[sdiy] precision/matched capacitors??

[John L Marshall]

If you need to get a precision match but not an exact value then build an AC
bridge. The signal source may be any oscillator, say 1 kHz. One leg will be
the reference capacitor. A second leg will be the test capacitor. The third
leg will be a resistor with a value close to Xc at 1kHz. The fourth leg will
be a noninductive pot to adjust for a null. The detector can be an AC
voltmeter or an audio amplifier. I would choose a fixed resistor of 0.9 R
and pot of 0.2 R for more greater precision from the pot.

John
----- Original Message -----
From: Jim Patchell <patchell at silcom.com>
To: <media at mail1.nai.net>
Cc: <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, October 31, 2001 10:36 AM
Subject: Re: [sdiy] precision/matched capacitors??


>     In general, 1% tolerance is about the best you will find, and even at
> that, 1% is difficult to locate at your convenient parts suppliers.  Also,
> when you are talking precision caps, you are talking about caps that use
the
> better dialectrics.  I remember on one job I did I used 1% polystyrene
caps.
> They were not cheap.
>
>     If you need matched caps, get a good capacitance meter, purchase a
bunch
> of 5% polystyrene caps (mouser sells these), and measure away.
Polystryrene
> caps are pretty stable, so once you know the value, it isn't going to
change,
> much....
>
>     Now, if somebody could suggest a capacitance meter....
>
>     -Jim
>
> media at mail1.nai.net wrote:
>
> > You can buy precision resistors, you can buy matched semicinductors, but
> > can you buy precision caps, or number of matched caps built into the
same
> > package??
> >
> > THANX!!
>