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Step Recovery

Discussion in 'Electronic Design' started by Andrew Holme, Dec 21, 2008.

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  1. Andrew Holme

    Andrew Holme Guest

    Just been hanging various junk box diodes between a pair of SMA sockets, one
    to my signal generator, the other to my 'scope, looking for step recovery.
    A varicap (BB405B, possibly) and an unidentified (channel switching in 50
    MHz XTAL oscillator) PIN diode salvaged from an old PMR set work quite well.
    The latter gave a 2ns step. The mainstream component vendors don't sell
    proper step recovery diodes, so my question is: which/what other diodes
    exhibit this behaviour most strongly? Can anyone recommend a good
    off-the-shelf device?

    See 'scope screenshot in a.b.s.e
  2. Andrew Holme

    Andrew Holme Guest

    It's the current. Here's the test circuit:

    R1=50 Diode
    | |
    /+\ .-.
    V1 ( ) | | R2=50
    \-/ | |
    | '-'
    | |
    | |
    === ===

    V1 = EMF of signal generator (sinusoid)
    R1 = Output impedance of signal generator
    R2 = 50-ohm coax to 'scope (50-ohm input)
  3. Guest

    As has been pointed out in threads past, PIN diodes intended for rapid
    switching can make decent Step Recovery Diodes. Both proper SRD's and
    band switching PIN diodes are designed with very abrupt transitions
    low to high doping, and band switching diodes for UHF and above will
    designed for low "off" capacitance.

    An easily available example in SOT-23 surface mount package is the
    MMBV3401, eg.:

    while this has been used to multiply by 16x to around 1GHz with some
    I don't know what the transition time is.

    Avago/Agilent/HP have a nice app note:
    in which they show how to use their HSMP-3820 and 3822 PIN diodes as
    multipliers, with some usefulness to >5GHz.

    SOT-23 packages have enough parasitic inductance that this might be
    dominating the useful risetime that you can get from these PIN diodes

    Depending on what you're doing with your SRD, you might care about
    time variation with temperature. HP worked around this in their
    oscilloscopes beginning in the 60's by using a brief forward bias
    pulse to
    build up stored charge in the SRD before applying the reverse bias
    that leads to the snap. If the forward pulse is much shorter than the
    carrier lifetime (at the highest temperature) then the lifetime
    with temperature has little impact on the recovery time.


    Sorry about the gmail address. I know that many people will have
    gmail for good reason.
  4. Tim Williams

    Tim Williams Guest

    Hey, weren't you going to send me a handfull once upon a time? Your server
    must keep dropping my e-mails or something, I never seem to get a reply.
    It's my understanding that snap diodes are just hyperabrupt varactors with
    an even more aggressive doping profile. So as the charge leaves, it kind of
    leaves in a plane wave, all at once, sluurrp, and it's gone. Then
    inductance says hello and...*whang*.

    The best edges I've ever found from diodes in my collection seem to be
    schottky rectifiers... and that'd be all from the capacitance, which does
    have a funny response (usually huge, a couple nF around 0V, for the bigger
    ones), but it's not a snap per se. I must have bad luck with my parts,
    haven't even seen a 1kV+ diode that snaps. Well, that's not entirely fair,
    there's that one diode that I suspect is a varactor, it went reasonably
    well, but about as the OP says, 3ns or so (which is still close to my
    observation limits, but not exceeding them, so...).

  5. Andrew Holme

    Andrew Holme Guest

    Thanks for that fascinating link. It was an advert for the 185A/187B, which
    I stumbled upon in a 1961 issue of Wireless World, that got me playing with
    this recently. That 'scope must've been a sensation in its day.

    I think the 2ns I measured for the rise time from my mystery PIN diode is
    limited by the 1.75ns rise time specification of my HP1725A and may actually
    be slightly faster. We have better 'scopes at work and I'll try it there.

    I have some test equipment here containing SRDs including a Marconi spectrum
    analyser and a RACAL true rms microwattmeter and the service schematics make
    interesting reading with their use of these exotic devices.

    I've also been playing with tunnel diodes and an avalanche transistor pulse
    generator circuit. I get sub nanosecond rises from the latter, and I've had
    the former oscillating at above 1 GHz; but I haven't tried generating fast
    steps using tunnel diodes yet.

    I also found quite an interesting paper "50 Years of RF and Microwave
    Sampling" by Mark Kahrs which mentions you, John, in the acknowledgments.
  6. Tim Williams

    Tim Williams Guest

    Alright, will do.
    Funny, I get at least three pages of academic journal whores, one Google
    Books hit which was interesting, or it would be if it had more pages in it
    at least, and another discussion between you and Terry Given about this
    effect in MOSFETs. Congratulations, you've come full circle!...

  7. Joerg

    Joerg Guest

    Yes, they do enjoy challenges. But the fun stops when a single-sourced
    part goes on allocation, your company isn't a high-roller in terms of
    order volume and thus low on the pecking order, and it's two days before
    your vacation trip with non-refundable tickets ;-)

    BTDT. I didn't design it in but I was the one who had to cancel the
    vacation trip in order to do an emergency re-design instead of playing
    at the beach.
  8. Joerg

    Joerg Guest

    But I bet you never had to order a batch of 35000 of them. That's when a
    PIN diode design almost flew into my face, on production ramp-up for a
    hybrid run where you must have the full batch or you can't do the run.
    If that happens the dispatcher at the hybrid line gets very pissed and
    you can end up in what equates to standby for air travelers. The guy
    from purchasing said that when he called in the order he could swear he
    heard someone falling off the chair at the other end.

    Wanted to take Friday off. No dice, no parachuting :-(

    Finally we found enough of them in Asia. Whew!

    Unorthodox designs are the most fun.

    They should really sell the 50-75c device at Digikey. Also gets it into
    the hands of hobbyists and many of those will be tomorrows design
    engineers. Plus I want to see a nice screen with the exact quantity in
    stock, preferably a five-digit number.

    Merry Christmas, Joerg

    "gmail" domain blocked because of excessive spam.
    Use another domain or send PM.
  9. Joerg

    Joerg Guest

    I used to back then. Pretty much until I married ;-)

    Static line isn't that much fun because it's done from less than 5000ft
    so that they all land on the drop zone, more or less. The real fun
    begins >12000ft. Sometimes when there was some thermal activity and
    nobody to do formation jumps with I went out last, pilot chute already
    in hand. Let the pilot chute go the millisecond I saw I was clear of the
    aircraft and then enjoyed 10-20 minutes of sightseeing from way up
    there. One guy even netted >30mins. It was an industrial area so you
    could do things such as looking into one of those cooling towers from
    above. Once I and another guy miscalculated the wind when we still had
    slow parachutes. Had to land inside a huge scrap yard full of extended
    (!) old cranes, trying not to smack ourselves into a boom or get tangled
    up. Whew! Then, a growling and snarling shepherd came. Luckily I grew up
    with big dogs so after some time of getting acquainted with him he
    guided us to the gate (we had no clue where that was).

    Oh yeah! For those who don't know it:
  10. Guest

  11. krw

    krw Guest

    He spotted spot first.
  12. Joerg

    Joerg Guest

    Usually I did, like when the bush pilot type chauffeur dropped us
    somewhere into the fog. Looked for logging roads, bus stops, and such.
    Then we landed, packed the chute, trudged to the bus stop, read the
    schedule, sez "except on weekends".

    But here I was frantically busy avoiding all the booms. Had a
    Paracommander (round) which wasn't exactly a Ferrari.

  13. Joerg

    Joerg Guest

    Someone once said that his Lt.Col. countered with the same question. He:
    "Which one of these is perfectly good then?" Lt.Col.: "Ahm, well, good
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