Phase margin of 1st-order low pass filter

[milvapp]

Hi all,

i have this LPF in Cadence.
I want to check its stability so i want to calculate the phase margin.
However according to the ac simulation results of gain and phase,
i cannot figure it out.
Can someone help me?
I am quite new to this,so i would also like to know how to find the 3-db bw
in order to find the cutoff frequency.

res=500KΩ
cap=0.1pF

Thnx in advance,
Bill

 

[FvM]

A 1st order filter is a passive circuit with real poles and is unconditionally stable. I can't understand the question.

i would also like to know how to find the 3-db bw

For a low-pass, you have to find the frequency, where the gain has dropped by 3 dB related to the DC
respectively low frequency gain. You can read it from your diagram - or simply calculate it fg = 1/(2 pi R C),
about 3.2 MHz in your example.

 

[milvapp]

A 1st order filter is a passive circuit with real poles and is unconditionally stable. I can't understand the question.

i would also like to know how to find the 3-db bw

For a low-pass, you have to find the frequency, where the gain has dropped by 3 dB related to the DC
respectively low frequency gain. You can read it from your diagram - or simply calculate it fg = 1/(2 pi R C),
about 3.2 MHz in your example.

Thank you.You really helped me a lot.I didnt know that 1st order are always stable.

Added after 4 hours 50 minutes:

Added after 1 minutes:

You can read it from your diagram - or simply calculate it fg = 1/(2 pi R C),
about 3.2 MHz in your example.

Another question.Acccording to theory -3db is at 3.2 MHz while if i follow the diagram
the -3db is at about 1.5MHz.Is this happening because of noise?

 

[FvM]

Acccording to theory -3db is at 3.2 MHz while if i follow the diagram
the -3db is at about 1.5MHz.Is this happening because of noise?

I didn't notice it. There's no noise involved in AC analysis, and it wouldn't change the filter's frequency
response. I have no explanation for the deviation. How many frequency points per decade respectively
octave did you specify for the AC analysis?. The -45 degree phase point should also correspond with the
low-pass pol frequency, but it doesn't.