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Pulse Shaping for 1-10 ns pulses

Discussion in 'Electronic Design' started by OPE, Nov 26, 2007.

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  1. OPE

    OPE Guest

    I am generating 1-10 ns pulses that are roughly square, with rise
    times on the order of 500 ps, and TTL-type voltages between 2-4V, at a
    rep rate of 20KHz. I would like to change these square pulses into
    triangular, sawtooth, or Guassian pulses.

    I've tried a standard RC low-pass filter, but there was a fair amount
    of ringing. Since it is easy to design a Butterworth filter, I've
    done it, though I need to get the right inductors and build it. I
    suspect these will generate a roughly triangular pulse, though I don't
    know of any way outside some kind of fast DAC to generate a sawtooth
    pulse with a ramped front edge and fast back edge.

    I have not found any designs for Guassian filters. Are these easy to
    do with passive components? Anyone have any good references for these
    types of filters?

    Thanks in advance for any answers or ideas.
     
  2. John Larkin

    John Larkin Guest

    What chip are you using? What scope?
    An RC won't ring if the layout is tight. The capacitor needs low esl
    and must be grounded hard.


    Since it is easy to design a Butterworth filter, I've
    A Butterworth inherently rings. A Gaussian or Bessel filter doesn't.
    It's just a matter of the L and C values. Williams' filter book has
    all this stuff. Roughly, you could



    in-------R------L-------+--------out, hi-z load
    |
    |
    C
    |
    |
    |
    gnd


    where R=50 ohms maybe, R*C = 5 ns maybe, and

    L = K * R^2 * C, with K in the range of 0.5 to 1 maybe.






    For a ramp, you could...


    current
    source
    |
    schottky |
    diode |
    in-------|<|----------+------- out, hi-z
    |
    |
    C
    |
    |
    gnd


    where the current source could be something active and fancy, not
    trivial, or a resistor with an inductor in series.

    Williams and Taylor's filter book, worth having around.

    John
     
  3. Joerg

    Joerg Guest

    Yes, they are. And there is helper software such as this:
    http://www.aade.com/filter.htm

    I mostly do it the way John suggested, with the Williams Filter
    Handbook. Then I simulate and tweak with Spice. And yes, as John
    mentioned there is a zero tolerance policy for layout. You can't get
    away with Sauerkraut wiring for something that has to shape a 500psec
    slope. Meaning a full ground plane, controlled trace impedance,
    microstrip and all that. This is hardcore RF engineering. And, oh yeah,
    even 0603 capacitors do exhibit some nasties such as inductance and
    series resonance.
     
  4. whit3rd

    whit3rd Guest

    Look into the old literature on waveform generation, like the
    MIT Radiation Lab books. Pulse shaping is a venerable
    tradition.

    Triangle pulse is easy; just an RC integrator (which also attenuates,
    a lot).

    Gaussian is impossible, except as an approximation (the tails
    are infinite, after all, including the ONSET of the pulse).

    In the absence of devices that switch much faster than your
    target times, you will probably want to make multiple-transmission-
    line
    filters. A true wideband delay line, after all, is just a bit of
    stripline
    on a circuit board...
     
  5. Robert Baer

    Robert Baer Guest

    Pad over ground plane is rather useful to reduce such inductances;
    tweek the signal pad size on the capacitor, as well as the shape (narrow
    trace abrupt to middle of square VS triangular transition from trace to
    square; square pad VS rounded pad).
     
  6. OPE

    OPE Guest

    <SNIP>


    Thanks John for your suggestions. It has given me many good leads.

    I am using Phillips 74AHC08 AND Gates and Toshiba TC7SZ08 AND gates,
    which are both fairly typical high-speed TTL parts. I have also used
    a Stanford Research Systems pulse/delay generator, but it produces
    ugly pulses and can't produce pulses shorter than a couple of ns. I
    think we might need to send it in for tuning to get it working
    properly. We typically use a 500 MHz HP digital scope that leaves a
    few artifacts. We also have a really old HP 12.4GHz sampling scope
    that is much harder to use and collect data from, but usually shows
    significantly reduced overshoot and undershoot.
    and must be grounded hard.

    My layout is with generic copper-plated circuit board and an exacto
    knife, so I doubt it is tight. I think I will try using two caps in
    parallel to help reduce problems.

    I want to check to make sure I understand the circuit correctly. The
    schottky diode acts like a capacitor on the first edge of the pulse,
    with voltage ramping up at a rate determined by the pulse height and
    the current source. Then, when saturated, it acts like a diode again,
    and turning the circuit off. Thus, it will produce a right-triangle
    type pulse like this: ____/|___

    The Fairchild FYV0203S series and BAT54 series lists 10 pF of
    Capacitance and 5ns of recovery time. Are these the specs to look for
    in choosing a schottky for ramp generation?
    Unfortunately, it was already checked out. But, I sure appreciate the
    reference.

    Thanks again.
     
  7. Guest

    High order Bessel filters ring. I forget the breakpoint, maybe the 5th
    order on up rings.
     
  8. Guest

    RC filters can't ring. [Their impulse respond is always positive.] Of
    course, if you have parasitic L in the circuit, all bets are off.

    The best passive filter book is that by Zrevev.
     
  9. John Larkin

    John Larkin Guest

  10. OPE

    OPE Guest

    I am connecting the output of the pulse generators to SMA cables, then
    to a little board where I have the filter, then to a scope. I am
    wondering if maybe some of my ringing is coming from impedance
    mismatches between the boards and wires or something similar.

    For my case the attenuation is fine. We use external attenuators to
    scale down the pulses anyway. I just need to minimize ringing and any
    junk that happens after the main pulse. I am thinking I will just use
    an RC filter for ~200MHz, as that will correspond to ~5ns rise and
    fall times.
     
  11. Guest

    If you had more detail in the graphs, you would see the impulse
    response go negative. it is very small, but high order Bessels do
    ring. For things where you can't tolerate any ringing (weigh scales),
    you need a Gaussian.

    Looking at my copy of Huelsman&Allen, I'd say the 5th order is where
    it starts.
     
  12. Guest

    You probably have ground bounce. In theory, the TTL has Schottky
    diodes, but maybe you can clamp the line with an external Schottky.
    Make that a small diode, not a power Schottky.

    I know HP has some trick to make flat pulses using what they called a
    flat pulse diode. This datasheet mentions it
    http://www.picosecond.com/objects/AN-02c.pdf
     
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