# Ring oscillator phase shift

Thread Starter

#### 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.

I understand working principle of ring oscillator... I'm just confused about this theoretical part about the phase.

#### AlbertHall

Joined Jun 4, 2014
12,187
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.
Think about this oscillator in terms of time delay rather than phase shift.

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

Thread Starter

#### 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.
Think about this oscillator in terms of time delay rather than phase shift.

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
12,187
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.

Thread Starter

#### 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
12,187
But you do have 360° phase shift around the ring, 180° from the inverters, and 180° from the propagation delays.

Thread Starter

#### Nemanja_95

Joined Nov 14, 2017
24
But you do have 360° phase shift around the ring, 180° from the inverters, and 180° from the propagation delays.
Now it's clear.
Thak you very much, really appreciate your help!

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