In the past, I used three value-ranges of decoupling on high-speed designs:
- Bulk: A 10uF or two caps located anywhere
- Intermediate: Some 0.1uF caps located near the part
- High-frequency: Some 1000pF located right under the BGA or connected to the power pins directly
You could not get high-value ceramics in small packages, so you had to use low-value capacitors to get the low inductance associated with a small package.
Now 0.1uF ceramics in an 0402 package are a standard low-cost part. I recently had a notion to check the curves to see at which frequencies a 1,000pF 0402 had a lower impedance than a 01.uF 0402:
There is only a tiny frequency range where the impedance of a 1,000pF is better. I would be interested to hear from anyone who knows a good reason to use them low-value decoupling caps today.