4046 disable input

[dr pepper]

I want to use a 4046 Pll without the internal vco, however I do want to use the source follower for the demod output.
This means I have to have the vco enabled as the enable input enables the vco and the source follower, how do I stop the vco from running or make it stable so its doesnt interfere with the rest of the circuit?, can I just ground the cap inputs, or the r1/r2 inputs?
I guess I could use an extrnal op amp and disable the vco/source follower but if I can save a chip I will.

 

[JimB]

Just looking at a 4046 datasheet (NXP), there was no detail of the internal circuitry of the oscillator.
My best guess to disable the oscillator would be to connect R1 and R2 inputs to ground and leave out the capacitor.
Then check that there is no output from the VCO.

JimB

 

[alec_t]

TI's datasheet (page 11) gives details of the oscillator. Looks like shorting pins 11 and 12 would disable it?

 

[dr pepper]

Thats a handy schematic, dont know how I missed that one.
Shorting 11 and 12 to ground will set the current mirror to max, if theres no cap present wont I then get silly freq's, one datasheet I looked at says no cap is allowable if you want the max freq the chip will do.
I was thinking, and I might well be wrong is leave 11 and 12 open setting charge/discharge current to 0, then connect one side of what would have been the cap to + and the other to - through fairly high value r's.
Thanks for that chaps.

 

[Nigel Goodwin]

I've got a book about PLL's at home somewhere, and it does a lot about the 4046, I'll see if I can find it this weekend.

 

[alec_t]

Shorting 11 and 12 to ground

No. I meant short pin 11 to pin 12, not short them to ground.

 

[dr pepper]

Sorry yes I'm a numpty I didnt read your post properly.
Thats a good idea too.

Thanks Nige.