[time-nuts] 4046 replacement

[Tim Shoppa]

There is no dead time issue at all with 4046 PLL's using the built-in XOR
(Type I) phase detector.

There can be a dead-time issue with 4046 PLL's using the built-in type II
(flip-flop) edge detector.

The 74HCT9046A uses current sources instead of voltage sources in its type
II (flip-flop) edge detector to avoid dead-time issues with this phase
detector.

The Type I phase detector was most commonly used in most 4046 narrowband
PLL applications.

The Type II was mostly used for the applications where the VCO had to track
over most of an octave or more than an octave. Most of these Type II
applications were relatively insensitive to dead time. (Otherwise the
phase-nuts 40 years ago would've noticed. Yes you can hear phase noise even
if they didn't have a good systematic way to measure it back then.)

If your original 4046 circuit has been working fine for the past 40 years
(the 4046 must be 40+ years old now) I see no reason to rip it out and
replace it with the newer variant. You may have trouble finding
through-hole (non-surface mount) 74HCT9046A's at this date anyway.

Tim N3QE

On Wed, Apr 18, 2018 at 5:40 AM, donald collie <donaldbcollie at gmail.com>
wrote:

> Thankyou Attila. I remember reading a book on PLL theory, recently,writen
> by a very knowledgable fellow, but I didn`t think to make a copy of his
> critique of the 4046. I recall that the nub of it was that the 4046 isn`t
> suitable for some applications because of a design flaw. Perhaps somebody
> in this group could explain further.
> Thankyou Bill, for the datasheet on the 74HCT9046, and your comments
> Cheers!.....................................................
> .........................Don
> jnr ZL4GX
>
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> On Wed, Apr 18, 2018 at 8:28 PM, wb6bnq <wb6bnq at cox.net> wrote:
>
> > Hi Donald,
> >
> > You could consider the NXP 74HCT9046A as a replacement.  It is an
> improved
> > version of 4046.  However, you do need to study the spec sheet as it is a
> > bit different, but in a good way.
> >
> > I have included an attached PDF of the spec sheet, if it makes through
> the
> > mail list server.
> >
> > Bill....WB6BNQ
> >
> >
> > donald collie wrote:
> >
> > I have 4  frequency synthesiser projects, each using HEF4046BCN`s, but
> have
> >> recently read that this CMOS IC has a design flaw. What would be a
> better
> >> chip to retrofit? I`m thinking perhaps a HEF74HC4046AN [that`s if it
> >> doesn`t have the same bug], or a 74HC7046AN - which is similar. Both
> these
> >> chips are more-or-less pin compatable TIA for any
> >> advice!.....................................................
> >> ..................................................Don
> >>
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