Brad these are nearly always due to inductance and
capacitance in the layout of the board. Try shorter
connections and use point to point as much as
possible. Be sure to decouple VCC. When you get to
these frequencies you need to use RF rules for your
design. Connection to VCC should be ok as long as the
connection is short and decoupled
Bobby
--- Brad O'Dell <[EMAIL PROTECTED]> wrote:
> 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.
>
> I tried R1=6.8k and C1=220pF. They were supposed to
> give 4MHz, but only gave 1.3MHz
>
> R1=3k and C1=100pF only gave 3.7MHz instead of the
> desired 22MHz.
>
> I tried R1=3k and C1=39pF (both minimum recommended
> values by the datasheet) and they only give 8.5MHz.
> These values correspond to 51MHz, well above the
> operating range of the unit.
>
> I tried R1=47 ohm and c1=6.8pF (both way below the
> minimum recommended) and I could still only coax
> 16.5MHz out of it. I am very hesitant to run the
> device
> indefinitely with these values.
>
> I also had a similar experience trying to use a
> LMC555 near its gauranteed minimum max. frequency.
> What have I been doing wrong?!?
>
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--
Author: Bobby Walker
INET: [EMAIL PROTECTED]
Fat City Hosting, San Diego, California -- http://www.fatcity.com
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