x1 'scope probes

G

George Herold

Guest
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.
 
George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.
Click to expand...
A FET probe is better still. I got a couple of Tek P6201s in good shape
off eBay for $60 each a few months ago.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
 
George Herold wrote:

A times one (x1) ?scope probe has a capacitance of about 90 to 100
pF. Say I?m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ?scope input C?

I think it has to do with flat response across the bandwidth. A piece
Click to expand...
of wire can serve as a "probe," (#22 or #24 will plug right into a BNC
center conductor jack, I don't recall which) but you might get artifacts
at some frequencies because of strays and stuff.

It's been many years, but I think if you stick a resistor at the
end of the raw coax, and some kind of capacitor in shunt with the
resistor, it can flatten out the response.

Try each of these on your calibrator output, (1X probe, naked wire,
naked coax, coax with a series resistor, and coax with a shunted
series resistor) and see what the corners look like - that should
give you your answer.

If your 'scope doesn't have a calibrator, then slap together an
astable and see what _its_ corners look like. :)

Have Fun!
Rich
 
On Wed, 13 Jul 2011 11:46:22 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.
Click to expand...
Scope probes use special "coax", namely tiny resistance wire in a big
tube. This is supposed to keep the capacitance down and reduce
ringing.

A short coax can be better than a 1x probe, especially as regards RF
pickup.

As Phil says, fet probes are great. Much cleaner and almost no
loading. They even work well without a ground clip. They sometimes
work well with the tip just near the signal.

John
 
George Herold wrote:
On Jul 13, 3:05 pm, Phil Hobbs
pcdhSpamMeSensel...@electrooptical.net> wrote:
George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.

A FET probe is better still. I got a couple of Tek P6201s in good shape
off eBay for $60 each a few months ago.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net

Yeah, but.... Is there any use to a x1 probe?

I had* this old 'big' TEK x10 probe that only had 8.8pF of C. Now all
the new little x10 probes that come with scopes are ~16pF.

George H.

*had, because it seems to have walked away.. sigh.
Click to expand...
As old-time circuit columnist Tom Kneitel used to say, 'Scope probes are
marked "10:1" because they ten to one away when you aren't looking.'

I can't remember the last time I used a 1x probe. I have a few that
have 1x/10x slide switches, but they're always on 10x. Maybe you could
havn one out as bait to catch the guy that walked away with your good
one. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net
 
On Jul 13, 3:05 pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF.  Say I’m looking at a signal with a source impedance of 1 k ohm..
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.

A FET probe is better still.  I got a couple of Tek P6201s in good shape
off eBay for $60 each a few months ago.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net
Click to expand...
Yeah, but.... Is there any use to a x1 probe?

I had* this old 'big' TEK x10 probe that only had 8.8pF of C. Now all
the new little x10 probes that come with scopes are ~16pF.

George H.

*had, because it seems to have walked away.. sigh.
 
On Jul 13, 3:44 pm, Rich Grise <ri...@example.net.invalid> wrote:
George Herold wrote:
A times one (x1) ?scope probe has a capacitance of about 90 to 100
pF.  Say I?m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ?scope input C?

I think it has to do with flat response across the bandwidth. A piece
of wire can serve as a "probe," (#22 or #24 will plug right into a BNC
center conductor jack, I don't recall which) but you might get artifacts
at some frequencies because of strays and stuff.

It's been many years, but I think if you stick a resistor at the
end of the raw coax, and some kind of capacitor in shunt with the
resistor, it can flatten out the response.

Try each of these on your calibrator output, (1X probe, naked wire,
naked coax, coax with a series resistor, and coax with a shunted
series resistor) and see what the corners look like - that should
give you your answer.

If your 'scope doesn't have a calibrator, then slap together an
astable and see what _its_ corners look like. :)

Have Fun!
Rich
Click to expand...
That's a good idea (looking with the 'scope calibrator), I'll give it
a whirl.

Hmm everything looks the same...(I used a function generator and not
the 'scope cal terminals.) OK enough screwing around. It seems to me
that a x1 probe is no better than a piece of coax.

George H.
 
On 7/13/2011 1:46 PM, George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.
Click to expand...
I have seen a tutorial on scope probes that described all the whys
and hows, I think it was Techtronix, but could have been HP. The paper
describes probes made with coax for low impedance and high freq probes.
Good article, saved it on the harddrive that died :) I'm sure it is
still available.

I've been searching, haven't found the one I remember, but see these.

http://www.eng.yale.edu/ee-labs/morse/probe/probe.htm

http://www.e-sonic.com/whatsnew/FeaturedProducts/Tektronix/ABCsOfProbes.pdf
Mikek
 
On Jul 13, 4:15 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 13 Jul 2011 11:46:22 -0700 (PDT), George Herold

gher...@teachspin.com> wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF.  Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.

Scope probes use special "coax", namely tiny resistance wire in a big
tube. This is supposed to keep the capacitance down and reduce
ringing.

A short coax can be better than a 1x probe, especially as regards RF
pickup.

As Phil says, fet probes are great. Much cleaner and almost no
loading. They even work well without a ground clip. They sometimes
work well with the tip just near the signal.

John

"work well with the tip just near the signal."
Click to expand...
Ahh, a bit of capacitve coupling. I've recently been wrapping a bit
of insulated wire around the pin of an opamp that I want to probe.
Then a square wave on the wire gives you a pulse of current into the
circuit. You can send in a triangle wave and get a square wave of
current. Very nice for seeing if there is any difference between the
light response and electronic response of a photodiode circuit.

Sure you can just tack a small cap on instead, but not as much fun as
a piece of wire....
It's half a gimmick!

George H.


>
 
Phil Hobbs wrote:
George Herold wrote:
On Jul 13, 3:05 pm, Phil Hobbs
pcdhSpamMeSensel...@electrooptical.net> wrote:
George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.

A FET probe is better still. I got a couple of Tek P6201s in good shape
off eBay for $60 each a few months ago.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) nethttp://electrooptical.net

Yeah, but.... Is there any use to a x1 probe?

I had* this old 'big' TEK x10 probe that only had 8.8pF of C. Now all
the new little x10 probes that come with scopes are ~16pF.

George H.

*had, because it seems to have walked away.. sigh.

As old-time circuit columnist Tom Kneitel used to say, 'Scope probes are
marked "10:1" because they ten to one away when you aren't looking.'

I can't remember the last time I used a 1x probe. I have a few that
have 1x/10x slide switches, but they're always on 10x. Maybe you could
havn one out as bait to catch the guy that walked away with your good
one. ;)
Click to expand...

I bought these 100 MHz probes on Ebay:

<http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130522062531&ssPageName=STRK:MEWNX:IT>

--
It's easy to think outside the box, when you have a cutting torch.
 
Phil Hobbs wrote:

George Herold wrote:
A times one (x1) ‘scope probe has a capacitance of about 90 to 100
pF. Say I’m looking at a signal with a source impedance of 1 k ohm.
Is there any advantage to using a x1 probe versus just a few feet of
BNC coax, (C ~ 60 pF) and then the ~20 pF of ‘scope input C?

A x 10 probe is clearly better.

Thanks,
George H.

A FET probe is better still. I got a couple of Tek P6201s in good shape
off eBay for $60 each a few months ago.

Cheers

Phil Hobbs
Click to expand...
Will those work on any scope?

--
✝
 
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