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'[EE] How scope probes work?'
2005\10\14@120340 by Harold Hallikainen

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I've got a project where we need to watch a pulse on another piece of
equipment. We do not want to load the circuit in the other piece of
equipment. Running a piece of coax to my equipment results in considerable
ringing, since the coax is not terminated. So, the question comes up, how
does a scope probe get away with a 1M load in the scope? I doubt that's
terminating the coax with its characteristic impedance. The idea of a 10X
probe with compensation is pretty clear... the trick is how do they deal
with the lack of termination?

Ideas?

THANKS!

Harold



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2005\10\14@135053 by David Van Horn

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> I've got a project where we need to watch a pulse on another piece of
> equipment. We do not want to load the circuit in the other piece of
> equipment. Running a piece of coax to my equipment results in
considerable
> ringing, since the coax is not terminated. So, the question comes up,
how
> does a scope probe get away with a 1M load in the scope? I doubt
that's
> terminating the coax with its characteristic impedance. The idea of a
10X
> probe with compensation is pretty clear... the trick is how do they
deal
> with the lack of termination?

Lossy coax


2005\10\14@200032 by Dwayne Reid

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At 10:03 AM 10/14/2005, Harold Hallikainen wrote:
>I've got a project where we need to watch a pulse on another piece of
>equipment. We do not want to load the circuit in the other piece of
>equipment. Running a piece of coax to my equipment results in considerable
>ringing, since the coax is not terminated. So, the question comes up, how
>does a scope probe get away with a 1M load in the scope? I doubt that's
>terminating the coax with its characteristic impedance. The idea of a 10X
>probe with compensation is pretty clear... the trick is how do they deal
>with the lack of termination?

Center conductor is resistive.  You can (sort of) fake it by adding a
series resistor at the source end of the coax.  Start with a value
that is close to the characteristic impedance of the coax.

Or - just cut the probe end off a cheap scope probe.  I always have
guys damaging scope probes here and I keep (packrat that I am) those
where the BNC and cable is still in good shape.

dwayne

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2005\10\14@232800 by Harold Hallikainen

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{Quote hidden}

Thanks for the responses! I guess I'll start saving mangled scope probes...

Harold


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2005\10\15@144224 by Peter

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> I've got a project where we need to watch a pulse on another piece of
> equipment. We do not want to load the circuit in the other piece of
> equipment. Running a piece of coax to my equipment results in considerable
> ringing, since the coax is not terminated. So, the question comes up, how
> does a scope probe get away with a 1M load in the scope? I doubt that's
> terminating the coax with its characteristic impedance. The idea of a 10X
> probe with compensation is pretty clear... the trick is how do they deal
> with the lack of termination?

The compensation deals with the ringing. Just copy the standard 10x
probe circuit and it will work. *But* most high speed scopes have 50R as
input impedance (switched or option) because the compensation only works
for lambda << 4*cable length.

Peter

2005\10\15@150136 by olin piclist

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Peter wrote:
> *But* most high speed scopes have 50R
> as input impedance (switched or option) because the compensation only
> works for lambda << 4*cable length.

Actually lambda >> ...

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2005\10\16@124324 by Jochen Feldhaar

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Hi all, now I have to pitch in my 0.02 Euro worth....

The 1:1 probe is just a piece of coax, terminated with the scope input
impedance of 1M parallel to about 25 pF. So add the 200 pF for 1.5
Meters of coax and for low frequencies you will not have a problem at all.

Make the calculation yourself when the capacitive part will be lower
tham 1 MHz, at what frequency this will occur: youz will be surprised!!

Above this the 10:1 comes in use, and I personally use them up to about
30 MHz, in some applications even a bit more.
Here, take the 25 pF of the scope input, the 200 pF of the cable, and
add the :9 network in front - and you get 30 pF parallel 10M (about).
The same applies here for the crossover from resistive to mainly
capacitive impedance.

As I have scopes up to 1 GHz at home, I use a 50 Ohm cable and 50 Ohm
termination (can be put in front of any scope input...) and for logic I
simply use a 2K2 resistor for the adapting, for RF work I built a probe
with a GAAS FET in the input and a emitter follower stage converting to
50 Ohms. Works great also for the spectrum analyzer, up to 3 GHz approx.
This beauty loads only 2 pF capacitive, but only works for AC.

Hope this helps,

Jochen Feldhaar

Peter wrote:

{Quote hidden}

2005\10\16@124740 by Peter

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On Sat, 15 Oct 2005, Olin Lathrop wrote:

> Peter wrote:
>> *But* most high speed scopes have 50R
>> as input impedance (switched or option) because the compensation only
>> works for lambda << 4*cable length.
>
> Actually lambda >> ...

yes

Peter

2005\10\18@164043 by Jochen Feldhaar

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Jochen Feldhaar wrote:

{Quote hidden}

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