AW: [SI-LIST] : What speed scope should I consider?

Andreas Lenkisch (a.lenkisch@trenew.de)
Fri, 21 May 1999 15:44:38 +0200

500 Ohms 10:1 voltage dividers used on a 50 Ohm input work very well on
higher frequencies (e.g. TEK P6056). Due to their much lower capacitance =
to
GND you can allow a slightly longer ground lead if really needed, but to
make them as short as possible is ever to recommend to avoid signal
distortion.
The shop built probes as described in the "Black Magic" book of Howard
Johnson, mainly I use for "embedded" testpoints. They are extremely as ch=
eep
as good.

regards
Andreas

-----Urspr=FCngliche Nachricht-----
Von: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]Im Auftrag von Ingraham, Andrew
Gesendet am: Freitag, 21. Mai 1999 14:06
An: 'si-list@silab.eng.sun.com'
Betreff: RE: [SI-LIST] : What speed scope should I consider?

>Also note that the loading of the probe on the cirucit isn't just the DC
>impedance of the probe. A 10 MOhm, 10 pF probe represents an input
>impedance of
>26.5 Ohms at 600 MHz. (unless I've done the math wrong)

Your math is right, but it may be even worse than this.

I don't have a reference handy, but I have read or been told that many
10Mohm, x10 probes also have a significant resistive loss or shunt factor
(perhaps due to dielectric losses?) at high frequencies. Thus the input
impedance is something much less than 10Mohm in parallel with 10pF.

I haven't used a x10 probe in a very long time.

Get good probes, then pay utmost attention to grounding and probe leads. =
1"
is too much.

Regards,
Andy

**** To unsubscribe from si-list: send e-mail to
majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
si-list, for more help, put HELP. si-list archives are accessible at
http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****