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Passive f to 2f converter

Discussion in 'Electronic Basics' started by George Herold, Dec 11, 2012.

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  1. I got a request for a f to 2f converter for our lockin.
    (Freq = 3 - 3k Hz. )
    (Well a crappy pic, but I wanted to show that it has a filter module.
    (It stinks signal wise too, the photographer didn't like the second
    bnc cable from the reference output.))

    Passive would be the simplest. I thought of a diode rectifier, and
    when my boss mentioned that we could send it through the filter, I was

    I first drew it up with coupling caps, but that wouldn't float.
    So a transformer was needed.
    I've got some of these,
    Triad transformers. (in both 10k and 600 ohm flavors)
    And it worked great.
    I hung a 1 k ohm load on the rectifier output of the 10k transformer.
    (hmm, that might be my problem.)
    And at some low frequency ~30Hz, all hell broke out,
    with some jaggedie's during the sine wave.
    Did I mention I was driving it from an opamp?

    I was wondering if I should add some source resistance?

    George H.
  2. Hmm I don't really know. Here's a 'scope shot of the input to the

    At a slightly higher frequency the step and the jaggedies go away.
    (Jaggedies look like slew rate limitied oscillations ~2MHz.)

    I put 100 ohms between the opamp and transformer and that made
    everyone happy.

    Of course this is much below the spec for the transformer.
    (200 to 15 kHz.)
    Oh that's nice, then I wouldn't need the transformer.
    Any hints for an opto-isolater with back to back leds? I see nothing
    useful in digikeys search function.

    George H.
  3. Thanks John, If I can do it without the brdige and transformer that
    will be a win.

    I've got a single ended (is that the right term?) input and output.
    (Both are referenced to the same ground point.) So I like the double
    LED idea. I don't have that much voltage to play with (~ 6 V p-p). I
    guess I'll loose about the same on the LED's or bridge.

    Oh, but I'll need some voltage to drive the transistor.... but maybe I
    can use that idea sometime in the future.

    George H.
  4. Hey, is there an opto with back to back LEDs and just a photodiode

    Or maybe a dual, that I hook up back to back?
    I found this on digikey... a bit spendie

    But a lot less mass than a transformer!

    George H.
  5. OK thanks,
    The input is a sine wave 3-3kHz, ~6Vp-p. The output can be almost
    anything that has some 2f. The user can send the signal through a
    filter (with gain = to Q) and then the signal becomes the reference
    input for a switched gain lockin amp.

    George H.
  6. Ughh, that is slow. 1 uA into ~200pF, and with Vf= 8V, that's more
    than 1nC so sure more than 1ms response time.... but maybe I can
    reduce the load resistance (and forward voltage) and make it a bit

    Very interesting. I like the circuit apps! But I'll need power to
    turn the FET on. (not impossible, but not ideal either.)

    George H.
  7. Bob Masta

    Bob Masta Guest

    Hi, George. I don't have an answer to your question, but
    I'm just curious if lock-in designs have finally evolved
    since the 80's/90's to allow the reference oscillator to
    drive the multiplier (or whatever) directly. Back then,
    every model I could track down had the reference oscillator
    as pretty much a tacked-on free-standing circuit; the
    multiplier was *only* driven by the PLL. So if you wanted
    to apply a frequency step, you had to apply a step to the
    VCO, which was plumbed to the PLL, and wait for the PLL to

    But the PLL is really only needed for cases where you *must*
    use an external reference... the classic case being a "fan
    blade" optical chopper, where the frequency was determined
    by the motor speed.

    I imagine *somebody* still uses this arrangement, but in my
    case (hearing research) and plenty of others, what people
    really needed was simply a reference oscillator to drive
    their experiment *and* the lock-in multiplier to analyze the
    signal coming back from the experiment. No need for a PLL,
    since the reference oscillator can be a quadrature type
    right from the get-go (digital, these days), and everything
    is always "locked"... no waiting for the PLL.

    The solution for me was to go with synchronous waveform
    averaging, which has other benefits as well (like preserving
    a complex neural response waveform). But lock-ins would
    still have an edge in absolute S/N for those who deal with
    single response frequencies. So, can you now skip the PLL if
    you want to?

    Best regards,

    Bob Masta

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  8. Hi Bob, I have very little idea of what 'modern' lockin design looks
    (I'd be tempted to guess that lots is now done in software.)

    The lockin we sell is a throw-back. It's basically for students to
    follow the signal chain through the insturment. It's has no PLL
    inside it. (Gasp)

    There are two classes of lockin's that I know of. One uses the
    multiplier function to mix the signal with the reference. And the
    other uses a switched gain stage. The lockin we sell uses the
    switched gain stage topology. It's built around the AD630 chip. So
    sure it's possible to do a lockin w/o a PLL... the switched gain stage
    approach just needs a comparator to sense the zero crossing. With a
    quadrature oscillator (and two swithced gain stages) you wouldn't even
    need a phase shifter. Signal changes could be as fast as the final
    low pass filter.
    (The down side of the switched gain is that it also sees signals (or
    noise) at odd harmonics of the reference frequency.)

    My favorite lockin from grad school was the 124A from EG&G. I was
    talking with the design engineer from SRS at a trade show a year or
    two ago and he has redone the 124A for SRS. I think it's an 'all-
    analog' signal chain. I hear that the low temperature guys like that,
    because the digital based lockin's would send 'crud' back down into
    their cryostat's and screw things up.

    George H.
  9. whit3rd

    whit3rd Guest

    The use of an analog multiplier (the six-transistor kind, like in MC1496) will get
    it done without the (circa 4mA) current requirement of an LED solution. It's
    easy to drive a center-tapped transformer with the output.
    Of course, this is NOT a passive solution, it takes a power supply; the upside, is
    that you can bias it for 2 uA current draw and toss two in more AA cells each year.
    It's sensitive to level, you'd want an input attenuator, too.
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