[R-390] Two caps in parallel

Fri, 29 Mar 2002 15:10:26 -0800

The theoretical plot will be almost the same (1,000 won't affect 1,000,000
very much).  But using practical devices and high frequencies, yes, the plot
will be different.  The self-inductance (do capacitors have inductance, yes
they do, and in series with the capacitance, too) of the larger capacitor
will be overwhelmed by the capacitive reactance of the smaller value.   Same
goes for self-resonance of the larger cap, will be shunted by the smaller
cap.

At what frequencies?  Hard to say, depends upon the construction of the
capacitor.  20 MHz and above, though, is reasonable.  Is that frequency
significant in a digital circuit?  Yes, if square waves are involved (think
ten times the fundamental frequency).

Many people suggest tubular bypass capacitors in the front end of receivers
should be replaced by ceramic capacitors just for this reason.  Others state
no, the sets were designed with the inductance of the bypass capacitors
factored into the design of the equipment (I have a hard time believing
that).  Tubular capacitors, on the other hand are preferred in 455KHz IF
strips and in coupling between audio stages presumably because they are more
stable.

73 de Bill, AB6MT
[email protected]

----- Original Message -----
From: "Scott, Barry (Clyde B)" <[email protected]>
To: <[email protected]>
Sent: Friday, March 29, 2002 12:20 PM
Subject: RE: [R-390] Two caps in parallel

> Ok, so let's say I put a 1mfd cap and a 1000pfd cap in parallel and plot
frequency vs. reactance.  Will the plot look differently than a single
1.0001mfd cap (if one existed)?  From the responses, I assume the plot will
be different.
>
> Barry(III) - N4BUQ
>
> Yes, this is correct. A common scheme in high quality audio amplifiers so
> the  power supply doesn't "sing along" with the audio circuitry Always
bear
> in mind that capacitors are "frequency sensitive" devices.
> Chuck N1LNH
> -----Original Message-----
> From: WYsixK <[email protected]>
> To: [email protected] <[email protected]>
> Date: Friday, March 29, 2002 2:10 PM
> Subject: [R-390] Two caps in parallel
>
>
> >The high value caps don't look very capacitive at high frequency so you
> >need a lower value one that does look capacitive at the higher
> >frequencies.  This is why it is common to see .01 disc ceramics in
> >parallel with large electrolytics.  If caps were perfect you wouldn't
> >need the smaller one.
> >
> >> > Are you suggesting the two caps act independently?
> >>
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