[sdiy] decoupling on modulars power bus

[Tony Clark]

   Hi Julian,

> I'm having a few problems with interference between modules on my =
> modular.  Notably lfos affecting oscillators through the power bus.
> 
> At the moment, the bus is just a simple passive stripboard affair (with =
> a run of tinned wire along the back to increase the gauge of the ground =
> strip) - basically a clone of the doefper distribution board.
> 
> What are my options for decoupling?

   Your best bet is to decouple at the module, as well as adding some 
caps on your distribution strip.  In general, decoupling involves putting 
two or more caps on each power rail (one large value, one small value).  
This way you supposedly cover different noise frequencies that could be 
hopping across your power rails.

> I have a few such busses in my modular, each serving one 19" row of =
> modules.  What's the best way to connect these up to the psu?  In some =
> sort of star configuration, no doubt, but should i try and decouple =
> these from one another at this point also?

   Star configuration is always the best.  But from what you describe, 
even this will not help you with intermodulation via the power lines.  
Only good decoupling/bypassing will work.
   My solution, learned from having to isolate a very high-gain amplifier 
from 100mV of line noise from a computer 5V line, is to isolate all lines 
on the module with a small resistor and large capacitor (such as a 22 ohm 
resistor and a 100uF capacitor).
   In general, you don't need to do this with most op-amps, since they 
have a very good CMRR (common mode rejection ratio).  However, any 
circuit that is of high precision (such as a VCO's lin/exp converter) 
should not only be decoupled, but also should be further run from a 
stable reference voltage!

   Cheers,

   Tony

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