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measuring amplitude of small high-frequency signal

Discussion in 'Electronic Design' started by Tom, Sep 30, 2005.

  1. Tom

    Tom Guest

    Everyone,

    I need to measure the amplitude of an AC signal of variable frequency
    between 1 and 15MHz, with amplitudes in the range of a few hundred
    microvolt (say 100..1000 uV). Available supply voltage is nominally
    +-2.7..3.3V or +2.7V/-1.35V (battery powered circuit). The measurement
    itself does not have to be taken often, a few Hz refresh rate will
    suffice.

    What I'm currently contemplating is to use a high-frequency opamp such
    as the AD8061 (300 MHz GBW) to provide two stages of gain 20. The stage
    outputs would be AC-coupled to get rid of the DC offset. This would
    give me a signal of ca. 40..400 mV which I could rectify and filter.
    For better linearity I would use an active rectifier circuit, probably
    using Schottky diodes.

    Another alternative I have considered is to use an analog multiplier to
    square the signal, and then simply lowpassfilter it for the DC
    component (from which the amplitude is easily calculated). However, I
    haven't found analog multiplier chips compatible with my battery power
    supply.

    Does anyone of you have a better suggestion?

    greetings,
    Tom
     
  2. <>) about
    'measuring amplitude of small high-frequency signal', on Fri, 30 Sep
    2005:
    In that amplitude range, an ordinary 1N4148 has a square-law response. A
    Schottky would give more DC output. You can rectify the signal directly
    (you won't get much DC, but enough) and then amplify the DC. The usual
    technique is to use two diodes, one in shunt across the signal and one
    in series with the load resistor, with the signal capacitively coupled
    to their junction.

    Ancient (1950s) RF millivoltmeters, such as the Rohde & Schwartz URV
    used this technique. But DC amplification is far easier now.
     
  3. John  Larkin

    John Larkin Guest

    LTC and Analog Devices, and maybe Maxim, make RF detector chips.

    John
     
  4. Jim Thompson

    Jim Thompson Guest

    15MHz is such a low frequency, why not an active-device peak
    detector. You could even take some gain.

    ...Jim Thompson
     
  5. Guest

    Why not using a calibrated SW-receiver?
    Thats the simplest solution.
    Aperiodic measurement is difficult with tis small values.

    rgds Rudy
     
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