Date   
Re: Probe bandwidth

Tom Gardner
 

On 21/07/17 02:15, Bob Albert bob91343@... [TekScopes] wrote:

If the probe is to have an upper frequency limit I thought perhaps it was the frequency at which its input reached 50 Ohms reactance, and at 100 MHz that is maybe a little less than 20 pF. If indeed its input reactance is 50 Ohms capacitive, then along with the presumed source impedance of 50 Ohms resistive (such as is common at higher frequencies) that equates to a drop in amplitude of 3 dB. For years I thought that was how the upper frequency limit of a probe was calculated.
In many engineering systems, not just electronic, the[1] bandwidth of a system is traditionally defined in terms of the "3dB down" frequency. That is the frequency at which the power[2] has halved, hence its other name the "half power" point.

In electronics, since power is proportional to voltage squared, 50% peak power => 70.7% peak voltage.

There is also a useful rule of thumb (based on certain assumptions that are often but not always valid) that the risetime tr = 0.35/bandwidth. Hence 100MHz 3dB down <=> 3.5ns risetime.

In a sane world, if you cascade two units (e.g. probe and scope) with bandwidths b1 and b2, then the cascaded bandwidth would be sqrt(b1*b1 + b2*b2). Unfortunately in some cases marketing departments decided that it was better if people ordered a 100MHz scope and a 100MHz probe you would get a 100MHz bandwidth. Sigh.

For most probes, any reference to 50ohms is inappropriate, since none of the impedances are 50ohms. The exception are "low impedance Z0" a.k.a. "resistive divider" probes - which are very useful for medium speed digital systems (i.e. risetimes >5ns, say). They are also the only type of probe it is easy to make at home.

FFI, see the references indicated by other people, or those at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

[1] there can be several different bandwidths, e.g. noise bandwidth, but they are irrelevant to this discussion.
[2] power delivered to a load or indicated on a meter

Re: Probe bandwidth

Fabio Trevisan
 

Hello Bob,
I see that I didn't really get the rational behind your question when I
first answered, and I apologize for that.
I'm not sure though, if I already got what you meant with your question.
I don't understand why you refer to 50 ohms as a parameter that would
somehow conjugate with the 20pF to form the probe's frequency response.
There 's simply no 50 ohms anywhere in this equation when talking about
high impedance passive probes, when they're meant to probe circuits that
don't have or are not designed to have any particular characteristic
impedance whatsoever.
There isn't also a consideration for the source impedance, or for the
loading effect that the probe will impose on the source signal, when their
bandwidths are stated or advertised.
A passive probe is said to be 100MHz assuming the -3dB point of output in
relation to what's actually entering it (effective voltage at the tip) and
not assuming a source impedance of any value and, therefore, not including
in the -3dB point, any loss that could have been had already at the input
due to the loading that the probe is causing to the source signal of
non-zero source impedance.
Having said that, indeed you have a point in there is a difference between
2 different 100MHz probes, one having lower capacitance at the tip than the
other, when connecting that probe to a real-world circuit, with impedances
greater than zero.
Although both may be truthfully exhibiting -3dB loss between their tip and
the scope's input (@ limit bandwidth), the one with the lower capacitance
will load less the signal than the one with larger capacitance at the tip
and so, the one with larger capacitance will measure a lower voltage... But
not because its bandwidth is wrong, but because this model loads more the
source than the other one and the loading effect is not taken into account
to publish the probe's OWN loss.
But I think this is all sensible because you cannot specify a probe's
bandwidth considering *anything* before its input, simply because you can't
tell what's there to start with.
If the signal you're measuring has 1kohm impedance at high frequencies,
even an expensive 10:1 passive probe with, say, 9pF at the tip, will
exhibit a poor 17MHz cutoff point only for its 9pF loading effect on the
source signal, not computing the additional loss from the probe itself.
So, when comparing probes to be used on real circuits, one must consider:
A) its advertised bandwidth
B) its capacitance at the tip..

Brgrds,

Fabio



On Jul 21, 2017 1:10 AM, "Bob Albert bob91343@... [TekScopes]" <
TekScopes@...> wrote:



Dennis,
I downloaded the document but the pages you reference don't say anything
about frequency response. They mention rise time but don't explain what
controls it.
I will read further, but the original question is unanswered so far.
It seems to me that a properly compensated voltage divider will have an
infinite frequency response, although its input impedance will get very low
at very high frequencies. Complicating this statement are the 'stray'
impedances, such as input inductance, ground lead effects, and so on.
Tektronix mentions a source impedance of 25 Ohms, which implies a 50 Ohm
source and a 50 Ohm load. Compared to my original computation based on 50
Ohms, this would double the frequency response.
So my confusion persists.
Bob


On Thursday, July 20, 2017 8:36 PM, "'Dennis Tillman' @Dennis_Tillman_W7PF
[TekScopes]" <TekScopes@...> wrote:


Hi Bob,
One more thing: This was an excellent question!

Unfortunately few people give much thought to the probe they are using and
whether it is properly matched to the input amplifier of their scope. Each
time you relocate a probe to another plugin you should check its response
and adjust it with a good square wave for the flattest response (no
overshoot or undershoot). Most Tek probes can be adjusted to match the
amplifier they are connected to.

The Probe Measurements book is a quick read and I highly recommend reading
it all. Another excellent examination of probes can be found in the Tek
primer "ABCs of Probes". This primer has the advantage of constantly being
updated and refreshed by Tek to keep it very current. You can get the
latest
version of it at Tek's web site. The address is
http://www.tek.com/sites/tek.com/files/media/document/
resources/02_ABCs%20of
%20Probes%20Primer.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 7:52 PM
Subject: RE: [TekScopes] Probe bandwidth

Hi Bob,
I understand your confusion. For a thorough explanation of the reasons that
the input capacitance does not determine the bandwidth, and conversely, the
risetime please take a look at pages 5 through 16 of the excellent book on
this topic published by Tektronix. The book is "Probe Measurements"
(062-1120-00). It is one of the wonderful Concept Series of books Tek
produced in the late 1960s and early 1970s.

You can download a scanned version of it at
http://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 10:52 AM
Subject: [TekScopes] Probe bandwidth

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob
------------------------------------
Posted by: bob91343@...
------------------------------------
------------------------------------
Posted by: "Dennis Tillman" <@Dennis_Tillman_W7PF>
------------------------------------

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[Non-text portions of this message have been removed]



[Non-text portions of this message have been removed]

Re: Probe bandwidth

Bob Albert
 

Dennis,
I downloaded  the document but the pages you reference don't say anything about frequency response.  They mention rise time but don't explain what controls it.
I will read further, but the original question is unanswered so far.
It seems to me that a properly compensated voltage divider will have an infinite frequency response, although its input impedance will get very low at very high frequencies.  Complicating this statement are the 'stray' impedances, such as input inductance, ground lead effects, and so on.
Tektronix mentions a source impedance of 25 Ohms, which implies a 50 Ohm source and a 50 Ohm load.  Compared to my original computation based on 50 Ohms, this would double the frequency response.
So my confusion persists.
Bob

On Thursday, July 20, 2017 8:36 PM, "'Dennis Tillman' @Dennis_Tillman_W7PF [TekScopes]" <TekScopes@...> wrote:


  Hi Bob,
One more thing: This was an excellent question!

Unfortunately few people give much thought to the probe they are using and
whether it is properly matched to the input amplifier of their scope. Each
time you relocate a probe to another plugin you should check its response
and adjust it with a good square wave for the flattest response (no
overshoot or undershoot). Most Tek probes can be adjusted to match the
amplifier they are connected to.

The Probe Measurements book is a quick read and I highly recommend reading
it all. Another excellent examination of probes can be found in the Tek
primer "ABCs of Probes". This primer has the advantage of constantly being
updated and refreshed by Tek to keep it very current. You can get the latest
version of it at Tek's web site. The address is
http://www.tek.com/sites/tek.com/files/media/document/resources/02_ABCs%20of
%20Probes%20Primer.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 7:52 PM
Subject: RE: [TekScopes] Probe bandwidth

Hi Bob,
I understand your confusion. For a thorough explanation of the reasons that
the input capacitance does not determine the bandwidth, and conversely, the
risetime please take a look at pages 5 through 16 of the excellent book on
this topic published by Tektronix. The book is "Probe Measurements"
(062-1120-00). It is one of the wonderful Concept Series of books Tek
produced in the late 1960s and early 1970s.

You can download a scanned version of it at
http://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 10:52 AM
Subject: [TekScopes] Probe bandwidth

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob
------------------------------------
Posted by: bob91343@...
------------------------------------
------------------------------------
Posted by: "Dennis Tillman" <@Dennis_Tillman_W7PF>
------------------------------------

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[Non-text portions of this message have been removed]

Re: Probe bandwidth

 

Hi Bob,
One more thing: This was an excellent question!

Unfortunately few people give much thought to the probe they are using and
whether it is properly matched to the input amplifier of their scope. Each
time you relocate a probe to another plugin you should check its response
and adjust it with a good square wave for the flattest response (no
overshoot or undershoot). Most Tek probes can be adjusted to match the
amplifier they are connected to.

The Probe Measurements book is a quick read and I highly recommend reading
it all. Another excellent examination of probes can be found in the Tek
primer "ABCs of Probes". This primer has the advantage of constantly being
updated and refreshed by Tek to keep it very current. You can get the latest
version of it at Tek's web site. The address is
http://www.tek.com/sites/tek.com/files/media/document/resources/02_ABCs%20of
%20Probes%20Primer.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 7:52 PM
Subject: RE: [TekScopes] Probe bandwidth

Hi Bob,
I understand your confusion. For a thorough explanation of the reasons that
the input capacitance does not determine the bandwidth, and conversely, the
risetime please take a look at pages 5 through 16 of the excellent book on
this topic published by Tektronix. The book is "Probe Measurements"
(062-1120-00). It is one of the wonderful Concept Series of books Tek
produced in the late 1960s and early 1970s.

You can download a scanned version of it at
http://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Dennis Tillman W7PF

-----Original Message-----
Sent: Thursday, July 20, 2017 10:52 AM
Subject: [TekScopes] Probe bandwidth

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob
------------------------------------
Posted by: bob91343@...
------------------------------------
------------------------------------
Posted by: "Dennis Tillman" <@Dennis_Tillman_W7PF>
------------------------------------

Re: Probe bandwidth

 

Hi Bob,
I understand your confusion. For a thorough explanation of the reasons that
the input capacitance does not determine the bandwidth, and conversely, the
risetime please take a look at pages 5 through 16 of the excellent book on
this topic published by Tektronix. The book is "Probe Measurements"
(062-1120-00). It is one of the wonderful Concept Series of books Tek
produced in the late 1960s and early 1970s.

You can download a scanned version of it at
http://w140.com/tekwiki/images/1/19/062-1120-00.pdf

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Thursday, July 20, 2017 10:52 AM
To: TekScopes@...
Subject: [TekScopes] Probe bandwidth

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob







------------------------------------
Posted by: bob91343@...
------------------------------------


------------------------------------

Yahoo Groups Links

Re: Probe bandwidth

Bob Albert
 

Fabio,
Once again the original question has been misunderstood.  I am not concerned with the probe's construction or its cable's impedance.  I am concerned with the external parameters.
If the probe is to have an upper frequency limit I thought perhaps it was the frequency at which its input reached 50 Ohms reactance, and at 100 MHz that is maybe a little less than 20 pF.  If indeed its input reactance is 50 Ohms capacitive, then along with the presumed source impedance of 50 Ohms resistive (such as is common at higher frequencies) that equates to a drop in amplitude of 3 dB.  For years I thought that was how the upper frequency limit of a probe was calculated.
However, it seems that some probes with that same input capacitance are rated for higher frequencies.  Those probes have similar input characteristic and yet a higher bandwidth rating.  These also cost more; instead of around $4 they are as high as $24 for one rated for 300 or 500 MHz.
I was wondering just what the increased price gives the user.  With the same circuit loading as the cheap ones, how can it have a wider bandwidth?  So maybe the probe has some magical compensation that extends its frequency response in spite of the fact that it loads the circuit to the same extent that the cheap probe does.  And it loads it more yet as the frequency increases, making me ask this question.
I viewed the Tektronix explanation of probes but that addresses somewhat different questions.  And I presume the tutorial from them (which I downloaded but haven't read yet) is more or less a printed version of the video.
Bob

On Thursday, July 20, 2017 5:44 PM, "Fabio Trevisan fabio.tr3visan@... [TekScopes]" <TekScopes@...> wrote:


  Hi Bob,

I don't master all the theory on the passive probes, but when you put on
the same sentence the 50Ohm impedance and the probe capacitance; 20pF
(arguably.... More on this further down), it seems that you think the high
impedance passive probes such as the P6100 (10megohm at the tip, 1megohm at
the scope's input) work at the probe cable's characteristic impedance.
This can't be farther from the truth.
High impedance passive probes can't work at their cables characteristic
impedance because it that would be the case, to avoid all sort of
reflections and impedance mismatches, they would have to be terminated at
the scope's end with that characteristic impedance... And definitely
1megohm is not that, because I haven't seen a coax cable that can be made
with such high characteristic impedance.
High impedance passive probes are made with a special type of coax cable
where its resonant characteristic have been *critically damped* so not to
exhibit the *transmission line* characteristic of a regular coaxial cable.
This critical damping is achieved by making the inner conductor very thin,
wrinkled and resistive, at values in the 100s of ohms range.
The first patent (I think) of the concept were from Tek's John R. Kobbe,
and William Polits. It's patent US2883619A and the patent's text is a piece
of work.
Having said that, the thin wire aims at lowering the cable's cacapacitance,
increasing the frequency response, and furthermore proper selection of the
dielectric's properties of the insulating material allows for using the
least possible resistance for the inner wire, increasing the frequency
response.
Talking about the capacitance.. This value of 20pF you mention, I suppose
you're talking of the scope's input capacitance (to which the probe must be
able to compensate), because the chinese P6100 capacitance at the tip (in
x10 mode) is 13~17pF (and not 20), and in x1 mode it's 70~120pF (and of
course, the stated 100MHz applies only for x10 mode).
The capacitance at the tip is largely minimized by the 10:1 attenuation, as
the cable's capacitance is largely isolated from the tip (by the 9meg
resistor) and the tip basically only *sees* the compensation capacitor,
which only need to be about 1:10 of the sum of the cable's capacitance and
the scope's input capacitance.
The frequency response, in the end, is much more related to how good they
can make the cable (construction and dielectric wise) so to keep the
damping resistance of the inner wire to the minimum.
As the other folk mentioned, the Chinese P6100 gets there at 100MHz.
I can't tell they are as free of aberrations and ringing than the good
brands, but they get the basics covered.
Brgrds,
Fabio

On Jul 20, 2017 2:51 PM, "bob91343@... [TekScopes]" <
TekScopes@...> wrote:

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob

[Non-text portions of this message have been removed]

d



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[Non-text portions of this message have been removed]

Re: Probe bandwidth

Fabio Trevisan
 

Hi Bob,

I don't master all the theory on the passive probes, but when you put on
the same sentence the 50Ohm impedance and the probe capacitance; 20pF
(arguably.... More on this further down), it seems that you think the high
impedance passive probes such as the P6100 (10megohm at the tip, 1megohm at
the scope's input) work at the probe cable's characteristic impedance.
This can't be farther from the truth.
High impedance passive probes can't work at their cables characteristic
impedance because it that would be the case, to avoid all sort of
reflections and impedance mismatches, they would have to be terminated at
the scope's end with that characteristic impedance... And definitely
1megohm is not that, because I haven't seen a coax cable that can be made
with such high characteristic impedance.
High impedance passive probes are made with a special type of coax cable
where its resonant characteristic have been *critically damped* so not to
exhibit the *transmission line* characteristic of a regular coaxial cable.
This critical damping is achieved by making the inner conductor very thin,
wrinkled and resistive, at values in the 100s of ohms range.
The first patent (I think) of the concept were from Tek's John R. Kobbe,
and William Polits. It's patent US2883619A and the patent's text is a piece
of work.
Having said that, the thin wire aims at lowering the cable's cacapacitance,
increasing the frequency response, and furthermore proper selection of the
dielectric's properties of the insulating material allows for using the
least possible resistance for the inner wire, increasing the frequency
response.
Talking about the capacitance.. This value of 20pF you mention, I suppose
you're talking of the scope's input capacitance (to which the probe must be
able to compensate), because the chinese P6100 capacitance at the tip (in
x10 mode) is 13~17pF (and not 20), and in x1 mode it's 70~120pF (and of
course, the stated 100MHz applies only for x10 mode).
The capacitance at the tip is largely minimized by the 10:1 attenuation, as
the cable's capacitance is largely isolated from the tip (by the 9meg
resistor) and the tip basically only *sees* the compensation capacitor,
which only need to be about 1:10 of the sum of the cable's capacitance and
the scope's input capacitance.
The frequency response, in the end, is much more related to how good they
can make the cable (construction and dielectric wise) so to keep the
damping resistance of the inner wire to the minimum.
As the other folk mentioned, the Chinese P6100 gets there at 100MHz.
I can't tell they are as free of aberrations and ringing than the good
brands, but they get the basics covered.
Brgrds,
Fabio

On Jul 20, 2017 2:51 PM, "bob91343@... [TekScopes]" <
TekScopes@...> wrote:



I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost
purely capacitive at high frequencies. as the frequency where the reactance
drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with
stated bandwidth somewhat higher. For instance I see some Chinese probes
advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is
still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's
keep it at a technical level and explain just what bandwidth means so I can
decide what to buy.

I see probes very cheap (not really rugged but still a good value) for
various prices from around $4 to $13 but they appear to be the same, except
for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this
is a somewhat important question. And of course, after a few years they
fall apart and I need to purchase again, but over my lifetime I don't
usually get to where I have spent the same money as for a Tek or HP probe.

Bob


[Non-text portions of this message have been removed]



d


[Non-text portions of this message have been removed]

Re: Probe bandwidth

bc
 

My take on the cheap probes:
I bought a pair of those cheap "Jinhao" P6100 x1/x10 "100MHz" chinese
probes, as my 20MHz scope would use them fine if they turn out to be
junk.  I also have some real Tektronix P6120's.  For the heck of it I
built an avalanche pulser on a solderless breadboard (yeah, yeah, cap
cap cap...) to try to measure rise time.  I measured on my 2465.
With some margin of error, I got about 2ns risetime with the P6100s and
1.9ns with the P6120.
Seems both are fast enough for their rating.  I suspect because of the
just under 2ns risetime, some sellers will fudge it to 200MHz despite
it's not quite there, or perhaps just so happen to have a bit less
capacitance and will do 200MHz.
I do have to warn about the P6100's : They will not take abuse.  Expect
the wires to break off of their connectors (the alligator clip and the
BNC specifically).  The hook clip head that came with them are really
"fat" though sturdy, I would have reservation using them on even 0.1"
DIPs for worries of shorts.  (I've had some hook clips that are small
enough that I would have no reservation hot clipping onto live
circuits, but the P6100 I'd not even try.)
I am overall satisfied with the P6100s, but I'm doing a disservice to
my 2440 and 2465, they deserve higher bandwidth probes :-(
Note: This is just one sample.  I don't know if all of these probes
have the same characteristics so YMMV.

On Thu, 2017-07-20 at 17:51 +0000, bob91343@... [TekScopes]
wrote:
 



  


    
      
      
      I am confused regarding the stated bandwidth of an oscilloscope
probe.



I thought that the bandwidth was computed from the input impedance,
almost purely capacitive at high frequencies. as the frequency where
the reactance drops to 50 Ohms.



This idea seems to work most of the time but I am seeing some probes
with stated bandwidth somewhat higher.  For instance I see some
Chinese probes advertised as 100 MHz, 200 MHz, even higher, yet their
input capacitance is still around 20 pF more or less.



Before you jump down my throat about how terrible Chinese stuff is,
let's keep it at a technical level and explain just what bandwidth
means so I can decide what to buy.



I see probes very cheap (not really rugged but still a good value)
for various prices from around $4 to $13 but they appear to be the
same, except for the bandwidth rating.  How can this be?



Since my use is very casual and I don't need to pay for high quality,
this is a somewhat important question.  And of course, after a few
years they fall apart and I need to purchase again, but over my
lifetime I don't usually get to where I have spent the same money as
for a Tek or HP probe.



Bob 





[Non-text portions of this message have been removed]





    
     

    
    






  
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Re: "Lamps" wiring on 7000-series mainframes.

Colin Herbert
 

Hi,

I wonder if anyone can see a reason why this circuit won't work to give a
minimum of 0V and a maximum of +5V for the plug-in lights.

The idea is that with R2 at maximum, R3 is adjusted to give a maximum +5V
out. Then R2 can be a screwdriver access control to reduce the voltage to
the lights. The 500R pre-set is used because +5V requires 360R and R2 is
chosen as 10k simply because that is what I had available. Any comments?

Colin.





From: TekScopes@... [mailto:TekScopes@...]
Sent: 14 October 2015 16:57
Cc: TekScopes
Subject: Re: [TekScopes] "Lamps" wiring on 7000-series mainframes.





Unless you like changing lamps, I suggest using a lower voltage than 5V...
at least put a diode or 2 in series.

New file uploaded to TekScopes

TekScopes@...
 

Hello,

This email message is a notification to let you know that
a file has been uploaded to the Files area of the TekScopes
group.

File : /+5V regulated001.pdf
Uploaded by : pervy_tango <colingherbert@...>
Description : Circuit for adjustable o to +5V regulated supply for 7000-series lights

You can access this file at the URL:
https://groups.yahoo.com/neo/groups/TekScopes/files/%2B5V%20regulated001.pdf

To learn more about file sharing for your group, please visit:
https://help.yahoo.com/kb/index?page=content&y=PROD_GRPS&locale=en_US&id=SLN15398

Regards,

pervy_tango <colingherbert@...>

Probe bandwidth

Bob Albert
 

I am confused regarding the stated bandwidth of an oscilloscope probe.

I thought that the bandwidth was computed from the input impedance, almost purely capacitive at high frequencies. as the frequency where the reactance drops to 50 Ohms.

This idea seems to work most of the time but I am seeing some probes with stated bandwidth somewhat higher. For instance I see some Chinese probes advertised as 100 MHz, 200 MHz, even higher, yet their input capacitance is still around 20 pF more or less.

Before you jump down my throat about how terrible Chinese stuff is, let's keep it at a technical level and explain just what bandwidth means so I can decide what to buy.

I see probes very cheap (not really rugged but still a good value) for various prices from around $4 to $13 but they appear to be the same, except for the bandwidth rating. How can this be?

Since my use is very casual and I don't need to pay for high quality, this is a somewhat important question. And of course, after a few years they fall apart and I need to purchase again, but over my lifetime I don't usually get to where I have spent the same money as for a Tek or HP probe.

Bob

Re: Just lost horizontal deflection on my 2465

rovmkr
 

Does anyone have any experience with the Thomas Lafay adapter?
Has anyone attempted to get this adapter working?
Peter.

Re: Curve Tracer Adapers

Tom Jobe <tomjobe@...>
 

Another thing to add to the Kelvin discussion, is that the Tektronix
013-0098-0x test fixture(s) Brent was asking about do not even have
Kelvin connections that go to the curve tracer.
They have either 5 or 6 connections on the back and connect to the both
the left and right CBE socket rows. The 5 connection model is missing
the E pin for the right side connections.
After thinking about which test fixture I find most useful, it would
probably be the diode fixture.
For example, you want to go through some zeners and find the one with
the 'knee' that is closest to some value or has the 'squarest' knee. It
makes that kind of job quick and easy.
The older style diode fixture 013-0111-00 has Kelvin connections. The
newer style 013-072 does not have Kelvin connections but it's my
favorite of the two.
There are usually some of the "Stud Diode" fixtures 013-0110-00 for sale
on eBay and elsewhere, and I find that they are fairly useless except to
steal parts out of.
tom jobe...




On 7/20/2017 5:44 AM, Chuck Harris cfharris@... [TekScopes] wrote:

It is not exactly difficult to make alligator clips with
two leads and banana plugs for those cases... however, in
the current ranges the plain old 576 is capable of generating,
you will notice very little difference between a 2 inch
lead made of #22 gage stranded wire, and the Kelvin clip...
I rather doubt you will notice any.

I have a fair number of tektronix adapters, but don't use
them much, using the alligator clip method most of the time.

If you use small, insulated alligator clips, you won't have
to bend up the DUT's leads to make it fit the tektronix adapter.

And, many of the tektronix adapters are only usable with full
length transistor or diode leads.

I made two sets of leads, one that uses micrograbbers, and
another that uses tiny 3/4 inch long alligators.

-Chuck Harris

'Craig Sawyers' c.sawyers@... [TekScopes] wrote:
I'm not sure why you would want to short collector to base for any
bipolar transistor measurement.
But shorting drain to source is needed for measuring gate-source
breakdown voltage V(BR)GSS and
reverse gate leakage IGSS. The bottom one re-wires the emitter or
gate connection, to swap from ebc
to ecb transistor (or equivalent fet) leadouts.

The benefit of using the correct fixtures for the 576 is that they
make true kelvin connections to
the DUT, and so improve measurement accuracy if significant currents
flow.

Craig






------------------------------------
Posted by: "Craig Sawyers" <c.sawyers@...>
------------------------------------


------------------------------------

Yahoo Groups Links





[Non-text portions of this message have been removed]

Re: Curve Tracer Adapers

Craig Sawyers <c.sawyers@...>
 

It is not exactly difficult to make alligator clips with two leads and banana plugs for those
cases...
however, in the current ranges the plain old 576 is capable of generating, you will notice very
little
difference between a 2 inch lead made of #22 gage stranded wire, and the Kelvin clip...
I rather doubt you will notice any.
There are many Kelvin clips from Chinese suppliers at exceptionally low prices. I've bought two entire
lead sets from this source (one with BNC's and one with banana plugs) equipped with Kelvin clips for a
fraction of the price you pay for a western supplied kelvin clip.

Craig

Re: Curve Tracer Adapers

Chuck Harris
 

It is not exactly difficult to make alligator clips with
two leads and banana plugs for those cases... however, in
the current ranges the plain old 576 is capable of generating,
you will notice very little difference between a 2 inch
lead made of #22 gage stranded wire, and the Kelvin clip...
I rather doubt you will notice any.

I have a fair number of tektronix adapters, but don't use
them much, using the alligator clip method most of the time.

If you use small, insulated alligator clips, you won't have
to bend up the DUT's leads to make it fit the tektronix adapter.

And, many of the tektronix adapters are only usable with full
length transistor or diode leads.

I made two sets of leads, one that uses micrograbbers, and
another that uses tiny 3/4 inch long alligators.

-Chuck Harris

'Craig Sawyers' c.sawyers@... [TekScopes] wrote:

I'm not sure why you would want to short collector to base for any bipolar transistor measurement.
But shorting drain to source is needed for measuring gate-source breakdown voltage V(BR)GSS and
reverse gate leakage IGSS. The bottom one re-wires the emitter or gate connection, to swap from ebc
to ecb transistor (or equivalent fet) leadouts.

The benefit of using the correct fixtures for the 576 is that they make true kelvin connections to
the DUT, and so improve measurement accuracy if significant currents flow.

Craig






------------------------------------
Posted by: "Craig Sawyers" <c.sawyers@...>
------------------------------------


------------------------------------

Yahoo Groups Links



Re: Curve Tracer Adapers

Brent Watson <brentleew2003@...>
 

Good info, thanks Craig

On Wednesday, July 19, 2017, 11:38:59 PM PDT, 'Craig Sawyers' c.sawyers@... [TekScopes] <TekScopes@...> wrote:

 
I'm not sure why you would want to short collector to base for any bipolar transistor measurement.
But shorting drain to source is needed for measuring gate-source breakdown voltage V(BR)GSS and
reverse gate leakage IGSS. The bottom one re-wires the emitter or gate connection, to swap from ebc
to ecb transistor (or equivalent fet) leadouts.

The benefit of using the correct fixtures for the 576 is that they make true kelvin connections to
the DUT, and so improve measurement accuracy if significant currents flow.

Craig




[Non-text portions of this message have been removed]

OT: Anyone in UK (or Europe) likely to be placing an order with Lemo in the near future

 

If so I'd very much like to piggy back on your order to avoid the GBP100
plus VAT MOV.

Thanks
David

OT: Manuals for Datron (Wavetek) 4808 Option 70 Wideband Source

 

Hi everyone,

I have the Schematics and Parts list for this on loan at present, but I have
had no luck finding any other documentation for it.

The edition of the user handbook for the 4808 that I have is too early to
know about this option.

I'm looking for a more up to date version of the 4808 User Manual, and the
User and Service documentation for Option 70

Hoping that someone can help with either real "dead tree" manuals or with
scans.

Thanks
Dave

Re: Curve Tracer Adapers

Craig Sawyers <c.sawyers@...>
 

I'm not sure why you would want to short collector to base for any bipolar transistor measurement.
But shorting drain to source is needed for measuring gate-source breakdown voltage V(BR)GSS and
reverse gate leakage IGSS. The bottom one re-wires the emitter or gate connection, to swap from ebc
to ecb transistor (or equivalent fet) leadouts.

The benefit of using the correct fixtures for the 576 is that they make true kelvin connections to
the DUT, and so improve measurement accuracy if significant currents flow.

Craig

Tektronix 454

erik erickson
 

Hey everyone my tektronix 454 channel two doesn't work. It worked for about 20 seconds when I reseated all the transistors and then slowly went off the screen. Sometimes it works but will always stop working within 20 seconds. The position knob would not bring it back. I'm new to this so all help will be greatly appreciated. I have the manual and I've been reading it.
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