Hi All,

I am using a software package for designing RF amplifiers. To test the stabilitiy of the amplifier there is a tool in the package that plots
the Nyquist Stability contour for the positive imaginary points in the
complex plane. Some notes that I have found on Nyquist stability state
that

1. the contour only needs to cross the x-axis to the left of (-1, 0) in a clockwise sense for the system to be unstable.

Other notes simply state

2. that the contour must encircle the -1 + j*0 point for the system to be unstable.

I have found this confusing since there are many curves that can statisfy 1. but do not seem to satisfy 2. For example, when i plot the Nyquist contour for the positive imaginary points of my amplifier design I get a contour that first crosses the x-axis to the left of (-1,j

Can anyone shed some light on whether this is an unstable system as per the Nyquist Stabiltiy Criterion? Cheers

Peter Vun.

I am using a software package for designing RF amplifiers. To test the stabilitiy of the amplifier there is a tool in the package that plots

1. the contour only needs to cross the x-axis to the left of (-1, 0) in a clockwise sense for the system to be unstable.

Other notes simply state

2. that the contour must encircle the -1 + j*0 point for the system to be unstable.

I have found this confusing since there are many curves that can statisfy 1. but do not seem to satisfy 2. For example, when i plot the Nyquist contour for the positive imaginary points of my amplifier design I get a contour that first crosses the x-axis to the left of (-1,j

***0) in an anticlockwise direction. Once in the lower-left-hand plane, the contour then turns around and proceeds to cross the x-axis to the left of (-1,j***0) in the clockwise direction. To me, this curve does satisfy 1. but it doesn't seem to satisfy 2.Can anyone shed some light on whether this is an unstable system as per the Nyquist Stabiltiy Criterion? Cheers

Peter Vun.