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trimpot guts

Discussion in 'Electronic Design' started by John Larkin, Jan 18, 2008.

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  1. John Larkin

    John Larkin Guest

    This is a Bourns 3314G. They work very well, but the hairpins on the
    ends add inductance, roughly 7-10 nH, and the substrate is fairly
    capacitive, so used as a pot there's a lot of leak-through when the
    output is supposed to be zero. With a 100 ohm pot, a 30 ps input step
    makes a 70 ps output blip of about 60% amplitude. That corresponds to
    a serious amount of transmission in vaguely the 5 GHz sort of area. So
    it might be usable to 1 GHz maybe.

    I'm looking for a better pot, or we might try using a phemt as a shunt
    element and control gain with dc gate voltage, like a classic jfet
    attenuator. I need gain trimming in a dc-1 GHz+ signal chain, and pin
    diodes and most attenuator ic's don't work down to dc.

  2. Does it need to be a rotary, user adjustable, potentiometer, or can
    you use a "slide" type of pot for factory adjustments? See:
    Allen-Bradley (and probably others) make these.
  3. Joerg

    Joerg Guest

    Why pots? I never use pots, creates service headaches and adds cost in
    production. What's that shiny surface you placed them on?

    Another option can be dual-gates such as the BF998. My favorite used to
    be dies such as the SD5400 with several on there because you can
    DC-servo them. But then they became expensive and hard to find boutique
    parts :-(

    Why don't they make something like the CD4007 in high-speed logic?
    Depending on your circuit you might be able to use an unbuffered
    high-speed inverter/buffer.
  4. Guest

    PIN diodes?
  5. Joerg

    Joerg Guest

    Nope, John needs to regulate all the way down to DC. Of course, one
    could use PIN diodes plus a CD4007 but then it would really become
  6. Guest

    Oh yeah, "step". My bad.
  7. Is there any way to treat the internal structure like a transmission
    line? Put a ground plane behind the pot and use stripline to match the
    unit to the rest of the circuit?

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
  8. Joerg

    Joerg Guest

    And then a guy at Bourns improves design reliability and ...
  9. Ben Jackson

    Ben Jackson Guest

    Are you trying to make 10 units or 10,000 units? If you can afford the
    extra time, you could use two parallel SMT resistors, one installed and
    one not, and use 1% resistors (glued to toothpicks for handles) to trim
    it. When you find the one you need, solder it in. With the right test
    rig you could measure the un-trimmed amplitude and compute the value to
  10. Howard Swain

    Howard Swain Guest

    Hi John,

    How about Bourns 3329H?
    Note that the leads come straight down out of the pot and are only
    0.1" apart.

    How many dB range do you need?
    Are you doing L pad, T pad, pi pad, or ??
    Or changing gain-determining resistors in an amp?
  11. John Larkin

    John Larkin Guest

    I actually have some of them, and will try them. I'd prefer surface
    mount, to avoid via capacitance, but I guess we could surface-mount
    them, too, if we bent the leads.
    It's just

    opamp output----------+
    o<-------------- next opamp + input

    where I actually need a "gain" between the opamps of between 0.25 and
    maybe 0.8, so I can adjust for photodiode sensitivity. System
    bandwidth is roughly 1.2 GHz.

  12. Joerg

    Joerg Guest

    1.2GHz with a potmeter?

    <goose bumps appearing...>

    How about these?

    Ok, a bit noisy and sets you back five bucks in qties. I you go with the
    pot I'd buy tons of them so you don't get a black eye when Bourns
    "optimizes" the potmeter design and all your calcs go out the window.
  13. John Larkin

    John Larkin Guest

    Sure, why not? The 3314G looks like it may work OK at that frequency,
    although it would be cool to find something faster. Hey, I could add a
    lowpass filter in front of my scope and see how that looks. I have an
    870 MHz MiniCircuits gaussian filter around here somewhere. This pot
    definitely bleeds through around 3 GHz or so.
    The DC situation looks dicey, too. All you RF guys ignore anything
    below a gazillion Hertz.

    1 GHz design is easy; DC-to-1GHz is harder.

  14. Joerg

    Joerg Guest

    Or use a TV amplifier. Most of those roll off rather quickly around a GHz.

    True. That's why I sometimes split DC from the RF path early on. You
    mentioned that it is to accommodate photodiode tolerances. Since those
    rarely exceed +/-15% couldn't you just use a resistive divider where the
    resistor to ground is a series combo of a real resistor and the DS path
    of a zippy RF FET? The BF998 comes to mind.
  15. John Larkin

    John Larkin Guest

    Yesterday we were playing with a circuit (on a whiteboard) that would
    use two gaasfets and an opamp. We would dump some dc current into the
    drain of one fet and the opamp would servo its gate voltage to get the
    saturated drain voltage to some target. Apply that same gate voltage
    to the other fet, and it becomes a stabilized Rds-on to ground, part
    of an attenuator. Since the fets on one reel tend to match very well,
    that should work.

    The little NEC fet we're using elsewhere (NE3509) make a nice variable
    resistor. It behaves just about like a super-hot jfet. Rds-on is about
    6 ohms at zero gate voltage, 10 at -0.1, 30 at -0.2. Drain capacitance
    is almost constant at about 0.35 pF, about the same as a surface-mount
    resistor. Helluva part for 85 cents.

    But that's a lot of junk to replace one trimpot.

    Hey, just for fun while I was testing trimpots, I soldered two
    hardlines to a piece of copperclad, with a small gap between their
    center conductors, which I bridged with a 47 ohm, 0805 resistor. Then
    I did a TDR/TDT test on the mess. The thru signal had a 30 ps risetime
    (which is about what the scope is good for) and looked almost perfect,
    with a tiny bit of ringing, a few per cent maybe.

  16. Joerg

    Joerg Guest

    That's how I usually do that. But I won't rely on reel tracking. In
    cases where it's not all that hot you might want to consider these,
    under 10c:

    Of course the 9pf start to hurt at a few hundred MHz but sometimes when
    you only need a dB or two you can set it in series with a resistor. Man,
    I wish the SD5400 hadn't become boutique-ware. Stuff was so easy when
    you could buy them for around two bucks.

    However, you have to factor in the field returns where something changed
    after the unit got banged or the fighter pilot turned freighter pilot
    nailed it to the runway again.

    Sound like the electronics version of shooting a can down the road ;-)
  17. Guest

    Apologies for using google groups...

    Have you looked at this attenuator?

    In an ancient (paper) HP data book I found a DC-coupled (but specced
    to 26GHz) analogue variable PI-attenuator made from GaAs MESFETs, the
    part number was HMMC-1001. There are some interesting ideas in the
    datasheet. There is a replica attenuator on the same die and they use
    op-amps to servo the gate voltages so that the replica attenuator has
    the right attenuation and input impedance at DC, and therefore
    (assuming reasonable device matching across the die), so does the RF
    attenuator. I don't think Avago will admit that these devices ever
    existed so that part is mostly just interesting for the datasheet.

  18. John Larkin

    John Larkin Guest

    Again, these RF guys like to take liberties with the word "DC". The
    test circuit has 100 pF caps at the chip input and output, yet the
    attenuation accuracy spec is "DC <= 1GHZ", and the graphs all cruise
    smoothly down to 0 Hz. There's probably a big internal DC bias on the
    switches, but they don't say what it is.

    I used some Hittite terminating RF switches where the datasheet
    features said, in huge type, "DC-3 GHZ OPERATION". The internal
    off-channel terminations quit working below 100 MHz, at which the off
    channel signals blow straight through. They blamed me for not reading
    the return loss curves carefully enough to deceipher their definition
    of DC.

    We're considering doing this with discrete phemts, possibly servoing
    the gate voltage against a dummy part. The NEC fet I'm using has
    absurd (0.35 pF) drain capacitance, so should be good to many GHz, but
    its Gm is so high, pinchoff around -0.4 volts, that Rds-on is only
    linear to +-0.1 volts or so, so it's only good for very low-level
    signals, again stuff that would keep the RF boys happy. If I can find
    a not-going-obsolete phemt or mesfet with low drain capacitance,
    reasonable Rds-on, and pinchoff greater than a volt or so, we might
    reconsider doing it active.

    The alternate is to just use two resistors as a simple voltage divider
    and hope that the photodiode gain is close and, if it's not, let the
    test people start soldering. I don't like to do that.

    Or maybe I could build a step attenuator, with resistors and using the
    fets as switches. In the fiberoptics business, nothing is ever more
    accurate than +-10% or so, if that, so we wouldn't need a lot of steps
    to get the amp gain close enough.

  19. Joerg

    Joerg Guest

    Then you'd rely on close enough in-reel tolerances. That's dicey because
    they will reel up stuff from the next wafer in the middle of it every so
    many reels.

    No chance to servo or clamp onto sync signals, carriers or something
    else in the signal itself? With my last FO project I lucked out and
    could do just that. Or course with a LED and a message to the host if
    the sync could not be detected for x milliseconds because the fiber got
    ripped out or something.

    Regards, Joerg
  20. Fred Bartoli

    Fred Bartoli Guest

    John Larkin a écrit :
    How about using the R-R linearizing trick? (one R between DG and one
    between GS)
    Of course you'll need low values, like 100R or lower, but given the low
    targeted RDSon it'll surely won't matter.
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