I've had really good results with Tektronix P6245 / TDS500/600 active powered JFET probe.
<1pf, 1MOhm, 1.5GHz, 40v p-p max. Got 4 probes which are a perfect match with my TDS3054 scope. ____________ Brian Guralnick [EMAIL PROTECTED] Voice (514) 624-4003 Fax (514) 624-3631 ----- Original Message ----- From: "Ian Wilson" <[EMAIL PROTECTED]> To: "Protel EDA Forum" <[EMAIL PROTECTED]> Sent: Monday, September 09, 2002 9:16 PM Subject: Re: [PEDA] off topic supply SOS > On 02:24 PM 9/09/2002 -0400, Richard Sumner said: > >Brian, > > > >Measuring spikes on the supply lines is not trivial. Where you put the > >scope probe ground is everything. The best way to measure is with a > >differential probe, placed across the power pins of the chip in question. > >Anything less may not give you the true picture. > > Off topic but maybe of some interest...and I am sure many of you have > similar techniques. > > A trick I learnt way back in 1986 when designing my first switchmode supply > - make your own *low* impedance probe. A low impedance probe is less > susceptible to high speed transients nearby that cause false readings. A > series cap (with suitable resonance characteristics - possibly made with > multiple caps) and a shunt low impedance resistor. The CRO probe connects > across the resistor using the spring gnd clip that should be available with > any high speed cro probe. > > If a short spring gnd is not available make one using a paper clip bent and > cut to make as short a gnd spring as possible. Be careful if using the CRO > probe as a former when you bend the paper clip - it can break them. Choose > a slightly larger drill bit and then crush the small spring you have made > to make a tight fit onto the CRO GND sleeve. You now have a spring to slip > over the CRO GND sleeve (remove the scope condom to expose the GND sleeve). > Bend and cut the other end of the paper clip to make a nice stiff probe > point. If you cut at an angle it will be a sharp point. > > To choose the resistor you may want to try a range of values - there will > be a "flat" spot where the low impedance probe is not so low as to load the > cct but not so high as to give a false reading due to pickup. For power > supply work I sometimes go as low as 47 ohm (impedances are low so the low > R is not adding any loading). For decoupling tracing this may be a little > low but 1k or 4k7 may be OK. > > This is a useful trick, especially when you don't really trust the diff > mode of your CRO or when your high speed active probes have died. You have > to think about the results as the low probe impedance can affect the > results of course. > > Sorry for those that the above is old news but it may be useful for some. > > Ian > > > ************************************************************************ > * Tracking #: 53D7B142AFF8B646AB9B2FCDF73D75C2DE43004C > * > ************************************************************************ * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * To post a message: mailto:[EMAIL PROTECTED] * * To leave this list visit: * http://www.techservinc.com/protelusers/leave.html * * Contact the list manager: * mailto:[EMAIL PROTECTED] * * Forum Guidelines Rules: * http://www.techservinc.com/protelusers/forumrules.html * * Browse or Search previous postings: * http://www.mail-archive.com/[email protected] * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
