oscilloscope probes VS BNC cable

Scope probes have a capacitance adjustment. This is to compensate for cable capacitance and scope input capacitance. It can make square waves look more square.

But I guess the same circuit can be added to some qualified coax + a BNC connector. (Example: Maybe cable tv coax could be ok?)

D from BC

"D from BC"

** ONLY divider probes have that.

** ONLY against the effect it has on the divided down signal.

** ONLY divided down ones.

** NO.

Cos it ain't a divider.

....... Phil

Oops... That's right.. No comp on 1x probes (direct to scope).

I think I just assumed everybody uses 1x10x divider probes..

Divider probes and other reactances (without googling) is:

------probe---+---\/\R1\/\---+ | | |------| |---- +------+---------+ Ccomp | | | \ = Ccable = CinScope / | | R2 | | \ | | / | | Gnd Gnd Gnd

What I meant was R1,R2 and Ccomp could be put on a qualified coax to make a divider probe.

D from BC

...

What if my scope has 1M, 25 pF at its input, such that R2, 1M, is to the right of Ccable; Ccable is approx, 27 pF per foot, and it's just under

3' long - would R1 be 10M and Ccomp about 9.1 to 10.0 pF?

Is there a formula for gimmicks somewhere? :-) Do I have to buy a 10M resistor to make a proper probe?

Thanks, Rich

The resistor divider has to drop the voltage to 1/10th, so the resistors have to have a ratio of 9 to 1, since the voltage is proportional to R2/(R1+R2). So a 9.1 Meg resistor is pretty good. Likewise, the cap that parallels the 9.1 meg resistor has to be about 1/9th of the total input capacitance (cable capacitance plus scope input capacitance. Now you see why they go to so much trouble to build special low capacitance cable for scope probes. I wonder if you can pull out the center conductor and pull in a very slinky high E guitar string (tinned with silver solder at each end for electrical connections).

E 0.20mm

RG59 cable core wire AWG 22 = 0.64516mm

Diameter reduction figure: 0.65/0.2 = 3.25

100Mhz Scope probe core wire diameter ???

Not here

Not here

Nope... but it's a page on making a DIY scope probe...

Fk it... I'll guess 33 AWG which ~ the E string

Anyways... Getting an E string preferrably centered in 3ft of RG59 (or other) might be tough. Pulling out the coax core wire might be difficult. Some oven heating might help. :P

RG6 replaces RG59.

It has a fatter 18 AWG wire. The guitar string idea gets better stuck in RG6.

RG6 5.00 dB attenuation per 100' at 750 MHz

But I think that's with video source and load impedances.

D from BC

I would look for low capacitance (high impedance) teflon insulated coax. Besides, it is slippery when you try to push the E string through it. E.g.:

I'm getting a bad vibe about that RG-180...I think it's going to be pricey, min quality'd and difficult to find a source.. RG6 can probably be found a Radio Shack, London Drugs, Home Depot or a surplus electronics store. Or perhaps stop the local cable guy and give'm 5 bucks...

D from BC

Does D from BC mean you are in British Columbia? I have some RG-180 I got on eBay. It is sort of stiff, but not much worse than most coax that is not made to be an o-scope probe. If governments don't get in the way too badly, perhaps I could send you 10 feet, or so, in an envelope, to play with.

Yup..I'm in British Columbia. Land of really bad traffic, great skiing, marijuana and Chinese food. :P Surrey BC might still be car theft capital of North America. Weather is like 50% continuous sun and 50% continuous rain. And a V6 is must due to some roller coaster type roads. Also, the women are in great shape because they don't make enough money to eat alot :P

About the wire.. Thanks for the offer..but I have cheapo 100Mhz probes I got off ebay. I have an interest in DIY probes but stay away from making one :)

D from BC

Well, I _could_, but it's the only BNC I have and it came already attached to the coax. I guess I'll just save up my pennies and buy a proper probe when I can afford it. :-)

Thanks! Rich