[sdiy] 4046 PLL Usage

[Harry Bissell Jr]

I think it would be wrong to call the input stage 'amplifier'... buffer
  is probably better as it would not have any gain.
  
  The usual input voltage transition point should be 1/2Vdd... but the logic
  'high' and 'low' levels from that point are not guaranteed.
  
  You're lucky if you were really using  a 50mV signal... there is no guarantee
  but if it works, it works.
  
  The point of manufacturer differences is VERY important. Sometimes they
  make equivalent chips are ARE different.  One counter chip had an input
  with schmitt triggers on two inputs, which were then AND'ed. Another
  manufacturer used the AND gate first, and schmitt triggered the output.
  DUH... what good does the schmitt trigger do, if it is not the first
  stage.
  
  Anyone using the 4046 would be wise to use a comparator or other signal
  conditioner to guarantee logic levels first, before the signal input... and especially
  if using phase comparator II (which you all will :^)
  
  H^) harry

MBEDTOM at aol.com wrote:  Just an FYI...
  
There are different "flavors" of 4046 PLLs out there.  No, I  am not referring to 74HC4046 versus CD4046 families.  I am  referring to the fact that some 4000-series PLL chips are different  from others.  Specifically, some devices have inbuilt amplifiers  for the SIG IN (normally, pin 14) and some do not.  Those devices  having inbuilt amplifiers will function with AC coupled signals of  fairly small amplitudes.  The devices lacking such an amplifier  require much higher amplitude signals to function properly.
  
  I have an assortment of 4046 devices from several manufacturers.   The Fairchild brand units, do not appear to have the amplifier on SIG  IN as the data sheet says, for example.  Yes, the devices perform  to data sheet specs but the amplifier, if there is one, is  crippled.  Both RCA brand (really old chips!) and Motorola brand  (not quite as old as the RCA) chips do have the amplifier.  They  reliably lock onto signals of 50 millivolts RMS or less.  Other  brands can require a volt or more of signal to operate reliably.   Current data sheets spec minimum level signals far higher than what the  devices really do.  I no longer have my original RCA datasheet but  I am certain the stated input requirement is much less than what  currently appears on any manufacturer's datasheet.
  
  I did a commercial product some years ago that used a 4046 and I hadn't  seen these amplifier-crippled parts.  Once in production, a second  source for the Motorola 4046s was needed for lead-time reasons.   While qualifying other manufacturer's parts I discovered this  anomaly.  If On Semiconductor is really making Motorola-mask  parts, they do have the amplifier.  T.I. got the RCA masks (I  believe) so they should have the amplifier as well.  Philips,  Fairchild, and STM parts do not appear to have the amplifier in them.
  
  As long as I am writing this tome, watch input risetimes.  The  "good" amplifier devices also seem to tolerate much longer risetime  signals than do "crippled" amplifier units.  What used to be  in-spec signal levels and rise times are now out-of-spec for most  devices.  Might I suggest sharpening up SIG IN signals with a fast  comparator regardless of brand?  That should cure all the ills  related to SIG IN.
  
  The 4046 parts are good for 1000:1 lock ranges, and beyond, with a  prudent choice of passive components.  These are very handy parts  for synthesis, indeed.  As always, your mileage may vary.
  
  Peace.
  Tom Farrand
  

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