[time-nuts] 4046 replacement

[John Miles]

> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of
> donald collie
> Sent: Wednesday, April 18, 2018 2:40 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] 4046 replacement
> 
> 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

The tristate phase/freq comparator in the original 4046 had a dead-band problem that caused its gain to vary widely as it approached its normal operating point in a stable loop.  This wasn't so much a 'bug' as it was a consequence of the fact that there's effectively no gain when there's no phase error to correct.  

Ulrich Rohde's book indicates that this problem was first documented in 1978 in an EDN article by some authors named Egan and Clark.  Newer PFDs implement the 'antibacklash' logic that Rohde mentions.  If you really must use a 4046, I'd look for a newer version whose data sheet explicitly addresses this problem.  Better still, use a newer part.

Another workaround is to force the PFD to stay out of its dead zone by loading the output lightly, e.g. with a 1M resistor.  You can never compensate for this effect perfectly, though.  You can probably expect some downsides like worse reference suppression.  

-- john, KE5FX
Miles Design LLC