video transmitter-doubts

Discussion in 'Wireless & RF Design' started by deepak007, May 21, 2009.

  1. deepak007

    Thread Starter Active Member

    Sep 30, 2007
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    hello

    i am trying to build a video transmitter. i am posting the circuit. i have some doubts. the second stage which should be a rf amp,also has a feed back capacitor.is it correct?
    and the input signal is applied at the emitter of the first transistor and the base of the second transistor. anything wrong here? please help.
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Not quite sure what your worries are. Is this a kit?

    Operating the input transistor in common base is not unusual in a video amp. However it depends what comes before and after this module as to whether the impedances will be correct.

    This design provides an extra isolation stage.

    http://www.circuit-projects.com/miscellaneous/video-signal-emphasizer-and-amplifier-circuit.html

    Your circuit also seems to have rather a lot of tuning to set up, which might prove problematic.
     
  3. deepak007

    Thread Starter Active Member

    Sep 30, 2007
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    thank you for your reply studiot! this is not a kit. i am trying to build a wireles video transmitter. as you see the first stage is an oscillator and the second stage should be a rf amp. but,the secong stage also has a feedback capacitor between collector and emitter.then the second stage will be an oscillator too.isn't it? i think there should be no feedback on second stage ,as it is a buffer/rf amp. am i correct?
    about the circuit you gave me. is it a wireless transmitter?how about this circuit?
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Your second circuit is much clearer.

    The first transistor on the left is an oscillator, presumably at carrier frequency though I haven't checked.
    The last transistor on the right is an input buffer, similar to the one in the circuit I posted.
    The middle transistor is a mixer/amplifier which creates the transmitted modulated signal and feeds it to the antenna.

    *********************************************************

    No the circuit I posted will not do you, it is simply a video amplifier.

    *********************************************************

    I would say you are right to have doubts about the first published circuit.

    That is because it is poorly drawn and thus not clear.

    I have added some notes.

    At A it is unclear whether there is a connection intended or not. The drafter of the circuit appears to follow the connection blob convention (poorly) in some cases as at C but not in others as at B.

    Also at C I have shown on the left how a double connection should be shown unambiguously, so two separate connections are never made to the same blob.

    So it is unclear whether there was any miscopying in the original draft, especially as both transistors were configured apparrently the same.

    I would guess that the first transistor is meant to combine the functions of input buffer, oscillator and mixer and that is why there is a feedback capacitor (C1) between collector and emitter.
    There may therefore be a mistake in providing this coupling again at C7
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I'm assuming sound isn't going to be part of it.
     
  6. deepak007

    Thread Starter Active Member

    Sep 30, 2007
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    yes,this is only a video tx. no sound. can i use 2n2222 or 2n3904 transistors?
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes your general purpose transistors will happily replace the BC550.

    But the notes in the circuit tell you to use an NPN RF transistor to repalce the BFY90s, which are expensive and hard to find.

    Possible alternatives might be


    BF134
    BF152
    BF155
    BF196
    BF197
    BF202
    BF203
    BF306
    BF357
    BF507
    BF689
    2N3839
    2N6389
    MPS918
    MPSH10
    BFX89

    Beware of connection variations.
     
  8. deepak007

    Thread Starter Active Member

    Sep 30, 2007
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    can i use 2N918 for vhf npn ?
     
  9. bertus

    Administrator

    Apr 5, 2008
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  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes a 2N918 is a bit of an elderly type, but it is a 600+Mhz NPN RF silcon transistor.
     
  11. radiohead

    Active Member

    May 28, 2009
    474
    31
    In video transmitters, the transistors are critical. 3904 and 2222 are general purpose transistors and may won't work with video transmitters. Audio is much easier because you only have one signal to modulate and worry about pre-emphasis. Video has horizontal and vertical, PAL or NTSC as well as possible audio. Unless you have one of those old TV sets, you can't fine tune your receiver, you have to get your transmit frequency spot-on. All I can tell you is to experiment with different transistors, get one of those cheap transistor grab bags and get to work with the different types. Also, try to get one of those small, hand-held tv receivers with the dial-style tuner.
     
  12. hwy101

    Active Member

    May 23, 2009
    91
    28
    I agree with radiohead, the transistors are very critical
    I have a working video transmitter and it uses a 2SC1359 High Frequency Amp
    if that helps narrow your search.
     
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