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
> *
> ************************************************************************

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