oscilloscope probes VS BNC cable

"john"

** Yes - it is done all the time where a piece of equipment provides the signal for the scope. ** Might be - bit it hardly ever matters.

** Yes - they exist to allow you to " probe " inside equipment & and pick up signals from wherever you like.

Most scope probes are made with thin, low capacitance per foot cable ( using a very thin inner conductor) and are fitted with an adjustable head incorporating a 1:1 or 10:1 switch. The 10:1 position giving a 10 megohm load in parallel with a few pF of capacitance which is more suitable for probing sensitive circuit locations.

Also scope probes are *designed* to work with standard 1 megohm in parallel with 20 pF input circuits of typical scopes.

** If it has a BNC output - then use a BNC to BNC lead.

...... Phil

First up, "BNC " is a family of connectors, not a cable type.

But assuming you mean coaxial cable such as 50ohm transmission line, the use of a proper CRO probe system provides a lower capacitance cable. Part of the objective is to not load the device under test.

The shielding on the proper probe cable is indeed typically not as complete as even something as mundane as RG-58C/U, but that really IS secondary.

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

It's not necessarily "better shielded". Most scope probes have a very tiny center conductor in their cable, yielding a high-impedance, low-capacitance cable. As D said, the cable comprises part of a capacitive divider.

I have a salvaged piece of RG-58/U with a BNC on one end and bare wire at the other. I've soldered a 1K resistor in series with the center conductor, and it's "good enough", at DC and audio frequencies.

But I was thinking of making a capacitive divider with it - I've got a 3' piece of coax, which is listed at, I think, 27 pF per foot. So, with 3 ft, that'd be 81 pF. Which, if I remember my old formulas, I would need to stick an 8.1 pF cap in series with my center conductor to yield a 10:1 probe.

I haven't checked my numbers, or tried anything like this, primarily because I don't have an 8.1 pF cap lying around. And, of course, a trimmer would be better, to trim out the tolerances and square up those edges! :-)

But, the coax should be good enough for the audio range, but I recommend an isolation resistor between it and the DUT.

Hope This Helps! Rich

"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

You didn't really mean that, did you?

Cheers

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

Noooo... I don't mean it's going to make 45% duty look like 50% duty. :P

1x probe cable capacitance and scope input capacitance can have an RC effect on some circuits with fast edges. The scope can present a slower than actual edge times.

With a 10x probe the circuit sees less applied capacitance. The circuit sees a capacitor divider composed of Ccomp in series with Ccable + Cscope. Therefore a closer to actual edge time can be viewed.

Hope I got that right :|... D from BC

It can be if the device you are "probing" provides a low impedance signal output designed for driving a coax cable. For example, if you are measuring the output from a signal generator it's common to use a standard BNC cable rather than a probe because it's a more convenient and provides a better quality connection. Also, a standard BNC cable can give you more bandwidth for your $$, as quality high speed probes are expensive.

Yes, probes exist to allow you to more conveniently "probe" point of interest in a circuit with minimum loading (they have lower capacitance). Also, a 10:1 probe allows you to measure much higher bandwidth (faster) signals than just a standard coax cable can.

In that case you would use a BNC cable, with a BNC T piece if needed.

Dave.

...

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.:

Exactly. It is all about impedance and impedance matching.

one must be sure to make the proper settings choices on the scope as well.

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

There are some situations where a piece of cable can give much better results than a probe, where the circuit impedance is low (like your amplifier output perhaps).

A common situation for me is measuring mV ripple on the output of a switch mode power suppy. A normal x10 probe would attenuate the signal unnecessarily, and the ground lead would pick up a *lot* of magnetic noise from the switching inductor. Simply connecting a "BNC" (RG58) cable gives much cleaner results.

Also see e.g. Jim Williams' classic AN47.

--

John Devereux

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