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Light-sensitive XOSC

Discussion in 'Electronic Design' started by Andrew Holme, Feb 12, 2013.

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  1. Andrew Holme

    Andrew Holme Guest

    I've just built a 1.5 GHz synthesizer using a 10 MHz CFPT 9301 TCVCXO from
    IQD (Farnell order code 127 2080) as the reference. I've just powered it up
    for the first time and was disappointed to see 100 Hz sidebands at
    around -40 dBc on the 1.5 GHz output. That equates to a peak deviation of 2
    Hz at 1.5 GHz or 13 mHz on the reference. I have lots of low noise LDOs on
    the board powering all the different parts independently, so I was surprised
    that 100 Hz was getting in there; and I was about to try powering off
    batteries when, whilst viewing the output in FM Demod mode on my SA, I
    realised the XOSC was light-sensitive! I turned off the (halogen bulb) room
    light in my lab - and no more 100 Hz sidebands! Anyone heard of that: a
    light-sensitive XOSC !?

    TIA
     
  2. Andrew Holme

    Andrew Holme Guest

    "Andrew Holme" wrote in message
    I've just built a 1.5 GHz synthesizer using a 10 MHz CFPT 9301 TCVCXO from
    IQD (Farnell order code 127 2080) as the reference. I've just powered it up
    for the first time and was disappointed to see 100 Hz sidebands at
    around -40 dBc on the 1.5 GHz output. That equates to a peak deviation of 2
    Hz at 1.5 GHz or 13 mHz on the reference. I have lots of low noise LDOs on
    the board powering all the different parts independently, so I was surprised
    that 100 Hz was getting in there; and I was about to try powering off
    batteries when, whilst viewing the output in FM Demod mode on my SA, I
    realised the XOSC was light-sensitive! I turned off the (halogen bulb) room
    light in my lab - and no more 100 Hz sidebands! Anyone heard of that: a
    light-sensitive XOSC !?

    TIA

    More info: it's not sensitive to visible light, only infra-red! And I can
    detect the TV remote!
     
  3. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    did you try to block the emission from the halogen bulb to the XOSC? Is
    there any change of the supply voltage of the XOSC when switching the
    bulb on and off?

    Bye
     
  4. Jasen Betts

    Jasen Betts Guest

    Glass encapsulated diodes like 1N914, 1N4148, and the SMD variants of
    same are light sensitive.
     
  5. Tauno Voipio

    Tauno Voipio Guest


    An incadescent bulb will show the AC component of the supply
    as slight undulations in the light output.

    50 years ago, I built a speech transfer system using a car bulb
    with DC feed and the output transformer of a radio in series as
    the transmitter, and a scraped OC71 with an amplifier as the receiver.
    To my surprise, the audio came through pretty well.

    Without the DC bias, the output contains a full-wave rectified
    copy of the line voltage.
     
  6. Guest

    I used those diodes for limiting the input voltage range from 0 to Vcc
    in a very high impedance circuit. Moving my hand around the circuit
    caused a huge output swing due to those protecting diodes not shadowed
    by my hand.

    On an other case, a microwave source using a low frequency (<100 MHz)
    crystal and a few multiplier stages was very stable when left alone.
    However, when someone looked at the circuit with an "evil eye", the
    frequency started to swing at a few seconds interval. Later on, it
    became obvious that the person breathing into the circuit caused some
    rapid temperature and hence frequency changes :).
     
  7. Tim Williams

    Tim Williams Guest

    Also some pretty dramatic lowpass, I would think -- part of the reason
    incandescents work surprisingly well at that is because the filament cools
    off by radiation so much faster at high temperatures (thermal conductivity
    due to radiation goes as (T_abs)^3!). At yellow hot (say, 1200K), the
    temp change per watt is 8 times greater than at invisible heat (say,
    600K).

    Tim
     
  8. Tauno Voipio

    Tauno Voipio Guest


    To my surprise, it did not sound as muffled as a low-pass
    should make it. Maybe we were so far in the skirt that
    all the frequencies were equally attenuated.
     
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