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50MHz crystal video transmitter

Discussion in 'Radio and Communications' started by alphactif, Feb 21, 2012.

  1. alphactif

    alphactif New Member

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    Hello
    It's possible to build a simple low power (50 - 100mW) vidéo transmitter (with or without audio) but i need to use a 50MHz crystal to have a stabilised frequency.
    I don't found anything on web, anybody have an electric diagram ???
    thks
     
  2. davenn

    davenn Active Member

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    yes it is possible :) do you have an amateur radio licence? The 50MHz band is shared between commercial TV transmitters and amateur radio usage
    Us amateur operators dont use do TV transmissions on that band as we dont have the bandwidth allocation to be able to do so
    You should really be using 2.4 GHz where low power transmissions without a licence is allowable.
    Using 50MHz is likely to highly annoy many people in your neighbourhood, and under good propagation conditions 50MHz and 100mW can easily go 100's to 100km and you would be upsetting people even further away

    cheers
    Dave
     
  3. alphactif

    alphactif New Member

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    :confused:
    no, i haven't amater radio licence, I jut need to build a simple TV transmitter using crystal.
    Of course, i don't need 100km !!!!, I need only 50 or 100m, in this case, a 1mW transmiiter is sufficient, so how can i do this ?

    Build Tv transmitter on 2,4GHz is very difficult !!!
    I live in africa, and here nobody will be annoyed :rolleyes:
     
  4. dave

    Dave New Member

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  5. davenn

    davenn Active Member

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    why reinvent the wheel ??

    there are plenty of 2.4GHz TX and RX modules available

    http://www.ebay.com.au/itm/ws/eBayI...57412&kw={query}&sortbid=19&ff4=262691_262721

    dont make life difficult for yourself, specially if you have little or no electronics experience ??

    Dave
     
    Last edited: Feb 24, 2012
  6. alphactif

    alphactif New Member

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    Hello
    As I said, I m from africa, I don't have visa or master card, I can't make a paypal account and I can't buy anything from website (ebay or other...), this is why I must build my only Tx.

    I found many Tx schematic on the web, most of them have an RC oscillator, and I need a stable frequecy Tx.

    Building PLL Tx is also difficult, and crystal oscillator is easy and more stable than RC oscillator, so this iw why I make this thread.

    here http://www.seekic.com/uploadfile/ic-...1845950513.gif

    this is 440 MHz video transmitter use a 55 MHz crystal and 3 stages of oscillator doubler

    It's possible to eliminate the three stages of doubler ?
     
  7. alphactif

    alphactif New Member

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  8. RCinFLA

    RCinFLA Well-Known Member

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    Cheapest, easy way is to savage the RF modulator from an old VCR. It also generates the sound offset carrier. They output about 1 to 2 mV RF @ 75 ohms. You can put an amp after it if you want more output.

    This assumes you are talking about old analog TV, not the newer digital systems.
     
    Last edited: Feb 24, 2012
  9. alphactif

    alphactif New Member

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    i tried two VCR, and RF modulator is woorking between chanel 30 and 40 (500 to 600MHz)
    RF module output power is very low, and I can't found any RF transistor able to amplify RF output at this hith frequency.
     
  10. davenn

    davenn Active Member

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    there are dozens of devices available to give you enough power to cover up to ~ 100m

    any of the MMIC devices like the MAR1 to MAR8 series.

    The GALI series of MMIC's

    just to name 2 lots

    cheers
    Dave
     
  11. JimB

    JimB Super Moderator Most Helpful Member

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    So, how do you expect to build a complete transmitter if you cannot find suitable transistors for the amplifier?

    JimB
     
  12. alphactif

    alphactif New Member

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    I found many transistor at VHF frequency, but I can't found any thing at UHF !!!
    This is why I need a transmitter working on VHF frequncy, my TV tuner start research at 48MHz, and I have many crystal between 50 and 75MHz. this is why I need a Tx working at this fréquncy

    I dont need long range, but I need a good stable frequcy.
     
  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Why do you think that?.
     
  14. alphactif

    alphactif New Member

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    PLL and crystal oscillator give more stability than RC oscillator.
    PLL transmitter is too hard to built, so i think than crystal is the best solution

    I need only 50m distance, so I think than any simple transmitter, even built with simple BC547 or 2N2222 in RF output stage is able to give this range
     
  15. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Obviously, but you don't need that much stability, and you don't use RC anyway, you use LC for RF use, RC is audio.

    All old modulators (those with a screw adjustment) were just free running LC oscillators, and even the professional t-verters (down converters from UHF to VHF factory built for specific in/out channels) we used years back on a TV relay system just used free running LC oscillators as well.
     
  16. alphactif

    alphactif New Member

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    so you think that is possible to have a stable frequency only with LC oscillator (not RC sorry :rolleyes:) ???
     
  17. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Of course, most examples have always used LC oscillators, it's only the advent of PLL to allow digital channel selection that spawned crystal controlled ones, and that was for the PLL's sake, not to be more stable.

    Likewise TV tuners were always LC based until digital tuning came along.
     
  18. alphactif

    alphactif New Member

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  19. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    They are essentially the same circuit, except the first one has an added amplifier for greater power.

    Either should be fine, depending on the quality of construction.
     
  20. trash

    trash Member

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    Oh there is so many options even for Africa.

    Why you want to crystal lock it to 50MHz I have no idea.

    There are Soooooooooooooooooooooo many options.
    ok, starting with a ch0 or ch3/4 modulator out of an old video recorder.

    An OM350 or rather two of them stripped from old masthead amplifiers can be used as QRP amplifiers.

    2.4GHz TV transmitters are disgustingly easy to build. A simple common base oscillator is all you need with a little bit of pre-emphasis.
    The problem there is bringing S band FM video to baseband. The cheat is to make it L band (about 900-1800MHz) and use an old analog satellite receiver.
    There are LOTS of those in Africa. :)

    Ah Dave, don't you know about my cache of misc ATV circuits.
    Lots of mischief there... :)
    This is an example of a common base oscillator that can be used as the basis for a microwave FM ATV transmitter.
    http://home-and-garden.webshots.com/photo/1535422598048941318NLMheI

    But there's no reason why you could not just build a simple VHF or UHF TV transmitter.
    This one transmits about 300mW and will send a signal a couple of kilometres with an external antenna
    http://home-and-garden.webshots.com/photo/2229740740048941318spQqHK

    But if for some freaky reason you can about the quality of your signal and demand it be crystal locked, then the above circuit can be driven
    by a butler oscillator rather than a free running oscillator.
    http://home-and-garden.webshots.com/photo/2020691270048941318MjnSgk

    The best part is that all of these components can be stripped from any old TV, even and especially a turret tuner :D
    Africa is a place of raw materials if you know how to use them :)
     
  21. alphactif

    alphactif New Member

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    hi
    ok, crystal oscillator is not the best idea for home built video transmitter.

    The scond link posted by "trash" is very intereting, but what is the component indicated 5,5MHz, and how can I build the 2t 2t coil ? as I see on the schematic, it s not a simple air coil.:confused:
     

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