# Ring oscillator phase shift

#### Nemanja_95

Joined Nov 14, 2017
24
So, from oscillator theory I know that we need to have 360 phase shift from output to input. But, if we have 3 inverters, each of them make 180 deg phase shift, that is not 360 but it still oscillate.

#### AlbertHall

Joined Jun 4, 2014
9,666
When the input to the first inverter goes high (or low), the output of the third one will go low (or high) but only after the propagation time of the three gates.

The oscillator below is similar except here the time delay is given by the RC circuit.

#### Nemanja_95

Joined Nov 14, 2017
24
When the input to the first inverter goes high (or low), the output of the third one will go low (or high) but only after the propagation time of the three gates.

The oscillator below is similar except here the time delay is given by the RC circuit.
Thank you for your answer, but I actually understand what is happening in time domain

I was just wondering what is happening with phase in circuits like this.
Greetings!

#### AlbertHall

Joined Jun 4, 2014
9,666
Time delay and phase shift are related.
Say we have a 1kHz sine wave and a circuit which results in a delay of 0.25ms. After passing through the delay the phase is -90° compared to the original signal.

In the three gate oscillator, at some frequency, the combined propagation delay will equal a 180° phase shift and result in in-phase feedback and oscillation.

#### Nemanja_95

Joined Nov 14, 2017
24
It's definitly clearer now.
But what about the basic theoretical approach to the oscillators, where it says that we need 360 degrees from the output to input? Clearly, it's not what we need here.

#### AlbertHall

Joined Jun 4, 2014
9,666
But you do have 360° phase shift around the ring, 180° from the inverters, and 180° from the propagation delays.