Connect with us

Interesting feature of an inexpensive function generator

Discussion in 'Electronic Design' started by Charles, Jul 2, 2013.

Scroll to continue with content
  1. Charles

    Charles Guest

    Hi,

    I have been using an GW Instek SFG-2110 and find that the TTL output has a
    fast rise time. It is about 2 ns and this could be useful for oscilloscope
    bandwidth measurements for instruments of less than 100 MHz.

    The TTL option seems to be missing on more expensive function generators.
    Any thoughts?
     
  2. Phil Allison

    Phil Allison Guest

    "Charles"

    ** Using the rise time of some TTL gate to calibrate a 100MHz scope is not
    gonna be very accurate. How do you check the rise time of your source ? What
    about issues with the coax linking it to the scope ?

    The easiest way to check scope BW is to use an RF generator - any decent one
    with levelled output will do, long as it goes out to a couple of hundred
    MHz.

    Anecdote:

    On more than one occasion, techs have questioned the practice of using a
    10MHz or more BW scope to test a piece of electronic audio equipment for
    flat response from 20Hz to 20kHz and beyond.

    They asked :

    " How do you know your scope has a really flat response ? "

    I replied:

    " Square waves ( audio frequency understood) look perfect " .


    .... Phil
     
  3. miso

    miso Guest

    They use a special pulse generating diode to calibrate scopes. These
    items are peddled all the time on ebay at exhorbintant prices. Like $250
    for what gets about $5 at the flea market.

    Just search for Tektronix tunnel diode pulser on ebay to see one.

    As far as I know, they are just diodes with RF connectors attached. You
    could probably roll one yourself if you knew the secret sauce.
     
  4. Jon Kirwan

    Jon Kirwan Guest

    Try:
    http://w140.com/tek_067-0681-01.pdf

    Jon
     
  5. tm

    tm Guest

    Try Ebay for the Russian tunnel diodes. Cheap and most work. Get the Ge
    ones, not the GaAs types.
     
  6. Search TekScopes Yahoo Group for "DIY TD Pulser". That is my version of the
    venerable Tek 067-0681-01 using russian TD still available from Ebay.
    Everything is there including all construction details, photos, and actual
    test results. YMMV but that one I built and put on TekScopes gives something
    like 35pS risetime and takes one evening to build.
     
  7. Charles

    Charles Guest

    "miso" wrote in message
    They use a special pulse generating diode to calibrate scopes. These
    items are peddled all the time on ebay at exhorbintant prices. Like $250
    for what gets about $5 at the flea market.

    Just search for Tektronix tunnel diode pulser on ebay to see one.

    As far as I know, they are just diodes with RF connectors attached. You
    could probably roll one yourself if you knew the secret sauce.

    http://www.jensign.com/avalanchepulsegenerator/index.html

    A way to do it using the avalanche mode of a readily available transistor.
    That's the good news. The bad news is that the avalanche mode is device
    dependent.
     
  8. Chris Jones

    Chris Jones Guest


    If you are satisfied with rise and fall times of about 250 - 300ps, then
    you can use a cheap 74LVC04AD or similar.



    Unfortunately that particular board didn't have sufficient low-frequency
    decoupling (it was intended for use at much higher frequencies where the
    decoupling was adequate), so the power supply cable inductance rings
    badly with the small decoupling capacitors on the board. If he had
    looked at the fall-time instead of rise time, and left my ac-coupling
    and back termination in, it might have been a bit cleaner. The best
    option would be to improve the local low frequency decoupling.

    Chris
     
  9. John K

    John K Guest

    Chris, how do you get 250 ps risetime? I tried to find the datasheet on
    Octopart and they have started using captchas for rate limiting. I could
    never get past that screen.

    I did get into Mouser and found the part is made by TI and NXP. Neither
    datasheet said clearly what the output rise and fall times were. The best
    I could find was a table that had numbers like <2.5ns, but it wasn't
    clear if that was for the input or the output.

    I'd like to find out for sure if that family really does 250ps risetime,
    but I'd be surprised if it does. Do you have any more information?

    Thanks,

    JK
     
  10. Chris Jones

    Chris Jones Guest

    The ~280ps risetime was found by measurement. If you want it guaranteed
    then you'd need a more expensive chip. It is only a typical value on
    even ADI's fast comparators ADCMP572 etc. In practice rather than paying
    a manufacturer extra to guarantee it, you might be better off buying a
    lot of 74LVC chips from the same batch, measuring a few and then knowing
    that this is what you have.

    You can see the risetime figure briefly on the screen of the scope in
    the video I linked. I measured it previously with a similar scope and I
    got about 280ps, depending on supply voltage etc. That was for an old
    Philips chip, so now that would be called NXP. I have a few of those old
    chips left over, and I know they are fast. No idea whether they use the
    same chip layout and fab now, maybe JT can advise.

    Chris
     
  11. John K

    John K Guest

    It was very difficult to follow the video. He kept rotating the time axis
    knob back and forth. Too fast to actually look at the waveform and try to
    understand what it is saying.

    I am surprised the risetime can be that low. It takes very fast
    transistors to do that. If that is true, why is the propagation delay so
    long? You'd expect with transistors that fast, the prop delay would be
    down in the picosecond range instead of 4.5ns at 3.3V as shown here

    "http://datasheet.octopart.com/SN74LVC04APWR-Texas-Instruments-datasheet-
    114799.pdf"

    I don't have any of these devices on hand and do most of my stuff in
    10GHZ ecl so I really don't have a need for them. If you have access to a
    fast scope, could you take a picture of the risetime?

    Thanks,

    JK
     
  12. Chris Jones

    Chris Jones Guest

    Note that the rise and fall time of the 74LVC04 is quoted in the
    datasheet with a 30pF or 50pF capacitive load. Instead, I measured the
    risetime with the output connected through a resistive back-terminating
    network to a matched coaxial cable, which would be expected to give much
    faster edges.

    Each inverter in those 74LVC04 chips would contain several (an odd
    number of...) inverter stages in a chain. They would probably also have
    ESD resistors at the input which would somewhat slow down the
    propagation delay to make it likely to be several times greater than the
    best possible rise and fall times. I think they probably use a 0.5um
    CMOS process guessing from the absolute max. supply voltage rating. On a
    0.18um process I think a single inverter stage has about 100ps
    propagation delay.

    Jim Thompson might know a lot more about it than that, and might even be
    allowed to tell us.

    I no longer have access to a fast scope so I can't give you any plots,
    but I can confirm that you could get a pretty clean looking square wave
    with <300ps rise and fall times out of that very board that was on the
    video.

    Chris
     
  13. John K

    John K Guest

    Where do you see a spec for the rise and fall times? I tried several
    different manufacturer's datasheets and all I can find is the prop delay.

    I can see the mfgr's wanting to eliminate every possible spec parameter
    that they can get away with. This reduces cost and removes the loss for
    devices that don't meet the spec.

    But I'm old school. I want as many spec parameters I can get:)

    JK
     
  14. Chris Jones

    Chris Jones Guest

    http://www.nxp.com/documents/data_sheet/74LVC04A.pdf
    Table 8

    In any case, removing the capacitive loading will speed up the output
    waveform, both risetime and propagation delay.

    Chris
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-