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From: "Bill Sloman"
Subject: Re: What kind cap should I choose?
Date: Sat, 2 Nov 2002 00:16:17 +0100
Organization: Planet Internet
NNTP-Posting-Date: 1 Nov 2002 23:16:18 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
"Bob Wilson" wrote in message
> In article <firstname.lastname@example.org>, email@example.com
> >Hi all,
> >I need help please. I am building a very simple transmitter. I have a
> >coilin in the Drain of 2n7000 (N type MOSFET) and a cap inparallel with
> >coil and the Source to ground. The Gate is pulse with an I/O of the
> >microcontroller. Let's say that I tune the resonance frequenct at 38KHz.
> >This means I have to choose 110 nF cap (F=1/2.0* PI*SQRT(LC)).
> >I tried two different cap.
> > I received two different signal base on the material type that I tested
> >Is it an standard cap material they select for transmitter type of
> >If yes which kind is the best, please?
> There is no special "transmitter" cap. All you need is a stable, low loss
> cap. This is pretty basic. Use an NO0 (also known as a C0G type) ceramic.
> the frequency you are working at (which is pretty low), even an X7R type
> will do the job.
The recommendation for NPO parts is dubious advice. A COG ceramic capacitor
would be admirably stable, but IIRR you can't get 110nF capacitors in COG
ceramic. X7R capacitors are much more sensitive to temperature.
In fact you can't get 110nF capacitors off the shelf from any supplier I
know. Farnell stock +/-2% polyphenlene sulphide surface mount capacitors up
to 100nF, which are probably as stable as anything that is easily available
at around that value.
The also have +/-1% metalised polypropylene film capacitors, but only up to
47nF - you've got to buy at least five, so get your 110nF as five 22nF parts
and wire them in in parallel.
Neither polypropylene nor polyphenylene sulphide dielectrics are
particularly lossy at 38kHz and both would seem likely to work well in the
Bill Sloman, Nijmegen
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