Imagine, if you will, some monkeys. Unlike the common stereotype of "monkey see, monkey do", these monkeys are rebellious, contrarian monkeys. Each one always do the *opposite* of what it sees the monkey in front of it doing. When one monkey stands up, the monkey who sees it insists on laying down. When one monkey lays down, the monkey who sees it insists on standing up.
Now place 4 of these monkeys, one at each of the 4 corners outside a skyscraper, so each one can only see the one monkey in front of him. When a prankster comes along and tickles the first monkey until the monkey rolls on the floor laughing (ROFL), the next monkey sees that and insists on standing tall. The following monkey diagonally opposite the laughing monkey can't see the laughing monkey through the building, but he can see the standing monkey on the corner in front of him, so he insists on laying down. The 4th monkey sees the laying monkey and insists on standing tall.
After the pranksters leave the first monkey, that monkey sees the 4th monkey standing tall and insists on remaining laying on the floor. We can summarize the situation as
- low High low High.
Later the pranksters find the second monkey standing tall, and tickle that monkey until the monkey rolls on the floor laughing. After the pranksters leave and the dust settles, a similar but not identical situation occurs. We summarize the new situation as
- High low High low.
What happens if the building has 6 sides? Or 100 sides?
Next the pranksters do something even more diabolical. They place some of these monkeys at one of these "modern" architecture skyscrapers has 5 corners, and place 5 monkeys, one at each of the 5 corners. While the pranksters are tickling the first monkey, we can summarize the situation as
- low High low High low
But as soon as the pranksters tire of it and wander off, the tickled monkey sees the monkey laying in front and insists on standing up:
- High High low High low.
Pretty soon the standing monkey that was watching the monkey be tickled notices that monkey is now standing up, so this standing monkey insists on laying down.
- High low low High low.
The process continues around the building
- High low High High low
- High low High low low
- High low High low High
- low low High low High
- low High High low High
- low High low low High
- low High low High High
- low High low High low
- High High low High low
- High low low High low
and so on. Even though the pranksters have long gone, the monkeys keep jumping up and laying down, over and over again, indefinitely.
The 5 monkeys in a ring around this building form a kind of ring oscillator.
There are a huge variety of other things we can use instead of monkeys. Electronic inverters must be fed a little electronic power, but that costs far less than the feed for a bunch of monkeys.
There are conceptually 3 parts to a ring oscillator:
- signal restoration: when the input of one stage is even a little bit low, the output goes practically all the way High; when the input is even a little bit High, the output goes practically all the way low.
- time delay: when the input suddenly goes high-to-low, it takes a short (but non-zero) amount of time before the next stage "notices" and goes low-to-high.
- a feedback loop, so that the output of the final stage is seen by the first stage, the output of the first stage is seen by the second stage, and so on.
In practice, the "time delay" part may not be a separate physical part, but merely the inherent light-speed delay in information passing from one location to some other physical location.