[sdiy] 4046 PLL Usage

[MBEDTOM at aol.com]

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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