Test Equipment Anonymous (TEA) group therapy thread

[mnementh]

mnem
"It was just a dream, Bender... there's no such thing as two."   

[BU508A]

2 + 2 = 5
for large values of 2

[Vince]

Thanks all, I stand corrected, my bad ! This round earth thing was indeed a scam then ?!    But how do they manage to put satellites in "orbit" then, I am all confused now ! 
These mad scientists really have powerful tricks up their sleeves, they could get you to believe anything, naughty mean scientists ! 

And how do scopes work then ? If electrons don't exist, what's drawing these fancy sine waves on the screen ? Has to be God drawing the pictures with his finger, hiding inside the CRT ! Yeah got it must be that !

[Vince]

2+2 = 4 in which environment ?

Does 2 even exist ?

Take a field with elements 1 and 0 only.

Well of course it does exist in this case : 1 and 0, that makes for TWO different states, it's a base 2 system ! 

[Cerebus]

Do you have anything with a input anywhere close to 3.9 puff...?

Or are you employing a variant of Bait Theory...?

mnem
 

Ah, so that's what the 3.9pf marking onthe probe means....

I was thinking of using them on my TDS 694C. Neither the front panel nor the user manual state what the input capacitance is, but it's a 3GHz scope so presumably it should do it ?!

3.9pF is the capacitance of the probe, not the scope. They don't need to match. High bandwidth probes always have low capacitance. Capacitance attenuate the signal at high frequency, resistors at lower frequency. That's why you have a mix of both in oscilloscope probes. The passive probe is a frequency compensated voltage divider.

Erm, there still seems to be some confusion here. I'm sure you all know what you mean, and are trying to say the right thing, but please excuse me if offer up a little clarification "for the young players".

Scope probes come in two basic flavours: high impedance probes which nowadays are almost always designed to work with 1MΩ scope input impedance, and low impedance probes that are designed to work with scopes with a 50Ω input impedance. In both cases the probe's impedance is not just resistive, but is reactive too (this is also true for the scope input the probe plugs into). Probes have stray capacitance, but also deliberate capacitance introduced to compensate for both the strays and in the input capacitance of the scope and produce as flat a frequency response as the construction allows. The compensation is designed to be adjustable to provide wiggle room for the tolerances involved in all the probe and input components.

The Tek probe in question is a high impedance one.

In the case of that 3.9pF 10MΩ Tektronix probe, the 3.9pF figure given is the input capacitance (I checked the datasheet for it) as seen by the DUT but there is a small range of input capacitances for the scope input that the probe is capable of being properly compensated for. For high impedance probes the input capacitance of the scope does need to match (within some range) the capabilities of the probe. (The TPP1000 data sheet does not state this range. That is unusual, it's a normal part of a probe specification.) The unspecified scope input capacitance will not be that 3.9pF figure; it's likely to be on the close order of about 20pF.

For low impedance probes matching is not really an issue because generally "as low a capacitance as possible" is the target and the issue of of HF and LF response balancing is less of a problem as the impedance seen by the probe input is both lower and flatter. To get a flavour of this, let's look at the impedance curve for that TPP1000 10MΩ probe:



You can see that as we get to higher frequencies the input impedance is dominated by the reactance of that 3.9pF input capacitance. The reactance of 3.9pF at 100 MHz is 408Ω, and it is no coincidence that's the region that the probe impedance is in by the time we get to 100MHz. Now, for a probe with 50Ω resistive impedance the 400Ω in parallel to that has a fraction of the effect (about 44Ω total) compared to being in parallel with 10MΩ (calculated as 407Ω for completeness sake) - a reduction of 12% compared to a reduction of 99.996%.

[xrunner]

2 + 2 = 5
for large values of 2

No, no ... for large values of 2

2
ENTER
2
+
5

[Saskia]

2+2 = 4 in which environment ?

Does 2 even exist ?

Take a field with elements 1 and 0 only.

Well of course it does exist in this case : 1 and 0, that makes for TWO different states, it's a base 2 system ! 

most definitely not. It cannot be base 2 if 2 is not defined.

[tggzzz]

So if you can't even rely on the fact that your kids will be taught that 2+2 makes 4

Douglas Adams famously defined "the answer" to be "42", and the question to be "what is 6*9?". They do match, in base 13

[tautech]

I still remember I had a laugh about that 20+ years ago. When I was in the UK studying EE, one of our electronics teacher, one day drew a NPN and PNP transistor symbol on the board. Then he tried to "help" us by giving us one of these strange phrase that would tell us which symbol is which.... only to get it wrong ! 

This one?

NPN: Not Pointing iN

PNP: Pointing iN Positively

 

Oh I couldn't tell you, I don't remember what it was at all... all I remember was the good laugh I was having when he made a fool out of himself !

Piss iN a Pot is the only one you need to remember......same sort of naughty stuff as Violet Gives Willingly. 

[mnementh]

On the Bench Today: UPS Batteries

So... I have this SMART-UPS 1500 I salvaged from a equipment refresh, and after 3 years daily use then plopping it in a trailer for over 2 years, shockingly even tho the pack was disconnected, most of the 7-ish-year-old batteries are not wanting to do anything of value. After zzzapp! -ing them all a wee bit several times using Terror Object #227 (A dim bulb tester connected to a bridge rectifier with some jumper wires) I finally got 3 of them to start making the bulb glow a wee bit. 

Those are the ones we're tormenting now. First in NiMH mode set to 11 cells, as this will allow the voltage to climb up as high as 18-19V before the OVP kicks in; experience has shown that if I'm lucky, the voltage will start to drop before then and the battery may start to charge normally, which all 3 have started to do. Now we're running them in Pb mode, and they seem to be taking some charge.

Time will tell; if I'm lucky, I can throw these in my underdesk UPSes and use them for my network infrastructure until I have a wee bit more discretionary funds for such things.

mnem
 

[Specmaster]

[...]It's not like you saw rainbows all over the world 10 times a day...then yes people would probably remember it. Personally if I see a rainbow once a year it's already quite good. So not frequent enough to expect people to know it by heart...

All the colors bleed into each other...   settling for six or seven colors seems a bit arbitrary!

Yebbutt... it was literally elementary-school stuff they drummed into our heads starting from first or second grade.

Yet another aspect of The Arts as it relates to science and the world around us which has been deemed no longer important by TPTB, I guess... as an interest in art is contraindicated in the indoctrination of pliant, docile, worker bees to support the hive the TPTB in the fashion to which they've become accustomed... 

mnem
 

When us crusty old farts pass over to the other side, all of these kinds of things will become lost forever in the annuls of time, there will be nothing magical for folk to learn and remember with IC's and SMD's 

[Robert763]

Just been checking new listings on ebay and it's full of over-priced tat. The penny dropped, checked messages and sure enought a 70% off listng offer 

[Cerebus]

[...]It's not like you saw rainbows all over the world 10 times a day...then yes people would probably remember it. Personally if I see a rainbow once a year it's already quite good. So not frequent enough to expect people to know it by heart...

All the colors bleed into each other...   settling for six or seven colors seems a bit arbitrary!

Yebbutt... it was literally elementary-school stuff they drummed into our heads starting from first or second grade.

Yet another aspect of The Arts as it relates to science and the world around us which has been deemed no longer important by TPTB, I guess... as an interest in art is contraindicated in the indoctrination of pliant, docile, worker bees to support the hive the TPTB in the fashion to which they've become accustomed... 

mnem
 

When us crusty old farts pass over to the other side, all of these kinds of things will become lost forever in the annuls of time, there will be nothing magical for folk to learn and remember with IC's and SMD's 

Oh yeah? Now you have to learn the EIA-96 resistance codes and use a loupe to read them off the devices, and that's definitely more brain-ache and eye-ache than the colour codes.  And they call that progress?

[McBryce]

I still remember I had a laugh about that 20+ years ago. When I was in the UK studying EE, one of our electronics teacher, one day drew a NPN and PNP transistor symbol on the board. Then he tried to "help" us by giving us one of these strange phrase that would tell us which symbol is which.... only to get it wrong ! 

This one?

NPN: Not Pointing iN

PNP: Pointing iN Positively

 

Oh I couldn't tell you, I don't remember what it was at all... all I remember was the good laugh I was having when he made a fool out of himself !

Piss iN a Pot is the only one you need to remember......same sort of naughty stuff as Violet Gives Willingly. 

So all this time med has actually been referring to Passive Infrared sensors?? Or was he really telling us to all PIss up a Rope?

McBryce.

[m k]

MuRuPuOrKeViSiVioHaVa

[AVGresponding]

So if you can't even rely on the fact that your kids will be taught that 2+2 makes 4

Douglas Adams famously defined "the answer" to be "42", and the question to be "what is 6*9?". They do match, in base 13

Something he stated was completely accidental; he chose the number 42 because it was the most amusing sounding one he could think of, and 6 x 9 because it was meant to indicate the fundamental ridiculousness of existence in general.

Incidentally I once made a 6-bit digital switch in a fps game level editor, and set the activation combination to "101010", only realising later that I'd made a combination lock that opened to 42 in binary...

Just been checking new listings on ebay and it's full of over-priced tat. The penny dropped, checked messages and sure enought a 70% off listng offer 

There's a batch of scam listings of high-end TE popped up today as well, the hacked seller account is "egg and chips clothing" or somesuch.

[Vince]

MuRuPuOrKeViSiVioHaVa

That one is easy ! It's the marking code for a SOT23 5.1V Zener diode ! Everyone knew that I thought ?! 

[Specmaster]

MuRuPuOrKeViSiVioHaVa

That one is easy ! It's the marking code for a SOT23 5.1V Zener diode ! Everyone knew that I thought ?! 

Nope, is that something you just made up for the sheer hell of it 

[med6753]

What have you all been drinking or smoking today? 

[tggzzz]

So if you can't even rely on the fact that your kids will be taught that 2+2 makes 4

Douglas Adams famously defined "the answer" to be "42", and the question to be "what is 6*9?". They do match, in base 13

Something he stated was completely accidental; he chose the number 42 because it was the most amusing sounding one he could think of, and 6 x 9 because it was meant to indicate the fundamental ridiculousness of existence in general.

I've no reason to doubt that, but I deeply and fundamentally don't care - and will ignore it

[mnementh]

What have you all been drinking or smoking today? 

Smurf pee. 

mnem
smoked...

[mnementh]

MuRuPuOrKeViSiVioHaVa

That one is easy ! It's the marking code for a SOT23 5.1V Zener diode ! Everyone knew that I thought ?! 

Nope, is that something you just made up for the sheer hell of it 

Gurrgle thinks it is a geocache of dubious content:

https://www.geocaching.com/geocache/GC4BFK4_murupuorkevisiviohava

mnem
I'd go, but I need to have my scales polished... 

[mnementh]

When us crusty old farts pass over to the other side, all of these kinds of things will become lost forever in the annuls of time, there will be nothing magical for folk to learn and remember with IC's and SMD's 

Yeah, I guess... And there will be nobody who cares, because there won't have been a actual 🌈 rainbow seen anywhere on the planet by anyone in living memory...

mnem
*toddles off to bleed quietly in the corner*

[Specmaster]

What have you all been drinking or smoking today? 

I was thinking much the same thing myself 

[Vince]

For low impedance probes matching is not really an issue because generally "as low a capacitance as possible" is the target and the issue of of HF and LF response balancing is less of a problem as the impedance seen by the probe input is both lower and flatter. To get a flavour of this, let's look at the impedance curve for that TPP1000 10MΩ probe:



You can see that as we get to higher frequencies the input impedance is dominated by the reactance of that 3.9pF input capacitance. The reactance of 3.9pF at 100 MHz is 408Ω, and it is no coincidence that's the region that the probe impedance is in by the time we get to 100MHz. Now, for a probe with 50Ω resistive impedance the 400Ω in parallel to that has a fraction of the effect (about 44Ω total) compared to being in parallel with 10MΩ (calculated as 407Ω for completeness sake) - a reduction of 12% compared to a reduction of 99.996%.

Honestly looking at those graphs just ruins my day.... it means basically you can't probe anything at all really... the impedance drops so quickly, and the phase shit moves so fast before after the narrow flat area, that unless you are looking at a pure sine wave... the signal you see on the scope is hardly what it is actually in the DUT, so making any "measurement" is just a joke 

You get a ballpark idea of what the signal shape might be, OK... other than that, it's just an illusion 

If your signal has any significant amount of energy in harmonics, then be it the rapidly changing impedance/amplitude, or phase shift... the signal displayed on the scope is potentially heavily distorted in all kinds of weird and wonderful ways.

That's mostly why I don't like modern high-speed stuff.... even simply looking at a freaking square wave/digital signal is day dreaming. It will never look like  a square and whatever overshoot or undershoot or dampening or any distortion anywhere on the waveform... you can't tell 100% if it is real, or if it's just the scope and probe setup that creates it for you out of the blue

It is very depressing....

Other reason for not liking high-speed is the price of probes and test gear rises exponentially with the bandwidth.

Sticking to 1GHz max, is about affordable to the hobbyist yet still plenty of stuff to do to learn and have fun with.  High-speed is just a money pit and a recipe for depression.... to me at least ! 

Some people have to do it as a job because all the gizmos are fast digital stuff nowadays.. but as far as fun goes in one's personal lab, I don't see the fun in high-speed digical stuff

Now that I have got that off my chest, let me go back to tying my power cords if I may.