[SI-LIST] Re: Measuring Power supply Ripple and Noise
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: "Santos Fernandez, Jesus" <jsantosfernandez@xxxxxxxx>
- Date: Fri, 14 Nov 2008 08:40:47 -0500
Well, as always, it depends. Mostly it depends on the frequency range
of interest, required sensitivity, and simply what is available in the
lab. There are multiple 'good' ways of doing the same thing, and we
need to understand the limitations of each to find the method that works
for the particular application. A few considerations: with active
probes, common-mode voltage range, and possible field pickup from nearby
strong sources (DC-DC or AC-DC converters) could be the problem. With
passive probes or direct coax measurements we may create unwanted ground
loops at low frequencies. Home-made passive probes can easily provide
flat response up to 2-5Ghz, but are cumbersome to move from point to
point. Commercial probes with browser pins are useful to poke around
quickly. It is always a good idea to make 'reference' measurements with
the given probe and setup, see past discussions in list archive.
Regards,
Istvan Novak
SUN Microsystems
Santos Fernandez, Jesus 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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