This article is the first of the four-part series on oscillators. We will look at square wave generators, sawtooth and triangle wave generators, sine wave generators, and finally, crystal oscillators. Let’s start with square wave generators.
What is an oscillator?
It is an electronic circuit that changes state from positive to negative in a repeating cycle without any stimulus other than DC power. This produces an AC waveform at the output.
What is a square wave generator?
Square wave generators are generally used in electronics and in signal processing. It is just like a Schmit trigger circuit in which the reference voltage for the comparator depends on the output voltage. It is also said to be an astable multivibrator.
A square-wave generator obviously produces a square wave. However, this may also be adjustable in mark-to-space ratio and is often used for timing, pulsing and clocking circuits. One of the easiest ways to generate a square-wave is by using a relaxation oscillator.
What is a relaxation oscillator?
Relaxation oscillators have two alternating states: a long relaxation period in which the system comes to rest and then a short change-over period in which the stable point flips over to a second stable state for a period then flips back again. The period is set by the time constant which is usually an RC or LC pair.
Some sort of active switching device is needed, such as a transistor pair or a uni-junction transistor or an op-amp comparator, or a custom chip such as a 555. The active device switches between charging and discharging modes, producing a repeating waveform.
For any oscillator to qualify as a relaxation oscillator, it must:
- Produce a non-sinusoidal periodic waveform like triangular, square, or rectangular wave.
- The circuit of a relaxation oscillator must be nonlinear. This means the design of the circuit must use a semiconductor device like a transistor, MOSFET, or OP-AMP.
- The circuit design must use an energy-storing component like an inductor or capacitor that continuously charges and discharges to produce a cyclic waveform.
Seesaw A shows the seesaw in a state of equilibrium and at “relaxation,” but as the bucket slowly fills up, a critical tipping point is reached. The state rapidly changes as the bucket end drops and the bucket tips out. As the bucket drains, the left-hand side is now suddenly much heavier and falls to the ground again, and then the bucket lifts and begins to fill again. (Let’s assume it corrects itself again). In an electronic circuit, this is what is happening: a capacitor gets slowly charged through a resistor until a non-linear part of the circuit is reached, causing a sudden discharge, and the cycle begins again.
In the waveform above and the multi-vibrator circuit below, the blue curve shows the voltage across one of the capacitors C1. It charges until the bias trigger point is reached, then suddenly turning the other transistor on, then discharges again. The black curve is the voltage at the collector, which is the output. In the multi-vibrator below, either collector can be used as the output. However, in this circuit, we are just flashing two LEDs alternately.
Shown below are the multivibrator circuit and a breadboard lash-up. The two LEDs flash alternately at about 1.5Hz. The transistors are any NPN GP transistors. The mark-space ratio can be varied by changing the C and/or R on one half.
Below is the collector voltage waveform.
Shown here is the square wave output of the above multivibrator circuit. You can see that the square wave is fairly good, but there is a slight charging delay. R1 and R4 are 560Ω, R2 and R3 are 47k, and C1 and C2 are 10uF. The LED’s alternate is at about 1.5Hz.
The period of each half is 0.69CR. So if R2 is 47k and C1 is 10uF, that would be 0.32S per half or 0.64 together. Then f = 1/0.64 = 1.5Hz.
A nice relaxation oscillator can be made from any inverting gates. Although two gates will work (NOR NAND OR Schmitt), three gives a better startup. The frequency is set by R1 C1 and
So, here we have
which gives 45Hz.
The frequency is adjustable over a 10:1 range and the output is set by R4. R3 is for feedback and is not involved in the timing. The waveform is nice and square.
With R1=100k, C=.004 f=1kHz, C=.04 f=100Hz, C=0.4 f=10Hz.
That’s all for relaxation square wave oscillators! A good oscillator can be made from the famous 555 timer and we will look at that in the next article on sawtooth and triangular wave generators.
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