Author Topic: 70 MHZ Oscilloscope with higher rated probes  (Read 1154 times)

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Offline jdutky

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #25 on: April 17, 2021, 08:00:13 am »
When I switch to SQUARE wave its where most of distortion is. I think my connections were bad when I first posted.

What you are seeing on the square wave is called "ringing" and it is caused by the way you have tried to probe the signal, not by the signal source or the bandwidth of the probes you are using. I'm guessing that you used the 3 inch or 6 inch ground leads that came with your probe to connect to the signal ground. That lead is adding a lot of inductance to your measurement and causing the ringing you are seeing on after the rise and fall of the square wave. If you really want to reduce the ringing you need to reduce the length of your ground loop, either by using a ground spring (which probably came with your probe) or by using a probe-tip to BNC adapter (if you are getting your signal from a BNC connector).

Here is a good video that spends some time talking about probe grounding techniques:

As to the question about using high bandwidth probes on a low bandwidth scope: it's better to use the probes that came with your scope, but if the "high bandwidth" probes are of the cheap Chinese variety, it may not be making all that much difference. Still, it would be better to get probes that are properly matched to the scope, both in bandwidth rating, and in capacitance.

Probes have a compensation network that allows you to match the probe to the instrument. Most of the compensation adjustment is done by an adjustable capacitor, and the probe documentation should tell you what the range of that adjustment is. What you need is a probe whose compensation range contains the input capacitance of your scope. Your scope's documentation should tell you what the input capacitance is. As an example, I have an old Tektronix 475, a 200 Mhz analog oscilloscope, with an input capacitance of 20 pF. I bought cheap Chinese probes on Amazon rated at 200 MHz with an input capacitance adjustable from 13 pF to 25 pF, so those probes can be compensated to match my scope.

Higher bandwidth probes often have lower compensation ranges than lower bandwidth probes, and higher bandwidth scopes have lower input capacitance than lower bandwidth scopes. If the compensation range for you probe does not encompass the input capacitance of your scope you will not be able to properly compensate the probe, and you will get distortion of your measurements. So, best to get probes that are well matched to your scope, both in bandwidth, and in compensation range.

As for using your probes to filter out high bandwidth components of your measured signals, that is probably not an issue. While I am quite happy with the 200 MHz cheap Chinese probes I bought (you can't go far wrong for $15), other people have reported that the Chinese probes are actually much lower bandwidth than their specs indicate. So, even though you bought 200 MHz probes, you may actually have only 100 MHz probes. In order to test that you would need a much better signal generator than you probably have on hand, and since your scope is only 70 MHz, it's not even worth worrying about. Suffice to say that your 200 MHz probes are probably not causing your noticeable problems, assuming that you were able to properly compensate them (which you said you did).

-- Jeff Dutky
 
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Offline David Hess

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #26 on: April 17, 2021, 03:43:13 pm »
What you are seeing on the square wave is called "ringing" and it is caused by the way you have tried to probe the signal, not by the signal source or the bandwidth of the probes you are using. I'm guessing that you used the 3 inch or 6 inch ground leads that came with your probe to connect to the signal ground. That lead is adding a lot of inductance to your measurement and causing the ringing you are seeing on after the rise and fall of the square wave. If you really want to reduce the ringing you need to reduce the length of your ground loop, either by using a ground spring (which probably came with your probe) or by using a probe-tip to BNC adapter (if you are getting your signal from a BNC connector).

That is usually the case but not always.  In the example I gave of a low bandwidth probe used with a high bandwidth oscilloscope, the probe's response can include ringing even with a coaxial connection to the probe.
 
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Online bdunham7

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #27 on: April 17, 2021, 03:57:57 pm »
What you are seeing on the square wave is called "ringing" and it is caused by the way you have tried to probe the signal, not by the signal source or the bandwidth of the probes you are using.

Well, it is ringing of a sort, but you aren't likely to see ringing due to ground lead inductance at that low of a frequency. 
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline radiolistener

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #28 on: April 17, 2021, 04:18:15 pm »
That is usually the case but not always.  In the example I gave of a low bandwidth probe used with a high bandwidth oscilloscope, the probe's response can include ringing even with a coaxial connection to the probe.

yes, the frequency response of a Chinese probes (which is usually included with 500-1000 USD oscilloscopes) is not flat and may be very terrible. Especially if you bought it on ebay or aliexpress for 10-20 USD as a "brand new 200 MHz probe".

But the probe ringing effect can be caught at much higher frequencies, more than 1-10 MHz. At low frequency it's influence is too low to be noticeable.
« Last Edit: April 17, 2021, 04:23:38 pm by radiolistener »
 
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Offline David Hess

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #29 on: April 17, 2021, 06:24:58 pm »
That is usually the case but not always.  In the example I gave of a low bandwidth probe used with a high bandwidth oscilloscope, the probe's response can include ringing even with a coaxial connection to the probe.

yes, the frequency response of a Chinese probes (which is usually included with 500-1000 USD oscilloscopes) is not flat and may be very terrible. Especially if you bought it on ebay or aliexpress for 10-20 USD as a "brand new 200 MHz probe".

Some probes are just defective by design.

Quote
But the probe ringing effect can be caught at much higher frequencies, more than 1-10 MHz. At low frequency it's influence is too low to be noticeable.

I agree.

When the probe and oscilloscope bandwidth are the same, then the limited bandwidth of the oscilloscope will conceal the poor response of the probe above the probe's bandwidth, for the most part; it can still be observed with a good enough signal source for testing.  When the probe's bandwidth is higher than the oscilloscope's bandwidth, then any aberrations in the probe's response are much higher and attenuated by the oscilloscope's more limited response even more.
 
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Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #30 on: April 18, 2021, 12:37:05 pm »
Hi,

I really appreciate all the information. I am kind of over my head; but that's how I learn.

I can't always trust YouTube reviews because many of them get free stuff for doing the reviews. What I get here is real world experience. Not all Chinese made "stuff" is bad. Before I retired I worked for a US monitor company and we had some made in China; to our spec and parts with US inspectors.

Keep in mind; as I said before, I am just experimenting and learning.

I will say with my limited knowledge and thinking through what you guys have been saying; I made new cables to connect to the PC DAC. Tested the cables with a Fluke 115 RMS multimeter.  I am getting very nice SINE waves and the SQUARE waves look much better depending on the frequency and TIME/DIV. At some point I will invest in a better signal generator.

What I want to do is learn and to try a real world application because the extent of my use will be troubleshooting a manufactured component. Mostly vintage. My end goal is to work on this equipment and possibly sell some of them at a good price, but I can't sell something that may or may not work properly. It's not in me. I will only sell something that I know is working and would use myself. I'm having a hard time parting with some of them though (i.e. 1963 Sherwood S-2100 valve tube receiver). So I guess at some point I will need an RF signal generator??

Keep it coming.

Rick
P.S. I can't seem to figure out how to get reply notifications to the post; or any. I will go back to check my settings; again.
 

Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #31 on: April 18, 2021, 12:39:57 pm »
P.S.S. I reset notifications so will see if it works.
 

Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #32 on: April 18, 2021, 12:56:28 pm »
Here are the probes characteristics. The one on the left came with the scope. The one on the right (P6200) are the ones I bought.
 

Online tggzzz

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #33 on: April 18, 2021, 01:30:05 pm »
What I want to do is learn and to try a real world application ... valve tube receiver

I'm glad you are making progress :)

Given those two points, I strongly suggest you understand the "effective use" and "safety" refeerences in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
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Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #34 on: April 18, 2021, 04:50:29 pm »

I'm glad you are making progress :)

Given those two points, I strongly suggest you understand the "effective use" and "safety" refeerences in https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

I have started reading The Art of Electronics by Paul Horowitz & Winfield Hill. It's an older eBook version, but helpful. I would like to get the 3rd edition, but it's expensive. Oddly a used copy is more than the new hard book copy on Amazon.

Rick
 

Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #35 on: April 18, 2021, 07:04:46 pm »
Hi,

Sorry for so many questions.

It's somewhat related to my topic. I made some cables to connect the PC signal generator to scope with braided wire. I'm curious; does this act like a coil and effect the signal; even though I securely welded the wires? Should I use a solid wire? If so, what gauge?  I have 24 AWG solid with silicon insulation.

Thank you.
Rick
 

Offline jdutky

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #36 on: April 19, 2021, 06:11:44 am »
I really appreciate all the information. I am kind of over my head; but that's how I learn.

jumping in over your head is a fine way to learn.

Quote
I can't always trust YouTube reviews because many of them get free stuff for doing the reviews. What I get here is real world experience. Not all Chinese made "stuff" is bad. Before I retired I worked for a US monitor company and we had some made in China; to our spec and parts with US inspectors.

This is very true, and it's not even all that easy to triangulate off of multiple reviews because so many of them are for hire. EEVblog is a good honest source for reviews and information. The fundamentals videos are quite helpful if you are just learning this stuff. There are a couple other YouTube channels that I rely on for similar content:

There are other good sources, but these are my most frequently visited, as they are full of so much information that I can't absorb it all in one viewing.

Good luck and have fun.

-- Jeff Dutky
 

Online tggzzz

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #37 on: April 19, 2021, 09:23:16 am »
I really appreciate all the information. I am kind of over my head; but that's how I learn.

jumping in over your head is a fine way to learn.

Provided, of course, you don't damage equipment or yourself in the process. With high voltages and old equipment with "variable" chassis/earthing, that really cannot be ruled out.

Quote
Good luck and have fun.

As I told my daughter when we were doing things that could have killed us, "have fun - safely".
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
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Offline Rick King

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #38 on: April 19, 2021, 02:16:17 pm »
I really appreciate all the information. I am kind of over my head; but that's how I learn.

jumping in over your head is a fine way to learn.

Provided, of course, you don't damage equipment or yourself in the process. With high voltages and old equipment with "variable" chassis/earthing, that really cannot be ruled out.

Quote
Good luck and have fun.

As I told my daughter when we were doing things that could have killed us, "have fun - safely".

I did purchase an anti-static wrist strap. I need to remember to use it. I also heard of the "one hand in back pocket"; so I end up working with my teeth. Oh; but then there is lead from solder; so the teeth are out. That just leaves my toes I guess. Haha
 

Online tggzzz

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Re: 70 MHZ Oscilloscope with higher rated probes
« Reply #39 on: April 19, 2021, 05:01:10 pm »
Neither an antistatic wrist strap nor RCD will make you safe!
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
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