On Tue, 16 Oct 2012 14:02:39 -0400, Phil Hobbs
wrote:
On 10/16/2012 11:56 AM, Jim Thompson wrote:
On Tue, 16 Oct 2012 11:25:52 -0400, Phil Hobbs
wrote:
On 10/15/2012 07:59 PM, Jim Thompson wrote:
Finally zeroing in on modeling the 74HC4046 after finding a
unpublished AppNote that gave more details on the innards. This is
what a fixed frequency looks like, simulation-wise...
http://www.analog-innovations.com/SE..._VCO_2_SIM.pdf
Comments? Scalings? (This is based on AppNote and Datasheets
claiming trip at VDD/2).
First release will be VCO only and will be in LTspice format. Once
you approve that, the PFD is virtually all logic.
...Jim Thompson
Different manufacturers give you a wide variety of ridiculously
nonlinear tuning curves for the VCO--the tuning sensitivity varies like
3:1,
I don't think most users fret over the incremental slope. The
"follower" variation is trivial to fix by adding an OpAmp. Sinking
one end of the capacitor into the substrate diode every half cycle is
something you have to live with if you like 4046's. I'm doing this
for fun (and requests from this group)... I wouldn't use one myself.
Get my MC4024 if you want better linearity. I think there's also a
PECL copy, but I don't remember the part number off the top of my
head. Or use a V-to-F chip.
and the oscillator quits below about 0.7-1.1V (@VDD=5) depending on
the device.
That's noted on the data sheet. Why does that give you such
heartburn? Do you really need zero frequency?
Which did you pick? 
I have the most complete data on the TI 'HC4046, but I was aiming sort
of average ;-) since I'm building it from behavioral blocks.
I would guess that you're one of the few people in the world that
would need a flat-ass accurate fit to one particular version.
(The metal gate 4046-style oscillators all stink on ice--HC4046, HC7046,
HC9046, all makers, all horrible. PD2 is nice if you stay out of the
dead zone.)
Cheers
Phil Hobbs
What do you really need? An accurate V-to-F?
...Jim Thompson
The loop gain is proportional to K_VCO * K_phi, so if the tuning
sensitivity varies all over the map like that, so does the frequency
compensation of the loop. That's what makes the HC4046 and its brethren
so sucky.
Cheers
Phil Hobbs
I've not even played with one, except to measure some DC. The data
sheets would seem to indicate that the non-linearity occurs at the
tuning extremes. Just bound your control voltage if you're getting
lock-in issues :-|
I don't know why the "designers" of the 4046 didn't do a better job of
copying the PECL core of my MC4024 (~1965). All current mode, no
diode clamping, etc.
...Jim Thompson
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