----- Original Message -----
Sent: Wednesday, August 11, 2004 6:50
AM
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.
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?!?
