[sdiy] Patchell's modules

[karl dalen]

--- Harry Bissell Jr <harrybissell at prodigy.net> wrote:
> --- karl dalen <dalenkarl at yahoo.se> wrote:
>  Decoupling caps
> > > are your
> > > FRIENDS !!!)
> > 
> > Only "if" power planes are low impedance.
> > If power planes are high impedance bypass caps are
> > terrible enemies.
> > They worsen the cirquit behaviour. So always low
> > impedance your 
> > powerplanes.
> 
> nah nah... they are still your friends. Its the power
> supply impedance that is your enemy !!!  :^P

Well, well, i forgot to include the supplies, sorry, but could
we say that inductive reactance of the tracing that usually 
dominates the impedance characteristic?

I had no idea of what Ian was designing, however a caos 
gen cant be high power so check inductances, if you do
a caos of high power check impedances! :-) 
 
> This is true only in very rare cases.
> In most cases
> adding the decoupling caps lowers the power supply
> impedance anyway.

Yes but asume we have a board trace net that is high in 
impedance, or un even impedance,now adding caps moves
the cirquits resonant points closer to various cirquit
frequencies wich in bad luck will create spurious resonances.
Wich would be less of an event if the impedance would be low,
because impedance varies with frequency!
Right, or im out and peddaling in the green?

The rule has been for long time that ground planes to be 
of low impedance 0.1ohm, and less.

> The "bite-in-the- at ss" comes if the power supply lines
> are HIGHLY INDUCTIVE (like big loops of wire enclosing
> a large area between current feed and return).

Well, agree! but these two are closely connected,
would it be wrong to say: inductance play larger
role in most low current,designs but imedance a 
larger role in power applications?
(asuming same frequency for both). To some extent!

> Adding
> a cap here might possibly make the circuit RING (a
> resonant condition).

Yeast, but what does the resonat condition depend on,
is it easyier to get resonant peaks in high impedance
nets rater in low, asuming low as 0.1ohm, and high as
in at least kilo ohm nets, however you could have a net 
of very high inductance and never go into ringing, wich
i sugest depends on the resonant frequency, wheter you
hit the resonant peak or not.
(i cross my fingers im not totally wrong here!!)

> Using staggered sizes of decoupling cap helps here, an
> electolytic, a tantalum, and a ceramic for low to high
> frequencies respectively.
 
> Most of the ringing problems would be at very high
> frequencies that the average synth-diy'er will never
> see... but for RF and some high-speed computer aps
> there WILL be an issue. 

And OP ams have Bw of several 100Mhz today, a current
feedback OP easily does 500Mhz.So the average synth diy can
see this problem if not properly designed.

Remembering the oscillator soft sync problem where the  slave
oscillator synced due to master osc integrator triggered the
slave trough the power lines, trough noise level at power 
supply lines, i wonder was this a inductive problem or
impedance problem or both?...........or something else!!?

I dont say this PCB thing is easy, it aint, and its complex.
I have a baad feeling that i have mixed up something here!

Reg
KD