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Inverting amplifier / positive pulse problem

Discussion in 'Electronic Components' started by O. P. E., May 3, 2007.

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  1. O. P. E.

    O. P. E. Guest

    I built a pulse generator that produces ~1.5 ns pulses that go from
    0-3V. (Use Linear app note 98 if you want to build one.) I would
    like to put it into an RF amp to produce the 7V pulses needed for my
    application. My problem is that my amp is an inverting amp. Thus,
    when I put in my positive pulse (actually attenuated down for the
    amp), it produces a negative pulse of ~4.5 V (corresponding to the
    bottom rail).

    I thought that I could switch the polarity of the SMA connector on my
    board -- attaching ground to the middle pin and the pulse output to
    the case. However, I don't think this will work as the ground from
    the amp and scope will then have to float. I thought of using a NOT
    gate, but wouldn't that just give a pulse that goes from -5 to 0?
    Also, the gate would likely increase the pulse width, which we would
    like to keep as short as possible.

    What would be ideal is to have some simple device that takes a
    positive voltage and flips it to negative, while keeping the pulse
    width short. At low frequencies, an op-amp could do it, but I'm not
    sure of any op-amp that will do it at these frequencies. Does anyone
    know something that will do it? Has anyone done something like this
    before?

    Obviously, it would be better to just use a non-inverting amp. The
    spec sheet on the amp (JDSU H301) doesn't even mention that it is
    inverting. Are all Optical Modulator Driver amps inverting, so that
    it is just an understood?

    Thanks
     
  2. LVMarc

    LVMarc Guest

    Most packaged rf amplifiers are not characterized as inverting or not
    inveting, just as an amplifer. ie gain input vswr output vswr and 1 dB
    compression point, plus "bandwidth".. You can make a modest gain 10-20
    db amplifier with a single active decie, some input match, bias and
    output match. with a single devcie if you wiggle the base (gate) and
    take the signal off the collector (drain) you have an inverter. you
    coul use two in series!

    For a 1-2 nS pulse you could use an avalanche tx and hen atenuate the
    signal down to 7 volts, as the avalacnhe tx will produce about 100 Volt
    1 ns rise 1.5 ns fall the signal has a nice impulse shape..

    Marc
     
  3. O. P. E.

    O. P. E. Guest

    Perhaps I should have said "GHz amp" instead of "RF amp." For most
    radio signals I doubt polarity matters. However, since this is an amp
    to interface a signal source with something else, the polarity does
    matter. Doing some more research, it looks like the amp expects a
    signal from -.7V to -2V. So, I think it may be expecting some kind of
    ECL signal. I hope that I can find some kind of translator circuit.

    I looked at a few circuits with avalanche transistors. However, with
    the delay line/AND gate circuit you can change the pulse width by
    turning a pot, and set the rep rate by using the square wave of an
    external function generator. The design I found with avalanche
    circuits with variable pulse widths required a BNC cable cut to
    length, and had much lower rep rates to allow capacitors to recharge.
    Also, I worry about the noise 100V pulses create, as I am getting
    enough with TTL.

    I'll have to look into your active device suggestion. That sounds like
    a circuit I looked at that was posted on Microwaves101. However, that
    circuit was to produce pulses of RF, while I am just looking to
    produce square/guassian pulses.

    Thanks for your response.
     
  4. O. P. E.

    O. P. E. Guest

    --------------------

    I found a nifty circuit for doing this while looking for something
    else. HP did it by using a simple transformer, then, of course,
    discontinued the product. Here is a link, and the data sheet shows
    how they hook it up.

    http://www.home.agilent.com/USeng/nav/-536902328.536881961/pd.html?pageMode=OV

    I think HP has discontinued more useful equipment than 90% of
    companies have ever made.

    Hopefully, this will help anyone in the future who follows this
    thread, and is looking for a solution. I'm going to look for some
    transformers to hook it up. As I've never worked with RF
    transformers, does anyone have suggestions?

    Thanks
     
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