[SI-LIST] Re: FW: Re: Passive Probe
- From: "Asbenson, Lyndell L" <lyndell.l.asbenson@xxxxxxxxx>
- To: Orin Laney <olaney@xxxxxxxxx>, 'steve weir' <weirsi@xxxxxxxxxx>
- Date: Sat, 26 Feb 2011 11:41:40 -0700
To think a simple probe! Excellent observations, If you look at different
vendor's recommendations for validating problems NIST tractable, some recommend
TDR other require VNA -Lyndell Lee Asbenson
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Orin Laney
Sent: Saturday, February 26, 2011 9:47 AM
To: 'steve weir'
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: FW: Re: Passive Probe
I totally agree. The RF communications paradigm of designing for flat
frequency response using a swept sine source is inadequate for probe design.
Scopes are time domain instruments, and the appropriate way to test a scope
probe is in the time domain, for instance using a TDR step for an input
waveform. Some of the lousiest RF connectors I've seen had a flat frequency
response but passed mutilated step edges, clearly the product of someone
only familiar with a VNA.
Fortunately, a lot of scope work is qualitative. I say fortunate because
clearly, few hi-Z passive probes are adequate for much more than that past
medium tens of MHz. The usual point of a 10X probe is that regardless of
faults it remains superior to attempting to use a 1X probe with a hi-Z
input(!). Which is why 500 ohm probes used with 50 ohm inputs are used for
wideband (mine is rated at 6 GHz), time domain sensitive work. This is a
great discussion for our wider audience. If you want to really give them an
earful, dive into proper grounding at the probe tip...!
Orin
-----Original Message-----
From: steve weir [mailto:weirsi@xxxxxxxxxx]
Sent: Saturday, February 26, 2011 9:05 AM
To: Orin Laney
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] FW: Re: Passive Probe
Orin, it all depends on how one evaluates the probe response. For
evaluating digital circuit rise-times, a 200MHz 10X probe from any:
Agilent, Tek, or ProbeMaster all give reasonable results that are fairly
represented by their specs. However, transmission line effects defy
passive compensation networks. One manufacturer tried to address this
with rather complex networks in their compensation boxes. The frequency
domain magnitude only results are OK, but the time domain results leave
a lot to be desired. If waveform fidelity beyond basic rise-time really
matters, a high impedance passive probe typically starts to noticeably
distort around 10MHz to 20MHz even when loading a low impedance load.
I've collected a lot of data and keep threatening to write-up a white
paper when I find some time.
Best Regards,
Steve.
Orin Laney wrote:
> -----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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--
Steve Weir
IPBLOX, LLC
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