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parametric detector

Discussion in 'Electronic Design' started by colin, Jul 9, 2005.

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  1. colin

    colin Guest

    Hi,
    Im looking at using a parametric method for a phase detector,
    The source (~4mhz) is already very strong but the sideband, @ 1-10hz, is
    extremely low, I am considering using a quartz crystal as the most sensitive
    slope detector but to get a significant signal to noise ratio I would need a
    crystal with a Q of over a million. I dont expect acheiving such a high Q is
    practical, unless cooling it drasticaly would help ?

    The main noise problem is 1/f noise especialy as a high Q circuit would need
    a high impedance amplifier and a mosfet would have too high 1/f noise at
    <10hz.

    However I cant see a way of using a paramatric method for a detector, at
    least not with any gain, I havnt realy looked at them much before, although
    I know the basic principles of operation of using as using variable
    reactance in a ac voltage dividor or charge/voltage pump.

    I came accros an old parametric amplifier by AD the 310/311 but I doubt they
    make this any more, at least something like this might be usefull after a
    diode detector, although I am not sure about how much the diode noise
    particularly 1/f noise would be.

    Colin =^.^=
     
  2. doug dwyer

    doug dwyer Guest

    I understand that you intend to apply a 4MHz signal with low sidebands
    to a high Q crystal offset slightly so that pahse modulation will appear
    as amplitude modulation and then perhaps employ a low frequency
    speactrum analyser to look at the phase noise. Is this the idea?
    If so:
    A good quality 4MHZ crystal wcan have a Q approaching 1000000.
    All depends on the diameter/contour surface polish and the quality of
    quartz used.
    The crystal could be operated at its series resonance and terminated in
    50 ohms so that a <1nVroot Hz amplifier could be used.
    The technique is not unknown, Marconi manufactured a tuneable cavity in
    the 400MHz range for the purpose.
    The orthadox approach would be to mix a high q crystal reference
    frequency with the "unknown" and explore the LF result with an analyser.
    I did well with this approach employing the steep side of an ssb crystal
    filter to attenuate the resultant mixed carrier whilst looking at the 0
    to 3kHz ssb noise.
    With careful use of low noise gain blocks stepped attenuator to overcome
    the dynamic range limitations of the 141T I could see down to -145db
    from the carrier.
    For another approach see error multipliers , these circuits are seldom
    seen now but you may be able to buy an old Tracor ? for the purpose.

    Finally a counter with low noise reference, high clock frequency and
    multiperiod capability can be programmed to convert jitter to ssb noise.
    See "AllensTime"

    note demon email address invalid
     
  3. colin

    colin Guest

    Hi, thanks, yes thats right :)
    well it is actualy a modulation signal im trying to detect rather than just
    noise but it is very similar problem, of course the phase noise of the
    reference oscillator is a big issue but I intend to try and null most of
    that out I hope.
    I'l go looking for some nice shiny quartz rocks... I chose 4mhz as it seemed
    they had the lowest series resistance for the frequency coresponding to
    highest Q although manafacturers dont list Q on a per frequency basis, its
    generally 20k for off the shelf types, im not sure if its advantagous to use
    an overtone or not. Im not sure if i can get any better ones, but the ones i
    bought seem to demonstrate a higher Q than anticipated from the simulation.
    aha, I have looked at using it in parrellel mode trying to use the
    considerably high voltage generated from resonance to improve snr before
    detection, although I was woried about destroying the crystal I can easily
    see 50v pk-pk.
    I will experiment with the series mode in ltspice, the problem with most
    1nv/hz^-2 amplifiers is 1/f noise is stil quite high as much as 1uv total
    below 10hz, not that many manafacturers seem to specify noise below 10hz,
    but certainly it has meant i have had to rethink things, I was hoping a
    parametric amp would avoid the 1/f problem unless i am mistaken here ?
    I have managed to get significant gain using the reactance of a varactor in
    a tuned circuit, but there still the problem of the preceeding detector
    diode.

    so could a 400mhz cavity have a higher Q ?
    Ive seen test aplication where the same type of oscillator is used as the
    reference if it is a VCXO of course.
    Ive tried using a PLL with a OCXO but this proved disapointing, of course
    one never fuly knows if its the topology used or just unlucky construction
    with such low snr, from what I can gather I think 1/f noise can be degraded
    with handling/soldering etc.
    Thats quite impressive.
    I will look into this with interest.
    thnks

    Colin =^.^=
     
  4. doug dwyer

    doug dwyer Guest

    Intrinsic acoustic loss is per cycle so ltimate q reduces from about
    10000000 at 1 MHz to 100000 at 100MHz.
    Modern crystals use grown quartz which can be much lower Q however the
    best grown approaches the ultimate.
    A 5 MHz 5th overtone will have a q of 1 to 2oooooo but cannot be driven
    hard.
     
  5. colin

    colin Guest

    Hi again,

    I looked at using series resoance with a 50ohm resistor but the much lower
    voltage means a much smaler signal even tho the slope is higher, I did
    consider a curent transformer aproach but think it wpould be pushing things
    a bit to actualy get more SNR.

    I looked at auto zero amplifiers and found they auto cancel the 1/f noise
    along with the dc offest, the AD8552 looks superb, very low voltage noise
    1-10hz and very low input curent/noise too, so i can use a highly tuned high
    impedance/high voltage section to give maximum signal, so i got one of these
    now, (I already had one in my parts box !) and boarding it up now.

    I couldnt find much on error multipliers, maybe il look some more ...

    Colin =^.^=
     
  6. doug dwyer

    doug dwyer Guest

    I looked and did not find, the technique was to multiply a 1MHz test
    signal to 10MHz mix with 9 MHz clean reference filter the resultant 1MHz
    and multiply again to 10MHz mix with the reference again can be done 4
    0r 5 times.with increasing sidebands the exact opposite to dividing.
    Various manufactrurers in the 60s
     
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