[SI-LIST] FW: Re: Passive Probe
- From: "Orin Laney" <olaney@xxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Sat, 26 Feb 2011 08:46:19 -0800
-----Original Message-----
From: Orin Laney [mailto:olaney@xxxxxxxxx]
Sent: Saturday, February 26, 2011 8:45 AM
To: 'Doug Smith'
Subject: RE: [SI-LIST] Re: Passive Probe
Greetings, Doug.
Tektronix would take issue with that. Properly designed (in the full sense)
probes meet their published specifications. A key attribute of such probes
is a resistive center conductor in the coax that suppresses resonance
effects. This requires careful modeling of the distributed RC circuit and
sometimes a compensating network. I've measured end to end center conductor
resistances in the range of 100 to 200 ohms.
Now, the observation that the loading effects of typical passive 10X probes
is an issue at ~50 MHz and above, and can limit the circuit probing utility,
is certainly something to take into account, but that is not a probe issue
per se and more one of user acumen. Hi-Z circuits are always problematic;
low-Z ones can be tolerant.
Interpreting 'a few ohms' as two and taking the input capacitance at the
scope BNC as 15 pF yields a ballpark frequency at the probe tip of 1.2 GHz.
We agree that this is way outside the range of a practical, passive 10X
probe with a 10 megohm input at DC, and well into the territory where a FET
probe or a passive one that is 500 ohms at the tip when terminated in 50
ohms is the proper 10X approach. On the other hand, a user with a test
point that itself has a Z of a few ohms can intelligently use a passive 10X
probe well into VHF. Alas, few users understand the limitations of their
probes well enough to pull this off. Even fewer think to deliberately
design in suitable test points.
We should meet sometime and get to know each other better.
Regards,
Orin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Doug Smith
Sent: Friday, February 25, 2011 11:02 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Passive Probe
Hi Orin,
The accurate bandwidth of such probes is for the most part about 50 MHz
no matter what the manufacturer says. Resonance and loading effects
above 50 MHz can easily affect circuit operation (input impedance just a
few Ohms!) and errors of 100% or more in amplitude are often seen. The
published data is taken in a way that is not applicable to how they must
be used to be practical.
They are great for audio circuits.
Doug
On 2/25/11 3:38 PM, Orin Laney wrote:
> It's actually a 9Mohm resistor that forms a 10:1 divider with the standard
> scope 1Mohm input resistance. There is also a small cap in parallel so
that
> the time constant of the probe matches that of the scope input and
frequency
> response is flat. These probes usually have a small trimcap that the user
> can adjust for flattest response. There's more that makes a good scope
> probe, but like Steve said you should troll the Tektronix site for the
> applicable app notes.
>
> Orin Laney
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of See Hour
> Sent: Friday, February 25, 2011 2:30 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Passive Probe
>
> Hello Gurus
> 10:1 Capacitive-input probes has a 10Mohm series resistance at probe tip.
> What is the purpose of the resistor?
> The answers that I have in my mind are:1) to be able to measure high DC
> voltage2) so that the input capacitive loading is small3) to minimize the
> loading that the cable capacitance would impose on the DUT
> Am I right about?
> RegardsYing
>
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