[sdiy] Ratio switching!

[Louis van Dompselaar]

> If the resistors are selected by hand using a good meter then surely even resistors sold as 5% would give the reqired accuracy AT THE TEST TEMPERATURE ?
>   
Yes, but what are the chances of finding two resistors in separate 
batches that would give you a 0.01% accurate division?  Even if you get 
two batches of 500 that are guaranteed to be evenly distributed (so 1 
resistor for each 0.01%) you'll end up testing on average 250 resistors 
from one batch against a given resistor from the other.  But in practice 
you might end up with thousands of +1% resistors in one batch and 
thousands of +2% in the other and never get the division you want.

But all I'm saying is that it's unlikely you'll get 0.01% accuracy on 
all steps.  If you lower your target, it's very feasable.  The octave 
switch in the Doepfer VCO is just a bunch of matched 1% resistors on a 
voltage reference and that works fine.

Louis