[time-nuts] LF power supply noise

Mike Monett xde-l2g3 at myamail.com
Tue Jun 23 14:16:53 EDT 2009


  Magnus wrote:

  Mike wrote:

  >>> So about  the  only thing left of interest is  histograms  of the
  >>> jitter. Unfortunately, the 543310A cannot store enough samples to
  >>> really make an interesting graph. What I would like to be able to
  >>> do is  similar to an invention I made for the disk  industry long
  >>> ago, called  Phase Margin Analysis. There is a  brief description
  >>> on my web page at

  >>>   http://pstca.com/patents.htm#phasemargin

  >> Somewhere in  my map of apps there is a HP appnote for  doing the
  >> same, to discs, intended for disc industry, back in the days.

  > That may very well be a result of my invention, which  occurred in
  > 1970, was published in 1979, and was copied by IBM in the 1990's.

  > But I  have  all the HP appnotes for disk. I don't  recall  any of
  > them describing  what   I   show   above.   Can  you  provide more
  > information?

  I found it.

  AN 191-7, 8.34 MB PDF, for the 5370B Universal Time Interval Counter

  Dated 1987-06-01

  url: http://cp.literature.agilent.com/litweb/pdf/5952-7908.pdf

  web page:

 
<http://www.home.agilent.com/agilent/facet.jspx?c=153627.i.2&to=80030.k.1&cc=US&lc=eng>

  There are  so many thing wrong with that app note I don't  have time
  to list them all.

  First, it is dated June, 1987.

  That is 17 years after I invented Phase Margin Analysis, and 8 years
  after the Katz & Campbell article was published in IEEE Transactions
  on Magnetics.

  By this  time,  the technique of Phase Margin Analysis  was  used in
  every hard  disk  company all over the world. You  simply  could not
  design, analyze,  or manufacture a hard disk without it.  So  it was
  well known and understood.

  But in  this case, it looks like some  junior  applications engineer
  got a  brilliant  idea to advance his career  and  publish  this app
  note. He  has no clue about magnetic recording and what it  takes to
  store data on the disk and read it back.

  The article  was written for floppy disks. The maximum rate  of data
  collection is mentioned at 8,000 samples/sec.

  That is  useless. In order to detect the loss in margin caused  by a
  single bit  defect,  you  must   collect   data  on  each  and every
  transition. For example, see Figs 6 and 7 in the Katz paper:

  http://pstca.com/pdfs/katz.pdf

  Thus, 8,000  samples/sec  is  far below  the  data  rate  needed for
  floppy, and hopelessly inadequate for hard disk.

  The timing  measurement described in the app note  does  not clearly
  describe where  the data and clock signals come  from.  Ideally, the
  timing measurement should be made at the output of the PFD  in order
  to include  any  offsets in the phase detector, and  to  include the
  loop dynamics in the measurement.

  For example,  the heads on a hard disk flutter as  they  move across
  bumps and imperfections in the surface. This causes a phase error in
  the loop  that  can turn a minor disk defect into a  hard  error, or
  mask an  error  by shifting the phase in the  opposite  direction. A
  similar effect  occurs   on   floppy   disk  with  imperfections and
  variations in  the  coating thickness. The next time  the  sector is
  written, the  timing  shifts  slightly. So  what  was  previously an
  error-free disk suddenly turns into a hard error. Or vice-versa.

  For example, see my micro-defect patent at

  <http://pstca.com/patents.htm#microdefect>

  These issues  are not emphasized in the app note, so it  is unlikely
  the author is even aware of them.

  It is not clear how the 5370B is connected to the  electronics. Just
  blindly hooking  up  cables  could cause all  sorts  of  loading and
  grounding problems. These issues are not emphasized in the app note,
  so it is unlikely the author is even aware of them.

  The Gaussian curve is a parabola when measured on log-linear plot.

  Any deviation from a parabolic curve on the side of the  margin plot
  is an indication of problems that need to be addressed.

  The plots  in  the Katz paper all show the parabolic  drop.  But the
  plots in  the app note have any kind of shape you may want,  and the
  software allows you to select various parameters for the curve fit.

  This is wrong.

  The plots  shown in the app note are useless  for  making analytical
  measurements of the performance of the channel.

  The app  note  is wrong,  misleading,  propagates  false information
  about data  channels  and measuring Phase Margin,  and  should never
  have been published.

  I could  go  on, and will probably kick myself  for  forgetting some
  hugely important issue. But I have to stop and work on several other
  issues that suddenly came up.

  (Nothing terribly important. My landlord needs some help with urgent
  problems in the laundromat and kitchen. He is a nice guy so I try to
  help whenever  I can, and he counts on me when problems crop  up. So
  one minute  I  am  analyzing  one  of  the  most  exotic measurement
  capabilities available  on  the  planet, and the  next  minute  I am
  buried in  a front load washer trying to figure why the  drain valve
  is stuck open, then fixing a broken tomato slicing machine:)
  
  Regards,

  Mike




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