questions about differential VCO start up

[iccc]

Hi,
To generate 8 phases clocks, a four stages ring vco is designed as shown in below.

The latchs are supposed not to be too strong or too weak。
However, there is also a start up problem like other ring oscillators.
As I know, ac simulaiton in linear system to guarantee a circuit to oscillate is one way to check the start up problem according to Backhuasen rule.
Can anyone tell me the way to check inverter based ring oscillator strar up issues like this four stages osc?

Many thanks .
ICCC

 

[nschutten]

When using AC analysis, you need to ensure that:
- the loop is cut open (feedback trace disconnected; this is now input-output)
- voltage gain needs to be >1x (at osc freq)
- phase (out-in) needs to be 0 degrees at the oscillating frequency

 

[iccc]

Thanks Nschutten.
As you said, it's the general principle. While I found the way to check this differential VCO using AC analysis.