[Hammarlund] Paper caps

[Bill]

Now with the web, I suggest pulling down the manufacturers data sheets.
I strongly suggest this
because there are many types of ceramics out there with various
temperature and coefficient ratings.
There are a host of different profiles out there, e.g. z5u, y5p, y5u,
y5v, NPO. The better the cap, the more it
will cost.   

Saying that you measured a ceramic is not sufficient, you need to report
the type (material, type, tolerance temperature coeff).  

Also, loss and capacitance change are two different items.  A bypass
capacitor
providing -j0.5 ohms to ground, that changes capacitance by 50% changes
the impedance either to -j1 or -j0.25.
Likely not a big deal if it is bypassing a 100 or 1 Kohm resistor.  Now
if it was a tuned circuit, that
is a different story - and you would have chosen the wrong part for the
job.

As to impedance to ground, it does matter if the it is inductive or
reactive.  If you are just worried
about one particular frequency, then you might get away with providing a
low inductive or capacitive 
reactance to ground. But, what about the other frequencies, esp DC.
Providing a low inductive impedance
to ground at RF, may also imply providing a DC short to ground. I doubt
you meant that, but you were not clear.
On the other side, once the reactance of a capacitor turns inductive,
the reactance increases with frequency.
Generally this is undesirable unless the frequency that this occurs is
way outside your frequency range of interest.

You also need to be very careful with resonances. That cap that just
started acting like an inductor may form
a resonant circuit with another cap at some frequency. Even in modern
systems, it is all to easy to get the 
decoupling caps to resonate in the power plane, and low and behold the
power rail oscillates.

BTW: In our modern cellular radios we use ceramic disk caps esp at RF
and esp in tuned circuits.
  These cicuits go to 3 - 5 GHZ, and believe me, loss is important.


In short, read the data sheet and description/uses for the part and pick
the right part for the job.

Bill, ve3jhu

-----Original Message-----
From: hammarlund-bounces at mailman.qth.net
[mailto:hammarlund-bounces at mailman.qth.net] On Behalf Of Gottfried Ira
Sent: Saturday, December 30, 2006 6:32 AM
Cc: hammarlund at mailman.qth.net
Subject: Re: [Hammarlund] Paper caps


James A. (Andy) Moorer wrote:
 > Ceramics for the RF deck for bypass.
 > If you have to replace a mica, use a modern dipped mica for
stability.  >  > You shouldn't use the Orange Drops anywhere the
frequency is over 1 MHz  > or so. They have more inductance than, say,
the ceramics. I measured the  > self-resonance of a .01 uF Orange Drop
as 1.5 MHz, which means it is not  > acting like a capacitor any more at
that frequency and above.  >

For the purpose of bypassing RF to ground, only low impedance is
important, it does not matter if this impedance is capacitive or
inductive. This also applies to coupling capacitors - in most cases
(depends on the circuit). Usual disc ceramics are lossy (OK for
bypassing) but show excessive degradation at elevated temperatures (e.g.
-80% of the rated capacitance !)

Ages ago I did some measurements... the results can still be found here:
http://www.oe1ira.at/hc/hammarcapac.html

Gottfried Ira

-- 
http://www.oe1ira.at  <-- New Address!


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