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Lesson 56 - Oscillators

This is one of those sections that begins with a question, which when answered, only leads into more questions. So let us begin with the all important question, "What in the name of Bartholemew McGillicutty is an Oscillator?". Well to answer the question most simply, I have no idea WHO Bartholomew McGillicutty is. However, an oscillator is something that moves back and forth, swings to and fro, goes in and out, swings from front to back, or side to side, etc, in a repetative motion.

Picture in your mind a clock. It has an arm that swings back and forth. Every second swinging away with a dreaded, annoying tick, tick, tick in the middle of the night so that you can not get any sleep. Another oscillator would be the meteronome you learned to play piano to as a child. Ever measured, ever metered - it stood as a means of measuring just how imperfect and flawed your own timing is. It stood as a constant reminder that forever, your greatest accomplishment in life would be chopsticks. That is a basic example of what an oscillator does.It moves back and forth, ticking time away as it goes.

An oscillator in electronics does essentially the same thing. It ticks at regular intervals to set the beat or rhythm of something. In many cases, it may in fact act as the heart of a clock (think of your digital alarm clock by your bed), or as the "clock" signal being sent to the processor of your computer - setting the speed at which it operates. Also of importance is the fact we've already discussed, that time is the reciprocal of frequency. So if it ticks away time, it also sets a frequency. For our discussions we will be using oscillators in this fashion - as frequency setting devices. Lets study how they work now.

We have already discussed circuits that swing back and forth - in the form of filters. So let us begin there. Asume we have a capacitor and a coil in a fixed loop. We apply a charge to the capacitor. By its nature, the capacitor discharges, and current flows through the coil. The coil, having an increasing voltage applied generates a magnetic field until the field is saturated and the coil is at full potential. Then the field collapses, and the current flows out the other side of the coil into the opposite lead of the capacitor. This starts the cycle all over again, as the capacitor charges.

Now lets look at another "mechanical" oscillator - the guitar string. We pluck the string. If left alone, it plays on indefinately until finally its sound gets so quiet you can't hear it anymore. In all the time though, its frequency (pitch) does not change. The reason for this is that the string vibrates at a certain speed back and forth. However, because of friction with the air, it swings less far with every swing, but still at the same speed /frequency.

If we compare this analagy to our filter circuit - we see the same thing happens. There is a limited amount of energy stored in the capacitor. Yes, it realeases into the coil, which then releases into the cap - back and forth, sloshing around like so much water in a tank. But unless we introduce another "nudge" into the circuit - it will continue to lose energy (vial magnetic losses, heat loss through resistance, etc) until finally the circuit stops oscillating.

In order for an electronic oscillator to continue oscillating, it must have an external "nudge" come along every so often to keep it going. When I was first learning electronics, I had the perfect physical example to relate to this - but I find that most of my students will have never seen a "kicker" style butter churn. (It was less overall work than making butter with a traditional churn, but required that you kick it every time you walk by).

It wouldn't be practical for us to stand there with a toggle switch, and give the circuit a jolt every so often, nor could we ACCURATELY do it so it would trigger at the right moment. So we rely on the electronics at hand to do the job. Fortunately - this type of thing can be done by accident, and most likely - is exactly how it was discovered.

Have you ever put a microphone too close to the speaker of a PA system? Certainly you found as have I, that it makes a most annoying squeal. Kind of like stepping on the fingers of a young child by accident. It makes a noise that is loud, high pitched, most annoying, and won't go away with any amount of candy or ice cream.

This is an example of a phenomena called "feedback". Feedback happens when we take the output of any amplifier, and feed it back into its own input. In an uncontrolled situation, it goes into runaway and you get the hideous squeel. But in a more controlled situation, you can make it do exactly what you want it to do. For instance, if you put a filter in line with it - the oscillator will "ring" at the frequency of the filter, as set by the XL and XC of the coil and cap. Furthermore, because its output is fed into the input of the amplifier at EXACTLY the correct interval - it will continue on indefinately. The problem then becomes that it will continue to build in amplitude (volume) until it gets overheated and blows up the very components making the frequency. Because of this, we must also limit the AMOUNT of signal fed back into the amplifier. This is usually done with resistance, although as you will find in further descriptions, there are other ways to control both the filtering and amplitude of the fed-back signal.

As such, in order for any oscillator to properly work, it requires the following things to happen:

  1. Proper biasing
  2. Amplification
  3. Positive Feedback
  4. Frequency and Level Control


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