[sdiy] Hand-matching capacitors for filter stages

[thx1138]

On 2/9/10 3:20 PM, "Mike Pepper" <profpep at hotmail.com> wrote:

>> As for various capacitor dielectrics, I think what you're most likely
>> going to find is differences between the two main dielectric families
>> - ceramics and plastic film.  The differences between polystyrene and
>> polypropylene will be minimal compared to ceramic and poly[ester|
>> carbonate|styrene|propylene].
>> 
>> If anyone is bothered to check through the archives you'll find this
>> subject thrashed to death many many times.
>> 
> I agree somewhat, though there seems to be some justification for 1%
> polystyrenes in high end audio filters
> 
> To avoid a certain amount of archive trawling, the famous Cyril Bateman
> articles are here:
> 
> http://www.scribd.com/doc/2610442/Capacitor-Sound
> 
> The famous, (and famously argumentative), high speed circuit expert Ivor
> Catt, (and when it comes to discrete chip logic design, this guy actually
> DID write the book), totally disagrees with the capacitor model used by
> Bateman et al, making the point that capacitors have no self inductance, but
> that the industry preferred to sell "low inductance" caps, rather than
> advise users to crop the leads as short as possible. Another very good
> reason to use reverse board surface mounted mounted decoupling capacitors; I
> notice Jurgen Haible is a fan of this techniue, on all his recent board
> designs. I first used a similar technique with leadless disc ceramics
> mounted in slots cut in the PCB, for RF and some early ECL logic work.
> Catt's book was almost the bible for this type of work, though only amongst
> engineers - physicists have branded him one of the heretics. I never got the
> deep theory, but do I know that if I followed his guidelines, my circuits
> ran better. You can put most of what I built on a small FPGA nowadays, and
> it will run faster!
> 
> A minor reminiscence: ECL was very hot running, on my first ever run with
> the stuff I did a trick I used with TTL, of putting my hand flat on the chip
> side of a wire wrapped card, to see if anything was overheating - the board
> stuck to my hand, it was that hot: for days I could read the Motorola logos
> lightly branded into my palm. I later found that was the norm - it always
> ran hot, which is probably the reason Seymour Cray described himself as just
> as good a refrigeration engineer as an electronic one.
> 
> ||\/||ike
> 
> 
> 
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Another trick I learned in fine tuning filters is using Ribbon cable or
spectarstrip. 

Rule of thumb was 1 pair of ribbon cable 1 " long was approx. 1pf. I would
use a pair of cutters to trim until desired result was obtained.

Perhaps this might help someone tune circuits out there.

Regards,

Terry