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Offbeat microwave question

Discussion in 'Electronic Design' started by Phil Hobbs, Sep 27, 2013.

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  1. Phil Hobbs

    Phil Hobbs Guest

    So one day soon I'm going to have these two SMA-mounted photodiodes that
    work up to about 10 GHz. Conveniently, they have built-in bias tees but
    don't have built-in load resistors, so they need a decent-quality
    termination.

    They're part of a differential Doppler interferometer producing tone
    bursts of very roughly 200 cycles, immersed in wideband white noise, to
    the point of being completely invisible on a scope.

    I need to build/buy/cobble together a splitter/combiner that works from
    DC to 8 GHz. I can buy resistive ones, but they have a 6- or 7-dB
    insertion loss, which means that by the time I'm done attaching an amp
    with a 3-dB noise figure, I'll wind up with about a 3000K noise
    temperature. Those photons are expensive, so this is clearly a non-starter.

    I'd also like the two paths to be phase-matched to within 5 degrees or
    so, and the gains to be equal to within, say, 0.3 dB. That way I have
    some hope of getting 15 or 20 dB of laser noise suppression. On the plus
    side, because of the signal characteristics, I don't care much about the
    time domain response, and a few amplitude whoopdedoos aren't worrisome
    as long as they aren't too deep or too broad.

    So, I'm thinking of just siamesing the two detectors together via
    matched pieces of RG-402 semirigid coax (0.141 inch) and either a T
    connector or some sort of artistically sculpted microstrip thing, e.g. a
    vanilla Wilkinson coupler tuned to some higher frequency.

    I'm sort of leaning towards the tee connector to start with, because the
    actual input resistance of the average connectorized amplifier is around
    25 ohms, iirc, so the match should be reasonable, and the degradation in
    the noise figure due to going a bit off the noise match shouldn't be too
    bad.

    True? False? Smart? Stupid? Other suggestions?

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  2. Joerg

    Joerg Guest

    A Wilkinson is resonant when it comes to isolation but I guess you don't
    need isolation here.

    If you use coax make sure both end right at the amplifier input. If you
    can't do that go from the point where the two cables joint to the amp
    input with two parallel coaxes. So that the impedance is half. Otherwise
    you'd see a big reflection from that point.

    Regarding input impedance, are you sure the noise-optimal input
    impedance is 25ohms? If not, maybe it can be tweaked to be around there.
     
  3. Robert Baer

    Robert Baer Guest

    * DIY termination? Perhaps use two 100 ohm 0603 in a physical "Y"to
    ground plane (am presuming 50 ohm system)?

    ** snipped to keep aioe happy **
    * Am assuming a balun transformer is not much better..

    ** snipped to keep aioe happy **
    * Did not know about the Wilkinson coupler and was going to suggest
    tapered transmission line "Y".
    But hoo nose? maybe the darn resistor can be tossed (when properly
    matched) for better noise and maybe lower loss.

    ** snipped to keep aioe happy **
     
  4. Robert Baer

    Robert Baer Guest

    Joerg wrote:
    ** snipped to keep aioe happy **
    Tapered transmission lines are one conventional way for impedance
    "conversion".
     
  5. Phil Hobbs

    Phil Hobbs Guest

    Hard to do over multi-decade bandwidths, though.

    Cheers

    Phil Hobbs

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  6. Phil Hobbs

    Phil Hobbs Guest

    I'll have to get a few and do that. The difficulty with separate paths
    is that it'll be really hard to get the gain and phase to track well
    enough over frequency to have any decent subtraction performance.

    Ideally I'd like to use a split cell, but they're not that easy to get.
    Discovery Semi supposedly makes them, but they give you the third
    degree before they'll even give you a datasheet, and that's pretty
    off-putting.

    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  7. Phil Hobbs

    Phil Hobbs Guest

    I thought about wiring them in series instead, with some Type 61 ferrite
    sleeves over RG-405 coax for broadbanding, and a pHEMT or something as
    the front end. I'd do that at 1 GHz in a heartbeat, and I've done the
    copper tape and XActo thing up to about 3 GHz, but 8 GHz is a little
    more difficult.


    Cheers

    Phil Hobbs


    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  8. Joerg

    Joerg Guest

    Yes, and you optical guys often need "DC to visible light" bandwidth.
     
  9. Tim Williams

    Tim Williams Guest

    Needing something bigger than cookie tins? ;-)

    Tim
     
  10. Neon John

    Neon John Guest

    John, that kind of crap is why we went to China for our custom
    extrusions. The prices were less than one tenth that of anything we
    got quoted in the US and the vendors fell all over themselves to work
    with us.

    John
    fluxeon.com
    John DeArmond
    http://www.neon-john.com
    http://www.fluxeon.com
    Tellico Plains, Occupied TN
    See website for email address
     
  11. amdx

    amdx Guest

    Do think this RG-402 made in China has the quality desired?
    $1.50 per foot.
    http://www.ebay.com/itm/40-1M-3FT-s...888?pt=LH_DefaultDomain_0&hash=item4ab437ae50

    Mikek
     
  12. Guest

    Can't help but join the who-does-the-brodaest-band bragging
    contest by mentioning the Athena+ detectors, which are square-law
    from dc to two-and-half million terahertz. Well, input match is
    less than phenomenal in some sections of the band. Joel Ullom does
    even broader a band, but he does not write to s.e.d....

    Regards,
    Mikko
     
  13. Guest

    Quasi-optical, I suppose? What's wrong with Wolter optics?

    What constitutes 'circuitry' is an interesting question, actually.
    I wonder how much microwave gadgetry could be reproduced with
    grazing-angle techniques? Phase relations would be tough to maintain.
    Still, I recall vaguely some bold plans to construct an interferometric
    X-ray telescope. It was supposedly going to be able to resolve surface
    patterns on Sirius.
    " Äitis oli ", I guess. Fits well :)

    Regards,
    Mikko
     
  14. Guest

    Wow, that looks pretty neat. That's way above the superconductive
    gap frequencies, so there's no way for me to even try to counter. You're
    the champ. I feel too tired now to figure out how you manage low-loss
    propagation at such high frequencies. Reminds me about the suggestion in
    the Kraus' old book on antennas about harvesting visible light with
    antennas and rectifiers.
    Right. I haven't been following the subject, but a bit of digging
    revealed this...

    http://bhi.gsfc.nasa.gov/docs/vision.html

    ....which sounds remotely familiar.

    Regards,
    Mikko
     
  15. Charlie E.

    Charlie E. Guest

    Ah, the People's Republic of Irvine. Very nice place, as long as you
    like lots and lots of other people, and are not in a hurry to get
    anywhere!

    Lived there for ten years before moving out to the desert. In Irvine,
    it took me about 20 minutes to get to Costco (about 2.5 miles.) Today,
    it take me about 20 minutes to get to Costco (25 miles!)

    But, if you like High Suburban life, Irvine is a nice place. It has
    just about anything you might want close by...
     
  16. Phil Hobbs

    Phil Hobbs Guest

    Except restaurants that aren't part of a chain, or a used bookstore, or
    walking trails with trees, or anyplace that isn't covered in six inches
    of glitzy roofie plastic from China. (Or so it seems.)

    Cheers

    Phil Hobbs

    (Whose aversion to Orange County grows every time he visits the place.)

    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  17. Guest

    OK, but it seems that even though it is close by, you can't get there
    from here. ...and I suppose if it isn't close by, you don't want it
    (or better not)? No thanks. You can have the whole state.
     
  18. Charlie E.

    Charlie E. Guest

    Oh, there are non-chain restaurants, as well as some small, local
    chain places. My favorite mexican place is Jalapeno's, a small family
    owned chain there. I am not sure about a used book store, been a
    while since I saw one anywhere! There are several walking trails,
    such as the one along the creek, that have trees, but they are also
    right outside someone's windows!

    Have they ever opened the park yet? They took over the old El Toro
    airbase, and were supposed to build a park there...
     
  19. Charlie E.

    Charlie E. Guest

    Well, nothing wrong with Orange county that a few million people
    leaving wouldn't fix! ;-)
     
  20. Guest

    No, that's OK. They can stay there. You all can. Quarantine, as it
    were. ;-)
     
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