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Author Topic: Test Equipment: Oscillocopes  (Read 42517 times)

Offline MarkE

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Test Equipment: Oscillocopes
« on: February 14, 2015, 10:35:20 PM »
This thread is to discuss oscilloscopes and good measurement practices using oscilloscopes.

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Test Equipment: Oscillocopes
« on: February 14, 2015, 10:35:20 PM »

Offline Pirate88179

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Offline John.K1

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Re: Test Equipment: Oscillocopes
« Reply #2 on: February 14, 2015, 10:45:13 PM »
Now we need some tutor :D


Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #3 on: February 14, 2015, 11:45:59 PM »
The basics of your average passive oscilloscope probe:

Most passive probes come in x1/x10 switchable configurations.  The oscilloscope itself contains the amplifiers which  have a 1 megOhm resistance from the channel input to ground.  When set for x1, the probe is basically a coaxial cable connection from the probe to the oscilloscope.  The coaxial cable for a typical 1.5 meter scope cable has a total of about 50 - 100pF capacitance.  That is a parasitic that the circuit under test has to drive.

When the probe is in the x10 mode a 9Meg Ohm resistor gets switched in at the probe.  That's the good news.  The bad news is that 9Meg Ohms forms a low pass filter with the cable capacitance that kills the frequency response.  In order to restore the response:  the probe includes a small capacitance of 10 - 20pF in parallel with the 9 Meg Ohm resistor.  The circuit being measured now drives 10-20pF in parallel with 9Meg Ohms instead of driving 50 - 100pF in parallel with 1 Meg.  10-20pF may not seem like a lot, but that depends on how fast the signal is.  At 1MHz 16pF is ~10K Ohms, a far cry from the total 10M Ohms of the probe at DC.

The next problem is signal reflections.  Anytime a signal travels through a transmission like, such as the coaxial cable of an oscilloscope probe, whereever the impedance changes there is a reflection:  A ghost signal that travels in all directions from the point where the impedance changes.  When the cable is electrically short compared to the frequency content of the reflection, the reflection is not noticeable.  Where the cable is longer than about 1/10th the rise time of the signal, or about 1/3 the frequency of a sine wave, the reflections become a big issue.  Remember that the oscilloscope input is 1 MegOhm.  A typical coax probe cable has a transmission line impedance near 100 Ohms.  This is a very bad mismatch.  Without adjustment, the signal that hits the amplifier nearly doubles due to the reflection.  In order to get around that, the probe has a series resistor past the 10-20pF compensation capacitor that matches the transmission line (coax cable) impedance to try and cut the signal by half so that when the signal doubles at the scope input the result is correct when it first hits.  The ghost signal still needs to be absorbed by the time it gets back to the probe.  The series resistor would do that if the DUT circuit had zero impedance.  That means the DUT circuit sees that reflected ghost energy.  But it sees it through the probe.  When the probe is connected using a 4" or 6" ground clip, that introduces inductance that resists passing the fast changing waveform of the ghost reflection.  As a result, the ghost reflection is not completely absorbed and now what remains of it goes zooming back down the cable towards the oscilloscope where it will be seen as signal.

These reflections that are made much worse by the ground clip generates ringing that may not settle for 50ns or longer.  This makes it hard to measure narrow spikes accurately, especially using the oscilloscope probe ground clip, and where the circuit under test has a high impedance.  There are vaious techniques to mitigate these problems.

Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #4 on: February 14, 2015, 11:50:11 PM »
Good topic Mark.  This will help me a lot I am sure, as well as many others.

Bill

PS  http://www.ebay.com/itm/New-Function-signal-generator-source-frequency-signal-generator-finished-board/131063212194?_trksid=p2054897.c100204.m3164&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140407115239%26meid%3Ddee726ea5268418aaae964634ecd71a2%26pid%3D100204%26rk%3D7%26rkt%3D28%26mehot%3Dpp%26sd%3D361077176699
Well it's better than nothing.  What works pretty well are the 3D25 USB generators from Hantek.  They go for about $160. and produce decent signals from 1Hz to ~10MHz ( they say 20MHz ) and are easy to control.  The signal output is limited to +/-3V into 50 Ohms which is so-so.  but one can add a voltage amplifier behind them if one is so inclined.  The next step up is to purchase a standaolne FG.  New, a decent digial one starts at about $350.

Is this FG worth getting if you don't have any as of yet?  (In other words, better than nothing?)
« Last Edit: February 15, 2015, 02:57:08 AM by MarkE »

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Re: Test Equipment: Oscillocopes
« Reply #4 on: February 14, 2015, 11:50:11 PM »
Sponsored links:




Offline Pirate88179

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Re: Test Equipment: Oscillocopes
« Reply #5 on: February 14, 2015, 11:53:00 PM »
Thanks Mark.

Hey, if you repost your above response and requote my post, it is now sized to fit the page.  I have no idea why the link I posted ran off to the
right so far.  I split it up so now it fits the page.  If you re-do your quote of my post, it will now fit.

Sorry.

Bill

Offline TinselKoala

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Re: Test Equipment: Oscillocopes
« Reply #6 on: February 15, 2015, 02:13:19 AM »
@MarkE: Thank you for starting this topic. I hope to learn a lot from it, and may be able to contribute a thing or two myself.




@Pirate: If you're a fan of classic analog equipment...

http://www.ebay.com/itm/Interstate-Electronics-Corporation-F-34-Function-Generator-115v-230v-/271740054768

 ;)




Free Energy | searching for free energy and discussing free energy

Re: Test Equipment: Oscillocopes
« Reply #6 on: February 15, 2015, 02:13:19 AM »
Sponsored links:




Offline Brian516

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Re: Test Equipment: Oscillocopes
« Reply #7 on: February 15, 2015, 04:38:22 AM »

@Pirate: If you're a fan of classic analog equipment...

http://www.ebay.com/itm/Interstate-Electronics-Corporation-F-34-Function-Generator-115v-230v-/271740054768

 ;)

OOOOhhhhH...... I just might have to snatch that one out from under ya!!!      haha   just playin, just playin... it's all yours...
unless I get it first!! AHHHH   
kidding.  I do need a FG though. not yet though, already have enough to learn and a PC/amp will be OK for the time being, esp since I can shut off my DC offset and disable all enhancements.

I'm going to bring my reply regarding the scope and arduino over to this thread.
Quote
@Brian, yes, I think it would be a good value especially if you don't want or need the stuff you are trading for it. You are right that it may just need cleaning of the switch contacts. If it needs repair that too can be managed; as you found out there are replacement boards available for the vertical amplifiers and even if you had to replace the entire VA board rather than just a component you will still be way ahead. I haven't looked at a 465 Service Manual but the Tek manuals generally are very complete and will have troubleshooting and calibration guides along with complete schematics so tracking down the problem may be fairly straightforward.
I've seen a few scopes that were advertised as "not working" and it turned out that the sellers just didn't know how to get a trace to show up! Probably not that simple in your case though but one could always hope.

Arduinos are a _lot_ of fun and I'm always amazed at what can be done with them. And the programming language is essentially c++, which is great!

Wish me luck tomorrow with this O-scope!  Let's hope the repairs will just be a nice, thorough cleaning which I will be giving it regardless! If not, I'll go thru and test all the caps, resistors, transistors etc etc if I must, and worst case I'll just buy a new VA board.  I'm quite anxious, though.... I've never even touched a scope, so I've been watching vids non-stop about how to use it so I don't screw anything up before I even bring it home!   I'm going to have him start it up and do the first test run off my PC FG, just in case he's some nut con man trying to pull a fast one and toss off a broken piece of equipment on me!  Gotta be uber careful with craigslist people.. you just never know... 
As for the PC FG, I just wanna mention for the benefit of all, Visual Analyzer is pretty good software. if you haven't heard of it, you should def check it out!

I found the factory service manual for it and scanned thru it, it definitely seems quite thorough. It's got schematics, troubleshooting, and quite a bit more, so definitely good to have!   I wish I could run across some of those people that have stuff I need and haven't a clue and don't care to!  Maybe I'll luck out this spring at some yard sales....  $5 FG WHAAAAATT?? (that would be awesome!)

I have some books on C++ somewhere. Never did get around to learning it, though. I guess I have some motivation now.  What add-on board do you think I should get first, besides an LCD or a touch screen?

Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #8 on: February 15, 2015, 05:08:07 AM »
OOOOhhhhH...... I just might have to snatch that one out from under ya!!!      haha   just playin, just playin... it's all yours...
unless I get it first!! AHHHH   
kidding.  I do need a FG though. not yet though, already have enough to learn and a PC/amp will be OK for the time being, esp since I can shut off my DC offset and disable all enhancements.

I'm going to bring my reply regarding the scope and arduino over to this thread.
Wish me luck tomorrow with this O-scope!  Let's hope the repairs will just be a nice, thorough cleaning which I will be giving it regardless! If not, I'll go thru and test all the caps, resistors, transistors etc etc if I must, and worst case I'll just buy a new VA board.  I'm quite anxious, though.... I've never even touched a scope, so I've been watching vids non-stop about how to use it so I don't screw anything up before I even bring it home!   I'm going to have him start it up and do the first test run off my PC FG, just in case he's some nut con man trying to pull a fast one and toss off a broken piece of equipment on me!  Gotta be uber careful with craigslist people.. you just never know... 
As for the PC FG, I just wanna mention for the benefit of all, Visual Analyzer is pretty good software. if you haven't heard of it, you should def check it out

I found the factory service manual for it and scanned thru it, it definitely seems quite thorough. It's got schematics, troubleshooting, and quite a bit more, so definitely good to have!   I wish I could run across some of those people that have stuff I need and haven't a clue and don't care to!  Maybe I'll luck out this spring at some yard sales....  $5 FG WHAAAAATT?? (that would be awesome!)

I have some books on C++ somewhere. Never did get around to learning it, though. I guess I have some motivation now.  What add-on board do you think I should get first, besides an LCD or a touch screen?
eBay has some function generators for $60 - $80 that do two channels  to 2MHz or 5MHz and 20Vpp open circuit w/ 50 Ohms impedance.  That would cover a lot of needs.

Offline Brian516

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Re: Test Equipment: Oscillocopes
« Reply #9 on: February 15, 2015, 06:17:08 AM »
once I unload this mountain of stuff I have to sell on ebay, I'll grab up some more equipment. all my funds lately have been going towards getting caught up. sucks, but worth it.

Free Energy | searching for free energy and discussing free energy

Re: Test Equipment: Oscillocopes
« Reply #9 on: February 15, 2015, 06:17:08 AM »
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Offline Pirate88179

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Re: Test Equipment: Oscillocopes
« Reply #10 on: February 15, 2015, 06:21:50 AM »
@MarkE: Thank you for starting this topic. I hope to learn a lot from it, and may be able to contribute a thing or two myself.




@Pirate: If you're a fan of classic analog equipment...

http://www.ebay.com/itm/Interstate-Electronics-Corporation-F-34-Function-Generator-115v-230v-/271740054768

 ;)

That looks cool....thanks.  I love classic equipment.

Bill

Offline TinselKoala

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Re: Test Equipment: Oscillocopes
« Reply #11 on: February 15, 2015, 10:50:07 AM »
Quote from: Brian516
I have some books on C++ somewhere. Never did get around to learning it, though. I guess I have some motivation now.  What add-on board do you think I should get first, besides an LCD or a touch screen?

Depends on what you intend to do. There are literally hundreds of "shields" out there. The first "shield" that I got was the RuggedCircuits "Gadget Shield" just to play around with; it has 3-axis accelerometer, IR detector, IR emitter, RGB high-brightness LED, 2 potentiometers, 2 pushbuttons, 4 LED's, visible light sensor. The demo sketch for this one does things like have the RGB LED driven by the accelerometer, so it changes color as you tilt the thing around, etc.  I also have their "Aussie shield" which is just a "breakout" board with easy quick-connectors for external wiring. For the LCD, I have a Parallax branded thing that has a backlight and a little piezo speaker, that is part of the Parallax Propeller system, it uses just a single data line (serial interface) and the two power leads so you don't tie up lots of the Arduino output pins driving a raw LCD. I also have another LCD shield that has buttons but it uses the regular interface so it uses up 7 outputs IIRC. I don't have any experience with touchscreens so I can't recommend one. I've got a motor driver shield with high-output H-bridge motor driver chips on it for robotics stuff, rc servos and steppermotors. Also a prototype shield that has pads to build your own circuits on, I put a high power mosfet switch circuit on this one.

There are even signal generator shields and video displays that you can get for the silly things, probably even full oscilloscope shields.

http://playground.arduino.cc/Main/SimilarBoards  (lists many hardware items, shields etc with functional descriptions)
http://shieldlist.org/  (listed by manufacturer not type, unfortunately)

If you already have some programming experience but just not with c, or even if you don't, probably the best way to learn is just to look at example sketches and start changing them and see what happens. It's really pretty easy to do the basic stuff, the Arduino IDE comes with a lot of basic example sketches. Most people start with the "blink" sketch which just blinks an LED.  The Arduino online references are excellent and the programming interface is portable, runs on any computer OS, so you can write your sketches on a Mac, load them with Linux, run and display serial data on Windoze etc.

Here's a handy thing I made: 
http://www.youtube.com/watch?v=S6N8ys8FiA4

And just to steer back to the topic of oscilloscopes, here's a scoposcopy video showing how the Inductometer does its magic:
http://www.youtube.com/watch?v=Qx3B89379eQ

Offline TinselKoala

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Re: Test Equipment: Oscillocopes
« Reply #12 on: February 15, 2015, 10:54:45 AM »
That looks cool....thanks.  I love classic equipment.

Bill
It's kind of limited in that it only goes to 3MHz and has a low output level, but it's very versatile in its trigger/gating functions and it has the external VCO input so you can control it from an external variable voltage source. I have one of these in storage in Canada that I paid a lot more money for than the selling price here. My regular workhorse is another Interstate FG, the F43 model "High Voltage" FG, it doesn't do sweep but it has 40 v p-p into 50 Ohms output.

It is handy to have a higher voltage output level as well as a higher frequency range, so the more modern ones MarkE found on EBay are worth a look too.

Offline John.K1

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Re: Test Equipment: Oscillocopes
« Reply #13 on: February 15, 2015, 11:31:34 AM »
My question is than what setup (1x,10x...) to use when. Currently I have my probes permanently set to 10x.  Any general rule?
Next question - would it make sense to use de-coupling 1pf capacitor - sometimes I do use 1pF 5% 1KV  - but to be honest I do not see much difference on the screen when using it.
I am planning to buy 100x probe. - yes/no? suggestions?

TK, you was talking about integration MATH function, what is that good for?

50Hz modulation from the main- is it big issue?  any chance to filter it out?

Thanks.

Offline MarkE

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Re: Test Equipment: Oscillocopes
« Reply #14 on: February 15, 2015, 12:08:40 PM »
My question is than what setup (1x,10x...) to use when. Currently I have my probes permanently set to 10x.  Any general rule?
Next question - would it make sense to use de-coupling 1pf capacitor - sometimes I do use 1pF 5% 1KV  - but to be honest I do not see much difference on the screen when using it.
I am planning to buy 100x probe. - yes/no? suggestions?

TK, you was talking about integration MATH function, what is that good for?

50Hz modulation from the main- is it big issue?  any chance to filter it out?

Thanks.
Use 10X almost exclusively.  The only time 1X is useful is for low-level signals.  10X always offers equal or better fidelity and less loading on the circuit being tested.

Filters can be useful, there are several ways to go about them.  Most scopes have a 20MHz bandwidht limiting option on the front panel. From a noise standpoint they are best to put at the scope input. Inserting 1.5K Ohms in series with 100pF to ground gives a decent performing 1MHz cut-off with low DC error.  You can solder the parts between a male and female BNC connector, and then hot glue over it.

Integration of instantaneous power yields energy, and energy divided by time gives average power over the time interval used, such as one cycle of a periodic waveform.

If you are picking up 50Hz, then:

1) Your signals are very small and high impedance. OR
2) You have a ground loop and/or an open ground.

To test for a grounding problem connect the scope probe ground clip to the probe tip.  Without touching the probe tip to anything, place it close to where you would probe in your circuit.  If you pick-up a lot of hum, then there is a strong magnetic field from a transformer or such coupling into the loop formed with the ground clip.  Reduce the exposed area using a high frequency ground clip, and/or add magnetic shielding.

Touch the probe tip to the device under test circuit common with the probe ground clip still in contact witht the probe tip.  If you still pick-up a lot of mains voltage, then check the green wire grounds of the scope and any mains powered gear in the circuit under test.  With the AC power OFF use a DMM to check the resistance between the probe common of the oscilloscope and the circuit common of the circuit being tested.  If there is anything more than an Ohm or two find out why and correct the problem.


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Re: Test Equipment: Oscillocopes
« Reply #14 on: February 15, 2015, 12:08:40 PM »

 

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