[SI-LIST] Re: Measuring Power supply Ripple and Noise
- From: "DAVID CUTHBERT" <telegrapher9@xxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>
- Date: Sat, 15 Nov 2008 09:23:22 -0700
One thing about all of these probes for TDR launch (<50 ps rise time), is
that due to the probe ground being a wire, the probe becomes 'hot' for
several hundred ps. If the probe were completely coaxial to the PCB this
would not happen. This 'steals' energy from the measurement
thereby corrupting it. Ferrite beads slipped over the probe and coaxial
cable cool things down. I have also tried a broadband ground plane that
worked well. It is designed along the lines of one half of a wide band
dipole. The downside is that physically it obscures the PCB right where the
probe is placed.
Additionally the coaxial cable loss in the first 100 ps or so corrupts the
measurement. This effect can be deimbedded on paper or in software. Short
the coax, TDR it, do some math, and you have the numbers to deimbed the
measurement.
Dave Cuthbert
On 11/15/08, steve weir <weirsi@xxxxxxxxxx> wrote:
>
> Simplicity, yes.
> Good for looking at real time noise, yes.
> Good to have around, yes.
> Good for looking at impedance with a VNA sometimes.
>
> At milliohm / 100's of MHz land it's very easy to get enough coupling
> from the gnd / probe tip loops of the two probes to see substantial
> measurement error. This is the same problem that the Agilent probes
> have as they have quite a large gnd / probe open area. The TEK P6150's
> have a fairly small area, but even with that, copper tape is often needed.
>
> Steve.
> Hal Murray wrote:
> >> Both TEK and Agilent offer 10:1 passive probes. I far prefer to use
> >> these probes when possible rather than soldering up coax. ...
> >>
> >
> > It took me a while to appreciate the simplicity of coax and solder.
> >
> > For $10, you can get a 6 ft chunk of coax with molded BNC
> connectors. Cut it
> > in half and you have two coax probes. Remove a bypass cap and you have a
> > good place to solder it to.
> >
> >
> >
> >
> >
>
>
> --
> Steve Weir
> Teraspeed Consulting Group LLC
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