In practice, I have found small electrolytic capacitors to have quite a low
impedance to HF range RF signals (just by trying them for bypassing,not a
scientific study ;-) SO, I would suspect that you are right, Robert.

It all depends on the self-resonant frequency of the cap. Below that freq.,
it will act like a cap. Above it, it will act like an inductor. I don't
know what typical values are for a small electrolytic.

ESR might actually make a MUCH bigger differnce than inductance,come to
think of it. The ESR of a small electrolytic can easily be 20 ohms. So, if
I'm figuring correctly, it would never appear to have a lower impedance
than 20 ohms. I also think that ESR may increase with frequency (due to
dielectric losses).

Considering all of this, I would say that using only an electrolytic is
probably OK,BUT make sure it has a low ESR. One would wonder,though,what
the point of placing large capacitances on the output of a regulator would
be? For bypassing,you want to place the caps as close to the components
being bypassed as possible. The cap on the regulator output normally does
nothing except stabilize the feedback loop,as you said,and must be right
next to the regulator. In certain circumstances, a large cap would be
necessary to imoprove transient response,but that probably only happens
with LDO or micropower regs (can't remember which usually have the problem).

Sean

At 05:53 PM 11/9/99 -0500, you wrote:
>>Only if the CAPS are suitable for high frequency stuff. There is no need to
>>use a large cap (Say 1uF) that has a high impedance at RF. In this case a
>>small cap say 100pF ceramic would be better.
>
>This might be true at very high frequencies, where mica caps are required,
>but my belief is that it's not true for frequencies ~ 40 MHz and below. The
>bigger the capacitor, the lower the impedance. (E.g., 10 uF vs. 0.1 uF)
>
>
>================================================================
>Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
>Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
>824 Timberlake Drive                            Tel: 757-467-0954
>Virginia Beach, VA 23464-3239                   Fax: 757-467-2947
>
>"Vere scire est per causae scire"
>================================================================
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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