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DIY sampling "scope"

Discussion in 'Electronic Design' started by PFC, Dec 14, 2012.

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  1. PFC

    PFC Guest

    Hi guys !

    I'm looking for a solution to measure PCBs, power supply decoupling
    performance, input impedance of stuff that has inputs, etc...
    I've got a 60 MHz network analyzer, works great, but I'd like to go a bit
    higher in frequency, without spending megabucks.
    And suddenly, this post from 2008 :
    So, if someone here has interest in designing the fast stuff (or just
    throwing some schematics), I'd gladly volunteer for the rest (usb, micro,
    adc, software, gui, etc).

    Basically a board which :
    - outputs a fast pulse or edge
    - has an input with a sampler running synchronous to the output, with a
    delay sweep
    - ADC, usb, pc, display

    With a Cypress FX2 ez-usb chip, streaming USB2 at full bandwidth is
    simple, I've got a board with that running from a previous project.

    Anyone interested ?
     
  2. Nico Coesel

    Nico Coesel Guest

    Thats the easy part. I've been playing around with a similar idea. The
    biggest problem is triggering on signals with frequencies in the GHz
    range.
     
  3. Nico Coesel

    Nico Coesel Guest

    Aren't there a lots of TDR kits out there already? If I enter 'usb
    tdr' into Google I get lots of companies selling USB TDR kits.
    Software is an important selling point these days.
     
  4. PFC

    PFC Guest

    I wasn't thinking about building a GHz scope, that would be way too
    ambitious.
    What I'd like would be a small part of what such a machine can do, for a
    very small fraction of the price.
    I'd just like to send a pulse into stuff and look at what comes out, at
    hobby prices (ie, couple hundred bucks not thousands).
    That would be an extremely useful tool. With separate output and input, I
    could also send the stimulus to logic circuits or mosfet drivers, and see
    what comes out, probe the supplies with a wide bandwidth, etc.

    BTW it's not a commercial project, I live in a mostly communist country
    anyway, so profits ... lol.
     
  5. FR is more communist than CN if you look at the percentage of GDP that
    is government controlled.
     
  6. Nico Coesel

    Nico Coesel Guest

    It would make sense to make a GHz scope. There are so many TDR boxes
    out there already that it would be hard to come up with an ultra low
    cost solution that adds something to the current landscape.
     
  7. Guest

    Sounds like good fun. Do keep us informed!
     
  8. PFC

    PFC Guest

    That's way faster than I'd need, but if you can pull this off on FR-4
    without megabucks, then I'm drooling.
    What about using SATA, USB3, or other cheap, mass-produced, highspeed
    cables for signal ?
    I'll try to make a quick mockup.

    How many bits are realistic in the ADC ? 8 ? or is it possible to hope for
    more ?

    For a TDR, there is :

    - a clock, used to generate a pulse
    - and a controlled delay, used to trigger the sampler

    The delay could be a fast ramp + comparator, the ECL delay chip whose ref
    I don't remember, or one of the new, interesting fast clock buffer chips
    with embedded PLL and adjustable delay.

    Using 2 oscillators doesn't seem too good, since it relies on long-term
    jitter being low, a delay relies only on period-to-period jitter, which is
    easier to get low.

    For a scope,

    - a trigger (ie, highspeed low jitter comparator)
    - and a controlled delay

    An equivalent-time scope can only examine perfectly periodic signals, with
    a very short period, and for triggering, it needs an edge on the signal
    that is always at the same spot... that's a pretty large restriction...
    observing more kinds of signals means more trigger complication.

    OK, feature creep kills projects, so a possibility of plugging in a
    trigger/delay daughterboard (replacing the default clock/delay) would be
    nice.
     
  9. Tim Williams

    Tim Williams Guest

    I'm looking at the manual but don't see anything of the sort. Cite?

    Tim
     
  10. mike

    mike Guest

    That's restrictive. All you really need is jitter statistics such
    that you get enough samples falling in the view window to produce
    a trace in the time you're willing to wait.
    It's important to realize the difference between a hobby project and a
    business plan.
    A typical example is the spectrum analyzer.
    Yes, there are lots of hobby projects that take a TV tuner and create
    some blips on the screen.
    But 30 Hz. resolution bandwidth and -124 dBm noise floor and 21GHz.
    range is 40 year old technology.

    It's easy to think that you can engineer/produce/support a TDR system
    for less than an existing company with established
    marketing/service/manufacturing infrastructure.

    It's easy to envision simpler ways to do stuff...unitl you try to
    implement it.
    The devil is in the details.

    And, yes, innovation happens...but you won't likely get the secret
    sauce in a newsgroup...at least not from anyone who know how the
    patent system works.
     
  11. Bill Sloman

    Bill Sloman Guest

    Communism is dead. France is moderately socialist, but elitist and
    more inclined than most to believe in the advantages of central
    planning, but they aren't silly enough to let the highly educated
    bureaucrats in the central planning office execute any of the plans.
    That's left to regular commercial companies operating in the free
    market. Quite a few of them have significant state share-holdings, but
    French productivity is respectable and they have been known to do good
    stuff, though their "experts" do tend to be a little impractical.

    The French bourgeoisie hate income taxes, and the French collect most
    tax as valued-added tax, which is not progressive and in fact collects
    a rather lower proportion of income from the rich than from the rest
    of the population. Their income inequality - Gini index of 32.7% - is
    marginally worse than Canada's at 32.6%

    http://en.wikipedia.org/wiki/List_of_countries_by_income_equality

    but it's way lower than China (42.5%), the US (40.8%) or
    Russia(40.1%), where the ruling oligarchies reserve most the goodies
    for themselves.
     
  12. Bill Sloman

    Bill Sloman Guest

    Motorola/ON-Semiconductor MC100EP195

    http://www.onsemi.com/pub_link/Collateral/MC10EP195-D.PDF

    It does 10nsec, so you have to use it with a 100MHz oscillator,
    counting clock edges for longer delays. Temperature stability isn't
    great (see figure 4 in the data sheet) and when I was planning on
    using it's predecessor I figured on automatic recalibration every few
    minutes - you use it to set up a pulse-width modulated waveform and
    measure the DC level of the filtered output with a decent A/D
    converter - 16-bits IIRR.

    It might be easier to measure it's temperature with a thermistor and
    use a resistor to keep it's temperature stable.
    DDS chip?
    Protecting the fast comparator, and giving it a balanced signal -
    external trigger versus external ground - can keep busy for a while.

    <snipped the obvious>
     
  13. Fred Abse

    Fred Abse Guest

    PDF scan posted to a.b.s.e.

    Where you'd get the TDs at sensible prices, I don't know.
     
  14. Tim Williams

    Tim Williams Guest

    Ah, found it. But this is self-triggered, so it can't be used for
    equivalent time sampling a waveform, only the highest point on the waveform.

    You could add a pulse generator to it, but then you need the timing
    generation, and the pulse generator, which again necessitates an accurate
    trigger generator. I don't see how this helps.

    Tim
     
  15. Fred Abse

    Fred Abse Guest

    I have a Tek TDR/sampler plugin that does exactly that.

    Stated to be <25ps. rise. As far as I can measure, it is.
     
  16. Fred Abse

    Fred Abse Guest

    7S12. I don't have the manual for the pulser, only the main TDR, but the
    waveform looks very TD, and it's only 300mV.
     
  17. mike

    mike Guest

    Don't know which head you have.
    The S-50 is a 50mA TD.
    The S-52 is a TD, but couldn't find the spec in the time I was willing
    to spend.
    Probably 50 or 100mA.
     
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