[sdiy] 4069 vco gives v+ noise? - one last question

[Czech Martin]

Yes, seperate pathes. 
You have inductive coupling (magnetic field) and
capacitive coupling (electric field).
We call these wire loops or parasitic transformers and parasitic
capacity. The stray signal is transmitted via "air".
These become dominant for higher frequencies.
We can shield these.

And we have galvanic coupling, via wires etc.
This means that different currents from different
modules share the same wire, thus creating
error voltages by Ohm's law.
Of course, wires can have inductance, or we can assign
inductance to a piece of wire (we had this ealier
that the total inductance is a property of a closed
loop). So di/dt*L will add to Ohm's law: U= R*I + L*di/dt
High frequency currents will therefore have additional
error voltages as consequence.

The seperate wire approach simply tries to seperate
the different currents, thus avoiding error
voltages in "clean" wires.
A reference voltage will be passed on in a seperate wire
with no dirty currents in it, and also no large I*R drop.

But there is a little problem:

If you go too far in this sepration approach, every module
or circuit part will have separate GND, or reference,
especially due to inductance
and this can lead to common mode problems.
This depends of course on where your reference point (star point)
is. Some A/D-converter application notes show this problem
very clearly (because the converter will suffer badly, otherwise).

What is a cure for some problem can be poison if you get an overdose.


m.c.


-----Original Message-----
From: Magnus Danielson [mailto:cfmd at swipnet.se]
Sent: Donnerstag, 15. Mai 2003 03:01
To: _nial_ at yahoo.com
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] 4069 vco gives v+ noise? - one last question


From: Karl Ekdahl <_nial_ at yahoo.com>
Subject: Re: [sdiy] 4069 vco gives v+ noise? - one last question
Date: Thu, 15 May 2003 02:19:39 +0200 (CEST)

> Thansk to everyone who helped me with this problem, i
> temporarily changed my transformer to a 22v center tap
> that i found, so now it's working again!
> 
> I'm going to build a whole new powersupply based upon
> the information i've got from all of you so i might as
> well ask one last question that i've been thinking
> about; What the heck is separate grounds? I've read
> everywhere that i should have one "analog" and one
> "digital" ground. Could anyone please give me a short
> definition on how this works?

The point is that you want to keep the noisy part of the design (that digital
stuff) as separate as possible from the more noise-sensitive analogue parts.
One trick is to use a star style of powerfeeds, so at the powersupply they are
tied together, but on the board they run separately.

Cheers,
Magnus - who is impossible with numbers in my head late at night