On 09:49 PM 9/09/2002 -0400, Brian Guralnick said:
>DAMN ISP HEADACHES!!!
>
>Re-send...
> > This is one of the decoupling caps between VCCINT 1.8v & GND.
> >
> > As you can see, the 1.8v dips down to 1.625.  This creates havoc with 
> the processing inside the Altera EP20K200E which has clocks
> > running at 133MHz, 125MHz, 95MHz, & 24MHz.
> >
> > 16KB
> > ftp://ftp.point-lab.com/quartus/Public/ProtelUsers/NoiseVcc18-2ns.png
> > 14KB
> > ftp://ftp.point-lab.com/quartus/Public/ProtelUsers/NoiseVcc18-4ns.png
> >
> > Here is my current decoupling caps for the 1.8v.
> >
> > 4 x 0.1uf smd 0805 ceramic.
> > 3 x 470uf electrolytic.
> >
> > See the problem?

0805 is supposedly somewhere around 0.5 nH - add a bit for via inductance - 
lets say 1 nH.  With 100nF this resonates at about 15 MHz.  So you are on 
the rising X curve with your noise glitches.

What about paralleling 100pf caps across the 100nF decouplers.  I suggest 
changing the order so the 100p's are on the board and the 100n on top - or 
do what the very high freq guys do and mount the caps on their side to 
reduce inductance (have a look at the way multi-layered caps are made to 
figure this out).

You could also try pick a capacitance such that the total net cap and 
inductance is series resonant at your major noise freq.  This will put an 
AC short, limited by capacitance Q (assume inductance Q is high), across 
the supply at this freq.  I have seen this done but usually on supplies 
with narrow band noise.

Using high-Q caps may help as well as they have much lower ESR and usually 
lower ESL.  More $ of course.  Murata and Philips make ceramic caps 
designed for high freq work.  And of course companies like ATC make 
porcelain caps for microwave work but I am not sure how high they go in 
capcitance.

Does paralleling another 100nF cap (or 10 nF or 1 nf or 100 p ...) on each 
of the existing caps make any difference?  The sorts of noise freqs you are 
seeing look like small HF caps may help rather than bulk capacitance.

Guessing,
Ian Wilson



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