[time-nuts] 4046 replacement

[Charles Steinmetz]

See below for further information on working with the 4046/7046/9046 PLL 
families, including must-have design tools for anyone designing with 
these devices.

I wrote:

>> The "flaw" in the 4046 is a dead zone around zero error in Phase
>> Comparator 2 (the PC one generally uses).

Magnus responded:

> It is very bad indeed. Someone chose to use the 4046 to lock up a 155,52
> MHz VCXO to a 8 kHz reference, using a 4046 as a core. The charge-pump
> was then "accelerated" with a supposedly better charge-pump with a ton
> of passives. Turns out that the dead-band was still there to haunt the
> designers. The 155,52 MHz was further multiplied to become the 2,48832
> Gb/s clock, and as they measured this they had problems with the
> jitter/wander of it

Of course the dead zone was still there -- it is built into the 
4046/7046 phase comparator, and nothing you do after-the-fact can 
eliminate it (but see below re: linearizing the 4046/7046 phase 
comparator). Most of what is wrong with the circuit you describe above 
is simply bad system design, not any fault of the 4046.

While it is true that some people call the PC2 output of the 4064 a 
"charge pump," as a voltage source it is, at best, a very poor one. The 
9046 has a real, current-mode charge pump with tri-state outputs. The 
attached charts show the difference in linearity [1].

There are tricks one can pull to linearize the PC2 output of a 4046 or 
7046.  In particular, (i) injecting current into the PC2 output node 
biases the detector away from the dead zone at the price of a static 
phase error, and (ii) instead of using a passive RC filter, run the PC2 
output through the resistor to the virtual-ground input of an active 
filter, which effectively turns the PC2 voltage output into a bipolar 
current output.  Still, however, the 4046/7046 PC2 cannot overlap 
positive and negative steering pulses as the 9046 PC2 can, and the 9046 
thresholds are established by a real voltage reference, so the 9046 will 
always be better than the best that can be done with a 4046 or 7046.

I do not use 4046-type devices very often, but ever since the 9046 
became available I have used it exclusively in preference to the 4046 
and 7046.

Best regards,

Charles


[1]  The attached charts are taken from the Philips CMOS PLL Designer's 
Guide (1995), which is an absolute must-have for anyone designing with 
the 4046/7046/9046 PLL families.  List member Daniel Mendes pried the 
Guide and supporting files out of Philips a couple of years ago, and 
list member Oz from DFW hosts them on his site.  I cropped the pages of 
the Design Guide to eliminate the large white borders and re-posted it 
all as a zip file to Didier's site: 
<http://www.ko4bb.com/getsimple/index.php?id=download&file=03_App_Notes_-_Proceedings/74HC4046_7046_9046_CMOS_PLL_design_guide_and_files_Philips_1995.zip>. 
Enjoy!

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