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Discussion in 'Electronic Design' started by RealInfo, Apr 7, 2013.

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  1. RealInfo

    RealInfo Guest

    HI all

    What is the "RIGHT" gain of the RF front end amp stage
    in a radio receiver ?
    Or maybe it is not about gain but abou isolating the
    local osc from antenna ?


  2. Guest

    This is not so much about right gain (i.e. front end noise figure)
    rather than front end selectivity vs. overload.

    At high-HF and low-VHF a 1-2 dB noise figure with 30 dBm IP3 should
    be adequate.
    Any good RF amplifier and a double balanced mixer should be good
  3. amdx

    amdx Guest

    Here's an article from QST that shows the design for
    "A high-dynamic-range MF/HF receiver front end".
    It has a switchable 8db preamp, but is claimed to be
    sensitive even with out the preamp turned on.
    Plenty to be gleaned about interstage gain by
    reading the article.

  4. amdx

    amdx Guest

    The original article for this design was in QST Feb, 1993,
    with feedback in June, 1993 and something important in Feb, 1994.
    My memory of this is weak but ISTR the "something important" was a
    reversal of two wires. (?)
    If anyone is real interested in the crystal filter I'll dig out the
    QST article and let you know what it is. Or any other info.
  5. Guest

    -174 dBm/Hz corresponds to 3 dB noise figure (300 K).

    The atmospherics noise (lightnings) will increase the background noise
    levels _far_ above that on LF/MF/HF.

    One should also remember that a full sized dipole will have a capture
    area of about 0.12 square wavelengths or effectively high comparable
    to 1/4 wavelength.

    Thus, the power delivered to the mixer is huge on HF compared to UHF.
  6. Guest

    Looking at some old CCIR (now ITU-R) nomograms for noise levels at 1
    MHz at a summer afternoon, the levels in most of Europe and North
    America is 40-70 dB "above thermal noise". The other nomogram will
    give the corresponding values for 30 MHz as 15-25 dB, which is about
    the same as the space noise at that frequency.

    Quite a few HF receivers were implemented with noisy heptode mixers
    and only needed a low noise triode preamplifier for the 15-30 MHz

    One other example, a typical ferrite rod antenna for MW reception
    might have a -40 dB gain below a dipole, but still suitable for
    receiving AM broadcasts, due to the huge band noise.
  7. Phil Hobbs

    Phil Hobbs Guest

    Which is why all DXing is done on summer afternoons. ;)


    Phil Hobbs
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