Re: MM74HC4046 PLL design please help!

[Dave Baldwin]

Subject: MM74HC4046 PLL design please help! I am at my wit's end.  The output frequency isn't following the formula at high frequencies.  The MM74HC4046 has a garanteed minimum max. center frequency of 11MHz.  This corresponds to a max. VCO operating frequency of 22MHz when the VCOin pin is at Vcc.  According to the charts and formulas given in the datasheet, The values of 3k for R1 and 100pF for C1 should give this center frequency.  And with the VCOin pin tied high, I should get 22MHz off the VCOout pin, but I only get 3.7MHz!  I'm currently only trying to get the VCO section of the chip to work.  R2 is open, but adding a 300k resistor does not help. [snip...] I also had a similar experience trying to use a LMC555 near its gauranteed minimum max. frequency.  What have I been doing wrong?!? A Long time ago, I had similar experience with the National part.  I had to Specify a different brand to get the results I expected.  seemed that 4046's from different manufacturers had essentially the same datasheets but performed quite differently.  Sounds like that may still be true. -=-=-=-=-=-=- "http://www.psyber.com/~dibsed" -=-=-=-=-=-=- DIBs Electronic Design    | Dave Baldwin: [EMAIL PROTECTED] Adapt and overcome.       | Buy a book and read it. -=-=--=-=--=-=--=-=--=-=-=-=--=-=--=-=--=-=--=-=--=-=--=-=-