Which oscilloscope for hobbyist?

[nctnico]

another somewhat related question: is there any reason to have the 50 ohm adapter for a scope being used on relatively low frequencies (<100MHz) ?  what specific circumstances require a 50 ohm adapter?

It all depends on your frequency and cable length, say, for 30MHz, the wavelength in free air is 10M. Assuming you are using RG316 test cable, then the effective wavelength is 7M.

As a rule of thumb, 1/10 wavelength and longer cable calls for attention on signal integrity, so as long as your cable is longer than 2ft, you will need to terminate it properly.

There are some BNC terminators on eBay, rated up to many gigahertz. For SMA connector you will have more choices. They generally cost <$10 for a used branded or a new unbranded.

Yes and no. The real problem in this case is the input capacitance of high-Z (standard) probes and 1M Ohm oscilloscope inputs. As a rule of thumb: that input capacitance kills any chance of measuring signals (containing harmonics) over 100MHz correctly.
Even when using a 50 Ohm terminator parallel to tens of pf isn't going to work right so using a (feed-through) terminator isn't going to help at very high frequencies. This is why you'll find 50 Ohm inputs on any decent oscilloscope having a bandwidth over 150MHz.

[gojimmypi]

thanks for the replies!

Say I had a hi-z scope (the DS1054)... and a 50 ohm coax cable signal that I wanted to measure....

Would I be able to plug the high-z probe into a coax "T" with one side terminated with a 50 ohm terminator, and the other receiving the 50 ohm signal - and get a proper measurement?

[gojimmypi]

for anyone looking for a hobbyist scope, I found this pdf file while googling for "50 ohm" stuff... lab notes from a 2007 UC Berkeley Class:

Function Generator and Oscilloscope - In this lab you learn how to use the oscilloscope and function generator
https://inst.eecs.berkeley.edu/~ee100/su07/lab/lab2-FunctionGeneratorOscilloscope/eecs100_eecs43_lab2-fncn_generator_scope.pdf

perhaps someone might find it useful

[TheSteve]

thanks for the replies!

Say I had a hi-z scope (the DS1054)... and a 50 ohm coax cable signal that I wanted to measure....

Would I be able to plug the high-z probe into a coax "T" with one side terminated with a 50 ohm terminator, and the other receiving the 50 ohm signal - and get a proper measurement?

Depends on the frequency - if it isn't too high this may be fine depending on what you want to do.

Can you give us a more detailed example?

[gojimmypi]

thanks for the replies!

Say I had a hi-z scope (the DS1054)... and a 50 ohm coax cable signal that I wanted to measure....

Would I be able to plug the high-z probe into a coax "T" with one side terminated with a 50 ohm terminator, and the other receiving the 50 ohm signal - and get a proper measurement?

Depends on the frequency - if it isn't too high this may be fine depending on what you want to do.

Can you give us a more detailed example?

Say a 50 ohm terminator like this: http://www.amazon.com/gp/product/B00G10MKCO

and a "T" like this: http://www.amazon.com/dp/B0000510YX/

plug the male end of the T into the scope, put the terminator on one of the female connectors of the T, and use a Hi-Z scope on the other female connector of the T.

as I talk through this, I think this is actually a silly idea, no? The scope will be an unterminated part of the transmission line causing problems.  (it has been a long time since my EE classes. did I mention I work as a software engineer these days?) 

[gojimmypi]

Added: https://www.youtube.com/watch?v=OiAmER1OJh4.

Another Dave video! Awesome. thanks

I also came across this National Instruments white paper "Select the Correct Oscilloscope Probe for Your Application":

http://www.ni.com/white-paper/14825/en/

edit: as internet links sometimes change, I've attached the NI Whitepaper

[alsetalokin4017]

another somewhat related question: is there any reason to have the 50 ohm adapter for a scope being used on relatively low frequencies (<100MHz) ?  what specific circumstances require a 50 ohm adapter?

I see you've gotten all kinds of theoretical answers to your question. Here is an actual test case. I already posted the scopeshot _with_  the 50 ohm thru-terminator up above, but I'll put another one here. The specific circumstance in this case is when you are scoping something with an output impedance of 50 ohms -- like most signal/function generators --, and with a cable connection of 50 ohm impedance.

Scope: Rigol DS1054z, unlocked to 100 MHz bw.
Signal source: DataPulse (Systron Donner) DP-101, output impedance 50 ohms, set to produce a 5 V (by knob markings) positive going square pulse train at 10 MHz.
Connection: 50 ohm coax BNC patchcords RG-58/U, two pieces "spliced" with a double-female BNC, total length about 6 feet.
Terminator: Rigol 50-ohm thru-terminator (aka impedance adapter), 18 dollars from TEquipment:
http://assets.tequipment.net/assets/1/26/DimRegular/Adaptor2.JPG

First shot below is with terminator at the scope channel input. Second shot is without the terminator, using the _exact same_ settings on the pulse generator.

Moral: You pays your money, and you takes your chances.

[alsetalokin4017]

And just for the sake of completeness, here's a shot using a BNC T connector and a 50 ohm end-terminator at the scope, as suggested above.
(ETA: But _not_ using a high-z probe (which would indeed be silly) , rather using the same BNC patch cable connection as my above shots.)

[gojimmypi]

another somewhat related question: is there any reason to have the 50 ohm adapter for a scope being used on relatively low frequencies (<100MHz) ?  what specific circumstances require a 50 ohm adapter?

I see you've gotten all kinds of theoretical answers to your question. Here is an actual test case. ....

That's great info, thanks for taking the time to post it.

In this case, you knew your signal source was 50 ohms. When poking around in a random circuit board, I assume the high impedance setting is preferred? Do you have an example of a circuit-analysis situation where there's not an obvious 50 ohm BNC connector.... where one would need an impedance adapter?

thanks again for your responses.

cheers

[alsetalokin4017]

another somewhat related question: is there any reason to have the 50 ohm adapter for a scope being used on relatively low frequencies (<100MHz) ?  what specific circumstances require a 50 ohm adapter?

I see you've gotten all kinds of theoretical answers to your question. Here is an actual test case. ....

That's great info, thanks for taking the time to post it.

In this case, you knew your signal source was 50 ohms. When poking around in a random circuit board, I assume the high impedance setting is preferred? Do you have an example of a circuit-analysis situation where there's not an obvious 50 ohm BNC connector.... where one would need an impedance adapter?

thanks again for your responses.

cheers

You are welcome. Yes, in most cases with low frequency signals -- defined by me as being 10 MHz and under -- one would normally use the standard 10x attenuated passive probe and the 1Megohm scope input impedance, as this setup will load the circuit under test the least. In most cases. This is probably why the Rigol designers didn't put a selectable 50ohm-1Megohm input impedance on their low-bandwidth scopes.

I'm not an EE (although I sometimes pretend to be one on YouTube, hah) but I'm sure that there are more qualified people here who can answer your question better when higher frequencies, active probes, and better scopes are concerned.

I will caution you with another quote from my favorite physicist, though...
"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
--Richard Feynman

Bottom line: Get the Rigol 1054z, unlock it to 100 MHz bandwidth, and also order the 50 ohm adapter-terminator of your choice, and use it when you are patching directly to your FG, or whenever you think it may be appropriate.

[alsetalokin4017]

And even more completeness:

First shot below is the PG's output, scoped with the stock Rigol probe, 10x setting, connected to the PG's BNC output jack, no other termination.

Second shot is the "silly" connection: the probe connected to the PG as above and connected to the scope with a "T" and 50 ohm end-terminator. Blecch fooey.

Of course the PG's settings have not changed in this whole test series.

[Howardlong]

You'll also find the response will change depending on the length of 50 ohm coax from your AWG to the 10X 10M ohm probe point due to the reflections incurred.

Another test you could do is to run an arbitrary length of 50 ohm coax from the AWG, use a 50 ohm thru terminator (or T piece plus terminator) at the other end of the coax from the AWG, and probe with the 10X 10M ohm probe at the thru terminator. The result should be pretty similar to probing with the 10X probe directly at the AWG with the thru terminator.

[TheSteve]

Hah - I thought he meant to use the T connector so he could have the load terminated properly and have a port to connect the tip end of the probe in to.  Which is totally fine until the frequency gets too high. Obviously connecting the BNC end of the probe into the T is just a bad idea all around.

[CustomEngineerer]

I don't know if the DS2000 is the same as the DS1000Z, but try this...

o 1MHz sine
o set the modulation to FM
o modulating frequency 1kHz
o modulate with triangle
o deviation 1MHz

Ahh, I see what you mean. That does work on the DS2000, thank you for sharing.

[nbritton]

Added: https://www.youtube.com/watch?v=OiAmER1OJh4.

Another Dave video! Awesome. thanks

I also came across this National Instruments white paper "Select the Correct Oscilloscope Probe for Your Application":

http://www.ni.com/white-paper/14825/en/

edit: as internet links sometimes change, I've attached the NI Whitepaper

[tggzzz]

I'm sorry but that's nonsense, BW is as much as important as sample rate.

The two should match. If there's a serious mismatch then that's a red flag. You have to wonder who designed the device.

Well, 25 years ago I knew the people that designed HP's front ends, and they certainly weren't fools. A 1GHz 40ns-25MS/s (or was it 25ns-40MS/s?) scope was very useful when observing the signal integrity of sub-nanosecond edges.

If you want to consider the maths of sampling real-time signals, don't forget that if you have an AM audio signal on a 100MHz carrier, you only have to digitise it at ~40kS/s to completely reconstruct the signal. Of course the sampler has to have bandwidth >100MHz.

[Wuerstchenhund]

Well, 25 years ago I knew the people that designed HP's front ends, and they certainly weren't fools.

No, they certainly weren't fools, but they had to work with limitations of the technology that was available back then (although 25yrs ago HP already made 1GSa/s scopes), and which no longer exist in 2015.

[G0HZU]

If you want to consider the maths of sampling real-time signals, don't forget that if you have an AM audio signal on a 100MHz carrier, you only have to digitise it at ~40kS/s to completely reconstruct the signal. Of course the sampler has to have bandwidth >100MHz.

Agreed. AFAIK HP had (low sample rate) sampling scopes with 1GHz bandwidth back in 1960.

My old HP vector voltmeter uses similar front end technology and offers 1GHz BW and it dates back to the mid 1960s.

[gojimmypi]

@gojimmypi:

Please allow me to recommend TEquipment as your USA vendor for the DS1054z. You really _can_ beat Amazon, I think. (Someone will send you the EEVblog TEquipment discount code if you ask for it....)

ETA: Two things: I had to return my original DS1054z under the TEquipment warranty (bad glitch on one channel) and had a replacement in hand in less than a week. And I just took delivery of an Extech LT300 light meter that someone else ordered for me from Amazon... and it arrived with a previously-opened and resealed box, a completely dead battery, and a clumsily reattached screenprotector film..... but it was sold as new.

I appreciate the recommendation. I still ended up ordering the DS1054z on amazon, but TEquipment was the seller. I added the scope to the cart from the eevblog page, as it appears Dave gets a percentage of the sale. I recently learned folks can donate to him as well, which I plan to do (this information is usually hidden in the "show more" section on youtube):

Support the EEVblog through Patreon!
http://www.patreon.com/eevblog

Dave is really awesome! I really like the videos and hope he continues to create content.

I did send an email to TEquipment asking about warranty repairs. Here's their response:

Please be advised that with Rigol you have an Exclusive 30-day money back guarantee will allow you to fully test your investment to make sure it works for you. If it does not meet your expectations, give us a call and we'll work out the return or offer a better solution.
In other words, for the first 30 days after you receive your unit, we will administrate the warranty repair or replacement.
After the 30 days, your warranty is with the manufacturer.

And so with amazon, I get the same 30 day deal - but return shipping is free. It is a ridiculously cool system that I've used on occasion. Amazon let's you print a return label online, then just take the box to a UPS store for free returns. (plus there's a UPS store walking distance from my home).

So TEquipment still received my business, just via amazon. I got a 3% cashback from amazon, Dave gets a cut, and I have piece of mind that I don't have to pay expensive shipping costs if I need to return it. Oh, and did I mention it will arrive on Wednesday? 2-day Prime shipping is awesome. Add to this I cut the cable a few years back and amazon Fire TV is the only thing on my screen.  But I regress, this is sounding more like an amazon commercial. 

This has been really crazy. I spent more time shopping for a $400 oscilloscope than I think anything else I've bought in recent memory. lol.  If it had been a Tek, I don't think I would have hesitated so much. My only concern is the tiny size. Again, an HDMI cable on the back would be just fantastic.

I'm really excited to play with my toy! I haven't been this excited since my Heathkit Multimeter package arrived in the mail when I was 14.     (and yes, I still have it!)

Thanks again everyone for their input, particularly since I certainly was not the first one to ask the "which oscilloscope" question.

[Rupunzell]

History of Sampling technology:
http://w140.com/kurt/50_years_sampling.pdf

hp used Ghz sampling technology in more than just the 8405A Vector Volt Meter, hp 3406A sampling RF voltmeter, hp 4851A Vector Impedance meter (study the 4815A probe) and others, circa early 1960's technology.

The video folks were pushing slightly past Nyquist using pulse discriminator and got useable information recovered.

Sampling information systems are not new, GHz sampling systems have been around in instrumentation and systems for over 50 years now. What has changed is the data centric technology world of today putting real world analog conversion to bits of data central to many information systems today.

-Yet most tend to forget the ways of Nature remains analog.


Bernice

If you want to consider the maths of sampling real-time signals, don't forget that if you have an AM audio signal on a 100MHz carrier, you only have to digitise it at ~40kS/s to completely reconstruct the signal. Of course the sampler has to have bandwidth >100MHz.

Agreed. AFAIK HP had (low sample rate) sampling scopes with 1GHz bandwidth back in 1960.

My old HP vector voltmeter uses similar front end technology and offers 1GHz BW and it dates back to the mid 1960s.

[gojimmypi]

I came across this article & thought I'd share for any future readers of this thread regarding the high impedance vs 50 ohm oscilloscope input topic:

"What's Wrong with my Function Generator? (hint: nothing)"
http://www.effectivebits.net/2011/12/whats-wrong-with-my-function-generator.html

[coppice]

Sampling information systems are not new, GHz sampling systems have been around in instrumentation and systems for over 50 years now. What has changed is the data centric technology world of today putting real world analog conversion to bits of data central to many information systems today.

As soon as Shannon formalised the sampling theorem in a way that engineers took notice of (as opposed to the various formulations of the same theorem by various people in maths and statistics many years before) people were busy making the narrowest aperture samplers they could for various jobs. Oscilloscopes were an obvious use.

[tautech]

I came across this article & thought I'd share for any future readers of this thread regarding the high impedance vs 50 ohm oscilloscope input topic:

"What's Wrong with my Function Generator? (hint: nothing)"
http://www.effectivebits.net/2011/12/whats-wrong-with-my-function-generator.html

On a similar topic there is a thread here:
https://www.eevblog.com/forum/testgear/show-us-your-square-wave/

[Wuerstchenhund]

IMHO people still build very low sampling rate and very high BW scopes nowadays. This is the only feasible way to get low cost and very high BW scopes. A good example is PicoScope 9000 series, with 10G BW and only 1M sampling rate.

Yes but these are essentially niche products which come with tons of limitations (plus the general annoyance of being USB scopes), however as you said they are also relatively cheap.

With real time technology there is no way to get such a piece of equipment at only $9000.

No, but then, low cost isn't always a top-most priority, and the benefit of having a capable real-time scope often outweighs the price difference.

At the end of the day it comes down what you want to do and how big your budget is.

[gojimmypi]

Now that my DS1054z is in transit, I realized I forgot to ask as important question: what sort of transient response should I expect from a digital oscilloscope? Will I be able to see switch bounce noise? or if some spike occurs between samples, I'll never know, right? I think I have only ever used an analog scope. But a 100MHz scope should see most transient stuff, right?