[SI-LIST] Re: Measuring Power supply Ripple and Noise

Steve Weir's suggestion of the Tek passive low impedance probe is right on.
Those are wonderful probes that should be in every SI engineer's tool box.
At $1200 for a fine cable and two probes it's a bargin. The 1:1 probe makes
a decent launch probe for TDR work. It is spec'ed to 3GHz while the 10:1 is
9.5 GHz.
One note of caution: 3 lb of sideways force will break the tip assembly of
the 10:1, 500 ohm probe.

The Agilent version of this probe is not as nice and the cable is not nearly
as flexible.

     Dave Cuthbert

On Fri, Nov 14, 2008 at 1:09 AM, Santos Fernandez, Jesus <
jsantosfernandez@xxxxxxxx> wrote:

> Hello,
> From the answers to Jaison, it seems to me that everyone has it's own
> little tricks and arts. I'm surprised that there is not a single reliable
> method.
> Some time ago, I used a (expensive) 3GHz Agilent scope with diff and single
> ended probes and, I think it was OK; the results matched the simulations
> quite well.
> My question is: Is there a single indisputably reliable method?
>
> J Santos
>
> Jesús Santos Fernández
> Laboratorio de EMC
>
>
> -----Mensaje original-----
> De: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] En
> nombre de Richard Feldman
> Enviado el: jueves, 13 de noviembre de 2008 21:37
> Para: DAVID CUTHBERT; Jaison Fernandez
> CC: si-list@xxxxxxxxxxxxx
> Asunto: [SI-LIST] Re: Measuring Power supply Ripple and Noise
>
> Jaison,
>        As others have mentioned, it's handy to use thin coaxial cable
> soldered directly
> to power & ground opposite the chip of interest.  Some caveats:
>
> 1. Without 50 ohm termination, ripple voltage at cable-resonant frequencies
> will be hugely exaggerated.
>
> 2. In many scopes with 1-megohm inputs, the AC-coupling time constant is
> reduced to a few microseconds when you also select 50 ohm termination.  That
> can mask important low-frequency details, e.g. in voltage regulator step
> response.  If you need AC coupling, use external or no termination (or a
> very large external DC-blocking capacitor) when looking at low frequencies.
>
> 3. When you probe across the terminals of a mounted chip capacitor, the
> response can be significantly influenced by the RLC of that specific
> capacitor, whose mounting inductance is shared with your probe.  Consider
> removing one capacitor if necessary to get suitable vias where you want to
> probe.
>
> Rich F.
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of DAVID CUTHBERT
> Sent: Thursday, November 13, 2008 11:43 AM
> To: Jaison Fernandez
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Measuring Power supply Ripple and Noise
>
> Jaiso,
> soldering a coaxial cable directly to the board works very well. The
> common-mode noise rejection is good. Set the 'scope to 50 ohms.
>
> Power supply boards commonly have CM noise on them and this is a real
> problem with single-ended X10 probes. An active differential probe can work
> well.
>
>   Dave Cuthbert
>
> On Wed, Nov 12, 2008 at 11:46 PM, Jaison Fernandez
> <jaisonf@xxxxxxxxxxxxxxx>wrote:
>
> > Hai all,
> > I am trying to characterize the power integrity of of a board (that
> having
> > a
> > system clock running at 400Mhz and DDR memory)
> > What is the type of probe (Single ended or differential) is best suited
> for
> > measuring power supply ripple and noise in the board.(I am using Tek GHz
> > Oscilloscope for this purpose)
> > Can any one give any document regarding measuring power integrity of a
> > board.
> >
> >
> >
> >
> > With Warm Regards,
> >
> > JAISON FERNANDEZ
> >
> >
> >
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