Re: Phase frequency detector

From: Mike Monett (mrmonett_at_yahoo.com)
Date: 06/07/04 

Date: 6 Jun 2004 19:56:00 -0700

Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<c973ke0n35@drn.newsguy.com>...
> Bill Sloman wrote...
> >
> > Mike Monett wrote ...
> >>
> >> Another possible confusion is shown in the data*** for the Philips
> >> HCT9046A chip:
> >>
> >> http://www.philipslogic.com/products/hc/pdf/74hct9046a.pdf
> >>
> >> On page 6, they claim that feeding a capacitor with a current source
> >> eliminates the deadband in the phase detector. They show the
> >> resulting performance in Fig. 11 on page 11.
> >>
> >> The truth is the cmos current sources still have a turnon and
> >> turnoff delay. If they are faster than the reset pulse from the pfd,
> >> there will be no deadband. But just because it is a current source
> >> feeding a cap does not guarantee this will be true. The prop delay
> >> of the phase detector has to be taken into account.
> >>
> >> So if the ic manufacturers can't get it right, it looks like the
> >> confusion over the deadband problem will continue as a topic in the
> >> newsgroup.
> >
> > You should have read a bit more of the Philips data ***. Their
> > trick, as detailed on page 8 of the application note, very
> > specifically in the note on Fig.8 at the bottom of the page, is that
> > the postive and negative current sources are overlapped by about
> > 15nsec, and their actual claim is that feeding a capacitor with their
> > pair of overlapped current sources eliminates the deadband, which does
> > sound consistent with your story.
> >
> > Are you really claiming that Philips didn't got it right on the basis
> > of an over-hasty glance at their data ***? Or do you have more
> > persuasive evidence, like some measurements on the 9046 in action?
> >
> > According to Tom Bruhns, Agilent list the 74HCT9046 as one of their
> > approved parts, so it seems likely that it does what Philips claim.
>
> Mike, I agree with Bill. After some thought, do you as well?
>
> Thanks,
> - Win
>
> (email: use hill_at_rowland-dot-org for now)

  Win, Bill,

  Thanks for your posts. Sorry for the delay - Google news was down
  recently but may be OK now.

  Don't get me wrong - I'm not claiming the Philips part doesn't work.
  But the data*** does not offer a way to prove it.

  Most large companies require a comprehensive incoming test procedure
  when qualifying a part to ensure it meets requirements. But the key
  parameters that guarantee zero deadband are not specified in the
  data***, and Philips offers no tests to confirm it.

  The "about 15nS" overlap is hardly a specification, and the
  data*** does not attempt to measure it. We might like to assume
  the risetimes of the current pumps would be faster than the overlap
  time, but these are not specified or measured.

  One could construct a special test fixture to measure the phase
  detector transfer curve and ensure it is linear through zero. But
  the test is slow and costly, fairly difficult to implement, and does
  not tell how close the chip is to failure.

  What is needed is a way to measure the overlap time and compare it
  to the current pump risetimes. If the risetimes are faster than the
  overlap time, the phase detector will be linear through zero. The
  difference between these two parameters tells how much margin is
  available before failure.

  It turns out there might be a way to get the needed information.

  Fig. 19 on page 19 shows the 3-state enable and disable times for
  PC2_OUT.

  These tests are intended to measure the prop delays, but they can be
  used instead to measure the risetime of each charge pump. The test
  for tPZH also gives the risetime of the positive current pump, and
  tPZL gives the risetime of the negative current pump.

  However, the overlap time cannot be measured in this test since the
  charge pump currents cancel at zero phase error.

  Fortunately, the lock detect output, PCP_OUT on pin 1, can be used
  to measure the overlap time at zero phase error. If needed, this
  information could be plotted as a function of temperature and supply
  voltage to ensure proper operation under all conditions.

  So with some ingenuity and a bit of luck, it is possible to measure
  the deadband performance of the chip after all. The tests are simple
  enough that anyone could afford to do them. With care, it should be
  possible to obtain a wealth of additional information, such as the
  degree of balance between the current pumps, the phase offset at
  zero error, and the amount of internal crosstalk in the chip.

Best Regards,

Mike

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