> 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
>
> --
> Dwayne Reid   <.....dwaynerKILLspam@spam@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
>

>> 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

> 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:
>
>>> 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
>
>
>
>