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Tek 2400 series digital scope usability question

Discussion in 'Electronic Design' started by Hawker, Mar 15, 2007.

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  1. Hawker

    Hawker Guest

    I spent this morning trying to track down a clock problem only to find
    out that it was the scope and not the clock. Wasted morning.

    I was looking at a 24MHz clock. The good scope (a TDS520B) was being
    used so I figured for only 24MHz I could use the Tek 2430A (a 150Mhz,
    100M/s scope. The clock looked like a sine wave (not square) had bad -
    visible, jitter and seemed to be amplitude modulating at some lowish
    frequency. Three issues. Looking at it with my trusty 2430A (also a
    150Mhz but analog scope) and TDS520B it looked as it should.

    I'm wondering if the issue was solely to low a sample rate (only 4
    samples per clock). I had hoped to upgrade the TDS scope and get rid of
    the 2430A but I can't afford what I want now. So I was thinking of
    getting a 2440 (300Mhz, 500m/s ) but if I can't look at a 50MHz square
    wave accurately with it then it won't do me much good. That would be 10
    samples per clock at 50Mhz. Since there were three issues with looking
    at it with the 2430A I wonder if all three can be explained and will not
    be issues with the 2440 at 2x the frequency and 5x the sample rate.

    Is this going to work or will I be disappointed?

  2. Joerg

    Joerg Guest

    To gauge the quality of a 50MHZ clock you'll need 1GSPS, at least.
    Better 2GSPS. Repetitive scans at any lower rate plus piecing together
    isn't going to work when you suspect a noisy clock. About a month ago I
    had to look at laser diode noise, became frustrated with their digital
    scopes and convinced that client to buy a used Tek 2465. One week and
    about $350 later that allowed us to get the job done ;-)

    Another downside of digital scopes is their rather limited vertical
    resolution or effective number of bits.
  3. PeterD

    PeterD Guest

    IMHO, a 100 M/s scope won't give you a very good indication of a 25
    Mhz waveform, especially a square wave. In fact, I'd expect it to look
    almost exactly like the way you described it! <g>

    Remember, a square wave consists of two frequencies, the slopes
    (leading edge, and trailing edge are very high frequency) while the
    middle part between the slopes constitutes a low frequency.
  4. Digital scopes with low sample rates like this have to use repetitive
    sampling techniques and not real-time sampling for high frequencies.
    This can be very troublesome and cause lots of issues, and is why
    almost all new DSO's are "real-time" sampling that have a sample rate
    at least 10 times the analog bandwidth.

    If your trigger point is stable then your 100MS/s 150MHz DSO *should*
    be able to display a stable *repetitive* signal at up to the 150MHz
    bandwidth. But once you single shot capture it, it is using real-time
    mode and you only get your 4 samples per cyle for a 25MHz signal.

    A 500MS/s scope will be *just* ok with a 50MHz signal, as 10 times
    minimum is the general rule of thumb. But you will most likley be a
    bit dissappointed with the 500MS/s, I would be aiming at a 1GS/s scope
    fro this kind of work. Even the cheapest sub $1000 entry level scope
    can do 1GS/s, albeit with limited sample memory size (a few KB) and
    analog bandwidth.

  5. Not if the trigger point is stable and you aren't using single shot
    capture, it will work fine in repetitive mode. That is why
    manufacturers actually make 150MHz bandwidth scopes with 100MS/s
    sampling, if it didn't work they would make the bandwith only 50MHz or
    so at best. They also make 20GHz bandwidth scopes with 100MS/s
    sampling etc.

  6. Joerg

    Joerg Guest

    It consists of a whole lot more frequencies. If you can't acquire up to
    the 7th-9th harmonic you won't get much of a display to write home about.
  7. Joerg

    Joerg Guest

    "Troublesome" and "stable repetitive" are the right words. If there was
    indeed a clock problem that can easily turn the displayed trace of a
    repetitive scan into pulp.
  8. john jardine

    john jardine Guest

    Wouldn't trust 'em as even a door stop.
    Last week I used a customer's expensive 100's of MHz TDS to look at a slow
    synching pulse alongside a 50Hz 5V sawtooth. At 100ms/div noticed bad
    distortion on the sawtooth. Did not waste time investigating as previous
    experience said "suspect the digital scope first". At 5secs/div my sawtooth
    magically reappeared in all it's glory but now apparently .05Hz. Damned
    waveform had been aliasing all the way down and was worthless. (I won't
    mention the hit and miss display of the synching pulse)
    Nice coloured traces.
    Nice printouts for those important management presentations and emails.
    Nice FFT display.
    Small and light.

    Tiny, lo-res, pixelly display. May as well be looking at a ZX81 screen. Kind
    of "is that my waveform or a display artefact"?.
    Unable to trace low duty cycles.
    Continual "am I seeing the real waveform or some alias"? and am I really
    generating all that noise?.
    As a technology demonstrator marvellous, as a 'scope f****** worthless.
  9. For sure.
    You get lots of nice random dots on the screen though, and it's kinda
    pretty to look at :->

  10. Joerg

    Joerg Guest

    Younger users may like to push the obfuscation button which collects
    lots of those random dots and makes it all look like graffiti :)))
  11. Hawker

    Hawker Guest

    That is the information I thought, but didn't want to hear.
    The reality is my 500Mhz 1 G/S TDS520B (I think) is starting to show
    it's age. There are signals I need to see but it can't show very well
    (esp outputs from CCDs in one sweep). I had hoped to make it my second
    scope, get rid of the 2430A and get a new scope (TDS 600 or TDS700
    series?) but looks like I need to shell out $4k-$10k for one of those
    and I don't have it now - probably not till the end of the year. I may
    get a 2440 just to get me by for now. I assume it will hold it's value a
    bit. I looked at TDS300 and TDS400 stuff but other than active FET
    probes they actually look like a step down from a 2440.

    Anyone have a suggestion for a basic feature, active FET capable scope
    with 500Mhz Bandwidth and 2 G/S (perhaps 4-5GS?) resolution? No fancy
    features, although FFT would be handy. Step up from a TDS520B. I assume
    I'm looking at used but if the price is right I could consider new.

  12. Joerg

    Joerg Guest

    Quite frankly I have never found the FFT feature on "modern" scopes that
    useful. If it was me I'd go for a used 2465 plus a nice 1GHz+ spectrum
    analyzer for any spectral investigation. Should be able to obtain that
    for under $2k total if you don't mind boat anchor size. That, plus a
    simple digital scope if you need to be able to see pre-trigger.
  13. For this performance you won't get new scope for under around $4K or
    But there are plenty of used Lecroy's on Ebay with 500-1GHz bandwidth,
    2GS/s+, and massively deep memories.
    They are a dog to drive, but the performance is there, and the price
    is right at under $2K
    This is but one example:

  14. Jim Yanik

    Jim Yanik Guest

    you can use a FET probe with a 2400 scope;you just have to power the probe
    externally.TEK used to make an external supply expressly for this.
  15. I just got my first DSO. Honestly so far I'm really impressed compared to
    my old Hitach V650-F. It's certainly allot easier to see what happened
    before the trigger event. ;-)

    I also got my first taste of aliasing too, kinda scarey. I was watching a
    PWM that updates ~80 times/second on channel 1 and put the CLKOUT from the
    PIC onto channel 2. CLKOUT was 2MHz. Instead of seeing the nice thick
    "bar" across the screen as I would expect on an analog scope, I could see
    the clock signal and it looked just like the clock signal, only running
    wayyyyyyyyyy tooooo slowwww. That's the part that really surprised me: It
    looked EXACTLY like the CLKOUT signal including the little voltage spikes.

    OTOH, it's the most amazing thing I've ever used. :)
  16. Hawker

    Hawker Guest

    I have a 2445 and access to a 2465b but honestly don't use it much.
    When I need high speed analog I need FET probes which the 2400 series
    stuff does not have. But more often then not I need to do fancy
    triggering and holding (trigger on an event and hold the capture) that
    can only be done with a DSO. The analog stuff gets some usage but not
    much anymore. I hate to admit it because for what it can do it usually
    is better than any digital scope.
  17. Joerg

    Joerg Guest

    Ok, depending on your typical design a DSO might indeed be the better
    choice. Definitely when chasing digital state changes where you don't
    want to crack out the logic analzyer.

    As Jim said a FET probe can be powered externally. I use a Philips FET
    probe on a Tek scope and they never even growled at each other.
  18. john jardine

    john jardine Guest

    Yes. Other than good sex, there's nothing finer than a new toy turning up in
    the mail :)
    My main scope is a straightforward 100MHz Hitachi V-1065A but look forward
    to the day I can dump it in favour of some digital model. Probably be a wait
    as they'll need to offer something realistic such as a 1THz sample rate at
    10bits and a display of 1024x768 with screen update at 70-100HZ
  19. Lionel

    Lionel Guest

    Three samples per cycle is inevitably going to give you a horribly
    distorted waveform. Ten samples per cycle won't be pretty, but should
    be usable.
    If you're pushing the sampling speed that way, an analog scope is
    probably more useful.
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