Need advice/help picking an o-scope

Discussion in 'General Electronics Chat' started by steinar96, Jul 20, 2009.

  1. steinar96

    Thread Starter Active Member

    Apr 18, 2009
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    Right now i'm looking at 2 USB Velleman o-scopes. One of them is 15Mhz while the other is 60Mhz.

    The 60Mhz is aprox 615$ while the 15Mhz one is slightly cheaper at 400$.

    I'm tempted to buy the 60Mhz one but i'm asking myself what i can do with the 60Mhz scope that i can't do with the 15Mhz one. If anyone could shed light on which kind of circuits i'm unable to analyse between this Mhz gap would be greatly appreciated.

    Looking at the Mhz alone i'm getting proportionally more for the money with the 60Mhz one. But then again if i can do almost any analysis on RF circuits with the 15Mhz buying the 60Mhz might be unnecessary:confused:
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    What is the application? You can't see logic with a 15 MHz o'scope, and 60 MHz is awfully iffy. Neither is likely to be satisfactory for RF - you can't see rising edges.

    I'm sure other opinions will show up...
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Remember that the 15 MHz scope will not give a good visual representation of a 7.5 MHz, think Nyquist, sine or square wave. I have always thought that it takes around 10 points for a period of a square wave to look visually nice. That leaves you around 1.5 MHz for digital signals if that is the direction you are going.

    So, back to beenthere's question, what is the application?

    I use a 100 MHz oscope at home that I got off the internet for $150. It has exceeded anything that I have needed it for to this point. But, the day may not be far off that I will be in the oscope market again.
     
    Last edited: Jul 20, 2009
  4. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    I'm buying a o-scope for current and future projects. My current project is a SMPS but that is only in the Khz range so the 15Mhz one should be enough. But a possible project coming up does involve some RF circuitry but at the moment i'm not sure of the frequency it will be operating at.

    Could you explain for me beenthere what you mean by "iffy" . Are you suggesting i try to get my hands on an even more powerfull scope. StayAtHomeElectronic's post seemed to point in that direction.
     
  5. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    OK, I'm not an EE, so I can't give you the EE's design perspective. But I have been using scopes for over 40 years and have some nodding familiarity with them.

    I consider the scope to be nearly the most important general purpose measurement tool available. The only thing that comes close to it is a good digital multimeter (asking me which one to give up first would kinda be like asking me which kid I want shot first).

    Clearly, what you want to use the scope for will define your needs. Alas, those of us who are hobbyists get our needs defined by our pocketbooks or what we can sneak past the wife without being eviscerated. We'll usually decide on a maximum amount we can spend, then look for what fits the bill.

    I'll give some of my opinions on what's important in buying a scope.

    My first scope was a 25 MHz Phillips dual channel delayed timebase analog scope. I bought it around 1977 or so for around $1700 or $1800 (I was single then -- i.e., I had no financial flywheel at the time). It was an excellent scope and I gave it away to a friend a few years back. He gave it back to me when he moved and it's sitting here next to me; still functional but unused.

    Reflections on that scope after 30+ years: the delayed timebase wasn't used terribly often. In retrospect, if I could, I would have rather had more bandwidth than the delayed timebase. The scope was (and is) quite handy; I still have and use the original probes that came with it. The manual is also superb -- all the schematics and troubleshooting information is given (but I've never needed it).

    My second scope was a used HP digital storage scope purchased for $600 on ebay. I bought it about 8 years ago and it was a great purchase for the money. It's a 100 MHz four channel scope and has an optional unit on the back that does math and nonvolatile storage. It was manufactured around 1991. I decided to purchase it because I had used a similar one at work; they were so easy to use that no manual was needed.

    I use a digital camera and a little plastic hood I glued up to take pictures of the waveforms on this scope, as I don't know how to get the saved waveforms (I don't have a manual and it's not that important to me anyway). I've taken hundreds if not thousands of pictures with it and it has paid its dues.

    I use digital storage virtually every time I use the scope, especially for transient or low repetition rate signals. I simply wouldn't consider an analog scope anymore, even an analog storage scope. The digital features are just too handy.

    I'd recommend you consider buying a good used digital scope on ebay over a new scope. If you're patient and study the market so you know what things are worth, you can get fabulous deals. I got an essentially brand new HP E3615A DC power supply for $100 delivered and it's the most used tool on my bench. Sometimes it can take years of watching and bidding. Most of us aren't that patient.

    Let's talk about bandwidth. In general, more is better. I use a rule of thumb to tell me how useful a scope will be: divide the bandwidth by 10 to tell you what signals you'll find it useful with. Of course, we all know a well-behaved scope will look at sine waves in a gentlemanlike fashion up to its bandwidth rating and display the amplitude at about 3 dB down. And the good scopes can look at and measure sine waves at twice their bandwidth as long as you've characterized their response (the Phillips scope mentioned above will trigger on and display over 60 MHz sine waves). But the signals that one looks at aren't typically sine waves. That rule of thumb tells me about what frequency of square wave I can look at and see a reasonable facsimile. Just as important is the risetime, which determines what kinds of measurements you can make (i.e., is the risetime of the signal you're looking at real or a limitation of the scope?). For digital scopes, the sampling rate and buffer sizes are important.

    Something that's hard to quantify is how well a scope can be triggered. Some scopes are horrible; some walk on water. Look for a trigger holdoff -- some digital scopes don't have it, yet it can be vital to get stable displays. I always test a scope on normal (i.e., not auto) triggering. If I can't get stable displays on practically anything my function generators can throw out, it's not a nicely-triggered scope. My experience is limited, but I almost feel the old analog scopes do a better job of triggering than today's digital scopes.

    On a digital scope, a deep memory (i.e., lots of waveform memory) can be a lifesaver when you're looking at transients and rare events. Sometimes you have to let a scope run overnight in the lab in the hopes of catching some event. I used to do firmware testing on SCSI disk drives and we were driven crazy by the lack of depth in our logic analyzer traces (we knew there were bugs in the firmware, but we couldn't track down what happened). Sometimes you want to see a pretty good distance behind the trigger event and that's where these deep memories help.

    I also use the digital math functions more than I thought, especially multiplication to get power waveforms. Subtraction has long been useful to get differential measurements using two single-ended channels. I personally don't use FFTs too much, as I can usually look at the time-domain signal and see what I need to see.

    I fondly remember a vacuum tube single channel Tektronix scope I used in the 60's in school. It had about a 4 MHz bandwidth and a small circular screen. The display was absolutely razor sharp and the thing was a dream to trigger. I don't remember the model number, but it was my favorite scope (within its operating range) of all I have used, even over the Tek 5000 and 7000 models. You could even pick it up and carry it around by one hand. :)
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,344
    Hello,

    Take a look at the PDF's from tektronix.

    Greetings,
    Bertus
     
  7. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    Both the 15Mhz and 60Mhz scope i'm looking at are USB one's connected to a laptop. Which makes analysis easier then a normal scope desk scope i would guess. So i'm not sure the thumb of rule devision 10 factor is actually so high as 10.

    Still having some difficulty making a decision whether i should try to find a higher then 60Mhz for reasonable price so every extra opinion is greatly appreciated :)
     
    Last edited: Jul 20, 2009
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    There is still plenty of functionality in a 15 MHz or 60 MHz scope. Analog signals in the 1 and 2 MHz range are still pretty fast. The entire ADSL band, which can bring up to 7 Mbits/second of data into you house for broadband internet connectivity, only uses signals up to about 1.1 MHz, at least that is what the standards had a few years ago.

    If portability is an issue, many of the ebay scopes will not qualify.
     
  9. knaaphix

    New Member

    Apr 1, 2009
    9
    0
    use a 100 MHz scope it's better than 15 MHz
     
  10. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
    141
    Good point; I left that out because that wasn't a choice when I was buying scopes. I think these would be wonderful for a field service person or when one had to travel light. But for a bench, I don't think I'd ever consider a USB scope; I'd rather have the knobs handy. Even with a touch screen, a virtual instrument is nowhere near as convenient as a real one with knobs (and I've used both kinds many times).

    If you're going to go the route of a USB scope, an absolute priority in my mind would be BNC inputs. The reason I say this is I've read of ones that don't have them. Now truthfully, the size of a BNC is from the vacuum tube era -- a much better choice from a size standpoint would be SMA for modern instrumentation. But BNC is the practical standard for probes and interconnection.

    As far as bandwidth, you will one day come up against a measurement problem where you hit your head on the measurement ceiling. It's guaranteed. So it's wise to get as much bandwidth as you can afford. But an enormous amount of work can still be done with even a 5 MHz scope -- just as long as you're aware of the limitations.
     
  11. steinar96

    Thread Starter Active Member

    Apr 18, 2009
    239
    4
    Thank you greatly someonesdad. Your input has been very helpfull;). At least i'm a bit closer to deciding. Wont be buying it for prolly a week or so. So i have some time to digest this. It's a quite expensive investment so it'll propably pay off to be sure that what i'm buying is enough for upcoming projects:)
     
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