Agreed. As 1GHz is approached, I've had problems finding beads that are
rated at that high a frequency. The chip beads that I have found work
pretty well. Mostly they've been used to kill supply path oscillations
on boards where there is no other option. They aren't cheap.
> As far as power delivery is concerned, ferrite beads have low impedance at
> DC and low frequency. Charging up decoupling caps at a slow speed is not
> degraded. High-frequency surge current will be blocked by the ferrite beads,
> but will be provided by the high-frequency decoupling caps placed between
> the beads and the VCC pins of the chip. This is exactly what the design goal
> is - don't want to see high-frequency current on VCC/GND plane.
Another way to look at it is that you are providing a definate, low Z
path for the AC return.
> When EMI problems are at close to 1GHz, caps are effectively inductors.
> Putting more caps down will lower the effective decoupling impedance, but
> limited by cost and space (if you can't get caps close to pins, it is a
> waste of money).
SMT 0603 ceramic caps are still pretty good at 1GHz+ as long as you keep
the value under a couple 100pF. 0402's go up through a couple GHz.
-- --------------------------------------------------------------- Mark Randol, RF Systems Engineer | Motorola SPS, Inc. (602)413-8052 Voice | M/S EL379 (602)413-4150 FAX | 2100 E. Elliot Road [email protected] | Tempe, AZ 85284 ---------------------------------------------------------------**** To unsubscribe from si-list: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****