4/29    I am confused about reading frequency response from a bode plot

        when the curve starts at a frequency other than zero.  Any pointer
        to reference on the net or book or kind enough to explain here?
        \_ Well you should be since measuring zero frequency is physically
           impossible.
           \_ DC is zero frequency. It's not impossible to measure.  The problem
              is that Bode plots are log/log plots.  What's log(0)?
           \_ DC is zero frequency. It's not impossible to measure.  The
              problem is that Bode plots are log/log plots.  What's log(0)?
              It's sort of hard to have a scale that goes all the way to
              -infinity, isn't it?  So you just stop at some other frequency.
              So I remember anyway.  I apologize if I'm wrong. --PeterM
                \_ To measure DC you must have an infinite period.
                   \_ are you a lawyer?  Yes, you're technically right.
                      In practice, you're an idiot.
        \_ Measuring DC is not the problem (you can measure the
           voltage of a battery, right?), it's displaying it
           that's a problem since log(0) = -infinity. So they
           just start it at some small value, for example 0.01Hz.
        \_ Ooops, sorry for the confusion.  I was really confused.  It
           should be read as "starts as zero degree..."  Help?
           \_ Do you mean a phase reponse plot?  Plotting phase in degrees
              starting from 0 and going to 360 is eminently sensible.  How
              *else* would you do it?  I suppose you could start with 45
              and go to 405.
              \_ YES!  So how does one interpret the phase margin when the
                 phase response start from 45 up to 405?  I know how to
                 identify the phase margin when starting from 0 dropping
                 down to -180.
                 \_ You're talking about a phase out vs. log(frequency in)
                    plot, right?  The phase is always with respect to input.
                    \_ Yes, for example, starting from 0 with unity gain
                       at -135.  I have 45 degree phase margin.  What if I
                       start from -90 with unity gain at -200?  What is
                       the phase margin in this case?  What about starting
                       from 100 with unity gain at 0?  What is the phase
                       margin in this case?