Design of sensitive low noise receiver in 500Hz-20MHz band

Discussion in 'Wireless & RF Design' started by Peter Pan, Feb 25, 2009.

  1. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Hello,

    I want to design a receiver of very weak signals with amplitude of about few (not more than 100) uV within 500Hz-20MHz band (or, alternatively, at least 5MHz-10MHz) and try to decide which active device to use.

    What you would suggest - bipolar transistor, FET, or op amp?

    Thank you!

    P.S. It's important that the project could be manageable by a beginner in electronics.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What kind of signal do you want to recieve ?
    A "nomal" SW communication reciever has a sensitivity of about 1 μV.

    Here is a link to the DX Zone, with a lot of RF information.
    http://www.dxzone.com/

    Greetings,
    Bertus
     
  3. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Hello,

    Thank you for the link.

    The signal which I want to receive is RF pulse with amplitude as large as noise level - one microVolt, rf frequency 7.5 MHz and duration of pulse is about 5 seconds. The signal is repeated many times, hence it's actually a train of such pulses which I will need to receive.
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    YIP....DXZone is one of my fav'rit places.


    You might look at a DIRECT CONVERSION receiver for such a wide range. We built them using DDS synthesizers as a local oscillator for ionospheric research. World's cheapest spectrum analyzer!

    eric
     
  5. KL7AJ

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    Nov 4, 2008
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    You got my curiosity glands going. Is this part of a HAARP experiment? If so, I have some really slick integration techniques I can share with ya.

    eric
     
  6. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Alas, my receiver is for low-field NMR (Nuclear Magnetic Resonance).:)
     
  7. KL7AJ

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    Nov 4, 2008
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    Ahh... Okay. Well the pulse length is still long enough to use some long integration and averaging...which can really to a lot for improving your S/N ratio.

    eric
     
  8. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Yes, there will be time averaging (to increase SNR) handled by digital part of NMR spectrometer (by its host computer actually) - it's just a way how we do it in pulsed NMR.
     
  9. bertus

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    Apr 5, 2008
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    Hello,

    What is the fieldstrenght of your magnet ?
    What nucleus are you trying to detect ?
    For 7.5 Mhz PROTON you will need a fieldstrenght of 0.17625 Tesla.

    Greetings,
    Bertus

    PS I am a NMR service engineer.
     
  10. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Sounds great, Bertus :)
    My recent question about power amplifier also relates to this low-field NMR project.

    It's for proton NMR and I have two different permanent magnets for this project - one is 0.2T (about 8.5MHz) and another 0.4T (about 17MHz). I will design 0.2T spectrometer as some of equipment which I have is limited to 0.1Hz - 15MHz band which is lower than proton NMR frequency in 0.4T field.

    P.S. I had mistyped in post #3 - the frequency of the signal which I want to detect is 8.5MHz (not 7.5MHz).
     
    Last edited: Feb 25, 2009
  11. bertus

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    Apr 5, 2008
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    Hello,

    Are you measuring the FID only or are you trying to do a fourier transform on the FID ?
    The resolution of an 8.51 Mhz NMR will be low.

    Greetings,
    Bertus
     
  12. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    When I was in the UCLA plasma lab, I got to do some work on a curious device called an ICRH (Ion Cyclotron Resoance Heating) chamber. We were attempting to isolate a certain calcium isotope, which had a resonance down around 40 KHz. We needed about 100 KW of power to excite this helical coil, and we ended up strapping a whole trailer full of CROWN sound reinforcement amplifiers together to get the job done. They were they only commercial amps that came close to running at that frequency! I don't think they got any isotopes they were looking for, but it was cool hardware anyway.. :)

    eric
     
  13. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Well, FT is carried out over envelope of NMR signal after detection - i.e. over signal whose line width in spectrum can be as low as one Hertz for liquid samples (when homogeneity of magnet is good enough).
     
  14. Peter Pan

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    Mar 24, 2005
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    100 KW is always cool ... i mean - hot :)
     
  15. bertus

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    Apr 5, 2008
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    Hello,

    Are you using the same coil for the pulse transmitter and reciever ?
    In our NMR's it is done this way.

    Greetings,
    Bertus
     
  16. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    Yes, the same coil. There were cross coil systems in early NMR (and in some of MRI machines now) but in modern (including commercial - Brucker and Varian) machines it's about one coil, as far as I know.

    Recently, I came across a simple NMR spectrometer in Web at http://www.geocities.com/norberto.geo/nmrito.htm and it describes a very simple receiver (it's basicaly just a 3-stage amplifier on 2N3904). What do you think of this design?
     
  17. bertus

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    Apr 5, 2008
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    Hello,

    I see there a diode switch is used to block the transmitter power to the reciever.
    In our systems a similar circuit is used (hotswitch we call it).

    Greetings,
    Bertus

    PS I am working for Bruker.
     
  18. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    I imagine that would be one SERIOUS PIN diode!
     
  19. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
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    That's good.
    Raggio's spectrometer also provides TDA pulse which makes Q-factor of probe's resonance circuit low when transmitter is on.
     
  20. bertus

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    Apr 5, 2008
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    Hello eric,

    The diodes are PIN diodes.
    At the highpower transmitters they are active switched.
    (You do not want to have 1 kW into the reciever).

    Greetings,
    Bertus
     
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