit behavior unintentionally. All oscilloscope probes are equivalent RC circuits, with the leads of the probe also providing some additional inductance. When used to measure the output and feedback to the input side of the oscillator, the capacitance of the probe will add to the intentionally placed capacitor in the Schmitt trigger oscillator.
The result is that the parasitics in the probe modify the oscillator frequency. Higher bandwidth probes will have lower capacitance and these should be used to probe in the event parasitic capacitance will alter the oscillator’s frequency. However, these probes will also have higher attenuation, so they cannot measure lower signal levels that might be used in some oscillators. The circuit diagram below shows where this capacitive loading can arise in a test circuit.
Here, because the capacitances appear in parallel, the total capacitance being loaded into the circuit is larger, so we would expect the square wave repetition rate to be smaller. From the frequency measurement, you could then calculate the true capacitance and frequency of the oscillator.
Schmitt trigger oscillators with 3 different loading conditions from oscilloscope probes. A larger probe capacitance produces smaller repetition rate.
The probe leads can also alter the ringing due to this capacitance and the load inductances. This is much harder to predict as it depends on the physical layout of the oscillator circuit. Lack of ground below the oscillator circuit is the biggest contributor to high inductance, as well as mutual inductance, that will create ringing in the output and in the feedback loop.
Therefore, there are a few simple steps in the probe hookup that will prevent excess inductance in these measurements:
- Use short direct probe points that minimize the loop in the probe connection
- Use a higher bandwidth probe with lower capacitance to resolve faster edge rates
- Follow the design points listed above
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