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Time Domain Reflectometry

Discussion in 'Electronic Design' started by [email protected], Apr 10, 2007.

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  1. Guest

    I'd like design a pulse generator, for the fun of it, and because I need
    to make some TDR work, together with my Tektronix TDS2024B oscilloscope.
    I saw the circuit by Tomi Engdahl, which uses a 74AC14 (schmitt inverter),
    but I am also tempted to use the Maxim DS1040Z-A15, which is a programmable
    pulse generator (from 5ns to 15ns, through five 2.5ns steps).

    My doubts are about driving the (50 ohm) cable:

    what if I connect the chip directly to the cable? It is specced of being
    capable of 50mA "short circuit current" which, if I am capable of doing
    the basic math, means that the driver has an impedance of 100ohm (at 5V),
    thus it is mismatched.

    If I connect two of them in parallel I should get a good match for 50ohm,

    And what if I only use one driver? The signal will be smaller, or there
    will be immediate reflections possibly destroying the IC?

    Last but not least, some theory: what's the point in having ~picoseconds
    rise time, when anyway you drive one hundred meters of 100pF/m cable with
    a 50 ohm impedance in serie?
    Before the impulse has traveled one meter, it has already been smoothed
    out to ns-range rising/falling times.. and much worse after tens of meters.

    Having a 1ns rise time is going to be worse than 10ps rise time just like
    adding a couple of meters of cable, or am I mistaken? I don't really get
    the point in having ps-scale impulses for such applications, even more
    because of obvious price considerations.

    About the 74AC14, Tomi's circuit uses 5 of them in parallel, with a 220
    ohm resistor each, giving a "cumulative" output impedance of 50 ohm. But
    what's the point in paralleling five buffers, to strenghten the current,
    when you add resistors in serie at each output anyway?

    And, if it makes sense, can I parallel even more cheap 74AC14's to improve
    the pulse generator performance?

    Tomi's circuit can be seen here:

    Thank you!
  2. Robert Baer

    Robert Baer Guest

    1) Forget non-existant Maxim crap.
    2) CMOS gaes can be paralleled to give whatever drive you want.
    3) You can use a resistive "tee" for matching.
    4) *NO* CMOS parts have picosecond rise, fall or delay times.
    5) Try ECL type logic (offset the supplies if need).
    6) Or try non-linear parts in the "tee" (eg: saturating L for the
    input element).
    7) Or try a snapoff diode.
    8) Or an Esaki diode.
  3. Guest

    If you are driving a 50R cable, it is a good idea to drive it from a
    50R resistive impedance, so that any reflection back from the cable
    is absorbed in the source termination, rather than reflected back into
    the cable, to be reflected back again and again.

    If you take the small signal output impedance of each 74AC14 to be
    about 30R, adding 220R in series gives a total output resistance of
    250R per gate. Five 250R resistive impedances in parallel is 50R.

    If you tried to make do with fewer gates, the gates would run out of
    current when the signal was close to Vcc or Vss, and their output
    impedance would be higher when the signal was in these regions.
    When you've got enough gates in parallel, adding more won't make any
    difference to the performance, though the individual devices will run
  4. John  Larkin

    John Larkin Guest

    No. A fast edge will propagate, as a fast edge, for great distanxes
    along a transmission line. The line has distributed inductance as well
    as distributed capacitance. Only non-ideal losses (dielectric and
    resistive losses) soften up a propagating edge. A 1 ns edge can be
    propagated through 100 feet of good coax.

    Tomi's circuit looks fine.

  5. colin

    colin Guest

    You could use an emitter folower with a 50ohm resistor down to ground,
    a very short high pulse will drive the cable hard,
    when the pulse returns the transistor will be off and so just see the 50r

    for such a pulse the cable is not like a capacitance, untill the reflection
    comes back from the other end
    it looks like a 50r resistor.

    Colin =^.^=
  6. Mike Monett

    Mike Monett Guest

    What is good coax? My experience is 1ns edges won't go very far in RG-58U.
    I forget exactly, maybe 10 ft.

    TDR also has round-trip, which doubles the distance.

    Mike Monett
  7. I don't think many people would consider RG-58 good coax. 9913, RG-213 or
    RG-8X are my favorites for 50 Ohm cable.
  8. John Larkin

    John Larkin Guest


  9. John Larkin

    John Larkin Guest

    That will be an asymmetric non-50 ohm impedance, and proper TDR needs
    a true 50 ohm source; most electrical TDR is done with a step, not a
    pulse. But you don't need a lot of voltage - 0.25 volts into 50 ohms
    is standard - so you can pad down a 5-volt swing and get a very good
    50 ohms.

  10. Mike Monett

    Mike Monett Guest

    No fair. Not many people get a chance to see that. Especially a 100 ft

    So you agree the ordinary everyday plain run-of-the-mill standard flexible
    coax that everyone and loves knows won't transfer a 1ns edge very far?


    Mike Monett
  11. Hi,

    I've made rather a lot of these generators -- both fast edges, and
    pulses. Over long (1000 metres) cables I personally prefer pulses. I
    have attached two small PDF files (am I meant to do that here ??)
    showing rather old versions of the circuits. The one using the S
    family TTL has worked for 20+ years, and I've used it to find faults
    at 750 m down a 75 ohm buried video cable. You need to check a test
    length -- the velocity factor is never quite right.

    The 2n2222 based pulse generator can generate any pulse length you
    like. The circuit originally had a avalanche transistor -- 2n2369
    from memory, and as long as you don't try a SOT23 2n2222 the can type
    ones work well. Layout is everything.

    The circuit came from a Jim Williams app note,C1,C1154,C1002,C1223,P1209,D4150

    Page 21.

    A later version is here.

    The coax sets the pulse length, and it can be quite long, or even very

    Some related links.,C1,C1154,D4183

    As some others have commented the attenuation can be fierce -- so 20
    volt pulses are good -- that way you might get 10mV back.

    Steps work just as well, and are always easier to see. Also easier to
    get the impedance from.
  12. John  Larkin

    John Larkin Guest

    For various values of very far. People could make 200 GHz
    oscilloscopes, but there's basically no way to get the signal to them.

    The edge gets ugly. On, say, 100 feet of RG58, a steep part is still
    transmitted, but its amplitude declines with distance and is followed
    by drool.

    __/ drool
    / steep

    I'll post some scope pics tomorrow maybe.

  13. Mike Monett

    Mike Monett Guest

    Thanks - that would be interesting.

    I'm surprised it would go that far. Your RG-58 must be a lot better than
    the cheap stuff everyone else gets.


    Mike Monett

  14. 16" wide brass waveguide.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  15. But the OP might be stuck with a particular coax type for his
    application. Remember, you TDR with the coax you've got, not the one you
    want (or something to that effect).
  16. For future reference: Your pdfs are not actually attached (which is not
    allowed here). They are links to a web site, which is the preferred
    method for posting graphics, binaries and such. Some of us who don't
    have access to binary newsgroups can still see them.

    If you must attach binaries, post to alt.binaries.schematics.electronic
    and refer to the subject line here. We can all hop over there and try to
    find them among all the jpegs of J.T.'s grandkids. ;-)
  17. Ian

    Ian Guest

    There's a 50 metre reel of Andrew's LDF5-50A in an adjacent cube,
    that was used for TDR work.

  18. Jim Yanik

    Jim Yanik Guest

    Tektronix sold a lot of their 1503 long-range TDRs,measuring up to 50,000
    ft of cable,IIRC. phone companies,cable companies.It used a 1/2 sine pulse.

    IIRC,the TEK 1502 measured up to a couple of thousand feet,using a fast-
    rise step pulse.
  19. Mike Monett

    Mike Monett Guest

    I used to own one. Can't remember if it was a 1502 or 1503. The
    resolution was so poor on long cables that I gave it away to the
    avionics company that maintained my Piper Malibu in San Jose.

    The 1503A used a tunnel diode step generator with a risetime of
    about 140ps. The B and C versions used a patented step generator
    designed by a rather famous individual whose name escapes me now.

    I tried to use his idea as a step generator in the Binary Sampler,
    but ended up using a simple ecl logic signal from a 100EP16. A
    simplified schematic omitting the 50 ohm output pad is shown here:

    Mike Monett


    Mike Monett

    Antiviral, Antibacterial Silver Solution:
    SPICE Analysis of Crystal Oscillators:
    Noise-Rejecting Wideband Sampler:
  20. Rich Grise

    Rich Grise Guest

    Most Time Domain Reflectrometrists wouldn't consider 1 ns to be
    an "edge". ;-)

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