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25MHz 2dBm sine wave to logic conversion

Discussion in 'Electronic Design' started by Andrew Holme, May 18, 2005.

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  1. Andrew Holme

    Andrew Holme Guest

    Can anyone suggest a good way to convert a 2dBm, 15-25 MHz sine wave to
    5V logic levels with very fast rise times and minimal noise / jitter.
    At the moment, I'm increasing the level to about 10dBm using an MSA0404
    MMIC, followed by a PNP differential-pair limiter (which is struggling
    a bit at this frequency). I'm wondering about ditching the PNP diff
    pair and replacing it with a SOT-23 single gate HCT Schmitt trigger.
     
  2. Andrew Holme

    Andrew Holme Guest

    Is there a good low-noise solution involving a broadband transformer
    e.g. wound on a ferrite bead? How about ditching the MMIC and going
    for a broadband step-up transformer, with load resistor, followed by
    the HCT schmitt trigger?
     
  3. Andrew Holme wrote...
    There are plenty of comparators that'll handle the task.
    For example, in the older high-voltage realm, there's the
    LT1016 and the TL3016. I also like the MAX903, which I'm
    using for a similar job at 80MHz.

    Or you could bias an unbuffered CMOS inverter into the
    linear region, and ac-couple the signal to it. A second
    inverter will sharpen the risetime.
     
  4. John Larkin

    John Larkin Guest


    Use a fast comparator or, even better, a 5-volt LVDS line receiver.

    For the LVDS, offset your sig to +2.5 volts and apply to the + input.
    Feedback the output to the - input through a slowish R-C lowpass. That
    will servo the input threshold such as to make a 50% output duty cycle
    output. Shouldn't need the MMIC. Expect under 10 ps RMS jitter if your
    input's that good.

    Actually, you could probably get away with just applying your sig to
    one input and grounding the other. The LVDS receivers seem to work
    fine comparing around ground.

    National, Fairchild, TI, ONsemi make LVDS receivers. They are fast and
    cheap.

    John
     
  5. Fred Bloggs

    Fred Bloggs Guest

    No need to broadband with differential pair, and Schmitt trigger is the
    last component you want to use for low jitter output. Lowest noise would
    be 50 ohm input single transistor CE with reasonably high impedance
    inductive load terminated in 2.5V ( a simple variant of broadbanded
    video CE amp) driving logic buffer.
     
  6. Joerg

    Joerg Guest

    Hello Winfield,
    Ah, my kind of stuff. It will burn a bit of quiescent power though since
    he wants to reach 5V. 74HCU04 and the like can get pretty toasty.

    Alternatively, Andrew could use a BFS17 or two to jazz up the amplitude
    and then run it into an inverter.

    BTW, some info on "not so digital" uses of HCU and other chips would be
    nice for the next edition of AoE. And no, I am trying hard not to ask
    again when it'll hit the book stores.

    Regards, Joerg
     
  7. Joerg wrote...
    Only the power necessary to operate at 25MHz. A 250mV signal is more
    than enough to drive the inverter out of class A.
    Yes, we're putting in some cool stuff, like actual measured MOSFET
    gate-voltage to drain-current transfer characteristics for the N-
    and P-channel FETs in the unbuffered CMOS logic ICs, both for older
    HC types and for the newer low-voltage CMOS processes. Want to know
    the class-A current for different voltages? Read it off the plots.
     
  8. Joerg

    Joerg Guest

    Hello Winfield,
    Not really for the unbuffered devices. Look at figure 8:

    http://www.semiconductors.philips.com/acrobat_download/datasheets/74HCU04_CNV_2.pdf
    That is great. One piece of info that would be nice is the min and max
    specs on class A current. Data sheets like the one above only state
    typical values and that is not quite enough information for a design
    that goes into production.

    Regards, Joerg
     
  9. Joerg wrote...
    With respect to my class-A argument, I stand corrected. As I
    mentioned, we've taken considerable data, but I was going from
    memory in making my remark. Looking at my supply-shoot-through
    current curves, I see they're more like those shown by Philips.
    However, note that the 11mA current we measured at 5V is only
    60mW, which isn't that toasty. :>) Also, we measured 2mA at
    3V and Philips shows 0.5mA at 2V, so the staggered-voltage trick
    could be used to reduce the supply current.
    Yes, it would be nice, but it's beyond what we'll be able to do
    with our limited time and more importantly, limited sample set.
     
  10. Joerg

    Joerg Guest

    Hello Winfield,
    Well, one of the newfangled TSSOP packs can get a bit hot. 2V would work
    but the 74HCU04 becomes really slow then, unless you take newer faster
    TI low voltage logic. More $$$ though.

    I'd probably opt for a discrete design in this case.
    No, I didn't mean sampling at your lab. That's hard to do because you
    may not be able to obtain the process extremes. But you could try to
    cajole the process mins and maxes out of a manufacturer with good
    specsmanship (but a sluggish website...) such as Philips. Your name
    carries a lot more clout with it while us regular guys will usually be
    turned down. Tried many times. Even the notion of the potential truck
    loads in sales volume or a letter to the big brass doesn't sway them :-(

    Thing is, the minute you are facing a regulated design history path, and
    nowadays that's nearly all of my designs, the absence of hard data on
    min and max ratings is usually the end of the road.

    Regards, Joerg
     
  11. Andrew Holme

    Andrew Holme Guest

    Thanks Win, and everyone else, for the suggestions.

    I've got an MM74HCU04N stuffed in a solderless breadboard - with all the
    strays that entails - and it puts my old circuit to shame. Experimenting
    with different feedback resistors, I'm seeing 80% rise times below 4ns on my
    100 MHz 'scope with 22k and larger. Does this wonder solution have a
    downside? I don't care about current consumption, but noise / jitter
    matters. Is it fair to say the unbuffered HCU gates have less in them to
    create noise?

    I've also got an AD8561 comparator, which is billed as an "upgrade for
    LT1016 designs", but I prefer the HCU solution.
     
  12. Andrew Holme wrote...
    The feedback resistor value isn't critical, if it's not too small.
    No. You should establish a jitter spec and a way to measure it,
    so you don't over-specify and reject an other-wise good solution.
    That said, the jitter is easily understood by analyzing the noise
    density of the amplifier performing the limiting function. The
    CMOS MOSFETs are running at fairly-high currents (we observed 11mA
    for an U04 at 5V), which does lower the FET's e_n, but perhaps not
    as low as a BJT running at say 500uA in a high-speed comparator.
    Yes, I was going to mention that one too.
     
  13. Joerg

    Joerg Guest

    Hello Andrew,
    Solderless breadboards give me the goose pimples. Wait until you see it
    perform on a real board ;-)

    The 74HCU04 is unbuffered, meaning it doesn't have much gain. So the
    slopes will be shallower than with an 74HC04. Jitter and noise should be
    fine, unless your application is really critical in which case I'd use
    discrete fast transistors. With logic it greatly depends on the quality
    of VCC and ground. Use at least one small ceramic SMT 0.01uF really
    close to the VCC pin, and a solid ground plane.
    Much cheaper, too. Also, you can "rent out" the remaining inverters in
    there.

    Regards, Joerg
     
  14. Joerg wrote...
    The hcU04's tranconductance g_m = 30mS at 5V supply, so its gain at
    frequency f is G = g_m Xc = g_m / (2pi f Cnode), which is not too
    shabby. For example, G = 19 at 25MHz into a 10pF node capacitance.
    Andrew's 250mV signal (125mV peak) has a zero-crossing slew rate of
    S = 2pi f A = 20V/us. According to the Philips' datasheet, the U04
    inverter's 25MHz gain of ~20, which would amplify this to 400V/us,
    implying a 10ns risetime for the middle 4V of the swing. A second
    inverter stage would further sharpen this up.
     
  15. Joerg

    Joerg Guest

    Hello Winfield,
    Yes, if you don't need to get too close to the rails with the output of
    this amp it is indeed not too shabby. The beauty of such solutions is
    that the 2nd stage costs next to nothing because it has six inverters.
    Or if it has to be priced out it would be 1/6th of 10 cents ;-)

    Regards, Joerg
     
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