[sdiy] Fixed filter bank questions

Tim Ressel timr at circuitabbey.com
Tue May 14 02:17:00 CEST 2019

Now we are into the realm of process control. Think about how a film cap 
is made: two layers of a conductive material sandwiched around a 
dielectric film, and then rolled up. Any thickness tolerance in any of 
those layers gets multiplied by the number of wraps. Controlling those 
thicknesses is possible but probably costs a lot.  You can get 1% caps 
but there does not seem to be a good selection, suggesting low demand. 
Also they are pricey. The same goes for SMT ceramic caps.

I think we get spoiled by resistors. They can be laser trimmed and tight 
tolerance parts are common and not too expensive. Even down to 0.1% they 
seem to be reasonable.

When designing we need to determine just how much tightness we need, 
then ask our wallets how much we can afford, and then balance the two. 
Just like brothels.


On 5/13/2019 4:45 PM, David G Dixon wrote:
> When I look at, say, those little blue or yellow film caps, it is simply
> inconceivable to me that they could be made 10% different from one another.
> If you've ever seen a continuous automated industrial process that cranks
> out millions of the same thing, the key feature is repeatability.  In order
> for every part of the process to work, and for every piece of the assembly
> to fit together, every part has to be identical from one piece to the next.
> That is a recipe for 1% tolerance, not 10% or 20%.
> Matthew said:
> On the contrary, I don't think the basic process of manufacturing
> capacitors has changed much since the 1960s - they are still made by the
> same kind of machines out of the same kind of materials - so why should we
> expect those processes to give much different results today?  Automation
> isn't magic, and it's likely to be used to reduce costs rather than to
> tighten tolerances, if the market seems to prioritize price.
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--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

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