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FM transmitter (mod4)

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, Oct 26, 2006.

  1. whiz115

    whiz115 Member

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    no... any other? :D

    you don't want a whatever power transistor...you need one which can have enough gain for the frequency range you want it.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Why not look at its datasheet from Philips?
    Its minimum transition frequency is only 50MHz so it has no current gain at 50MHz and above.
     
  3. whiz115

    whiz115 Member

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  4. dave

    Dave New Member

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  5. chingyg

    chingyg New Member

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    believe it or not, the only one i could find on the store is 2SC3807. haha. but on the data sheet they say its for low frequency?
     
  6. ebowlotus1960

    ebowlotus1960 New Member

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    Edit: A second read through the thread answered my question
     
    Last edited: Aug 22, 2009
  7. chingyg

    chingyg New Member

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    I found everything beside the FD700, an ultrafast diode. I think it is there to set a 50% duty cycle to bias the last transistor, but no matter how I cant find it. It is an 30V, 700ps switching time, 1pF capacitance. any suggestion on substitute. what about the 1N4148?
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    How can you make anything work properly without being able to get the correct parts?
     
  9. chingyg

    chingyg New Member

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    but aint kitchen experimentalist needs to improvises everything? where the fun if you could sit in an lab and get whatever part you want.
     
  10. whiz115

    whiz115 Member

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    can you improvise in your kitchen if you don't have kitchen utensils and food to cook? how are you going to build your linear amplifier if you don't have the needed?

    check this site... i believe it has everything you need, it's cheap
    and it can ship your order by airmail. The only thing you need is to have a Paypal account or a debit card and the right amount of money so you can place your online order. Ii prefer debit cards than credit cards because i believe it's more safe when doing online transactions.

    www.futurlec.com

    P.S. also why don't you join my topic since we are doing the same project?!
     
    Last edited: Aug 22, 2009
  11. chingyg

    chingyg New Member

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    hey that's a good point duh. yes i'll maybe use my debit card but, us in third world countries are kinda poor. but i tried futurlec just now, they still dont have that FD700 diode.
     
  12. chingyg

    chingyg New Member

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    Is there any other way to bias the last transistor in Class C without the use of that FD700. like an inductor from the Base to ground or something?
     
  13. chingyg

    chingyg New Member

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    oh, it is finally working. I only bulit one with the first 2 transistor though. it worked well, the range i havnt really tried but it is more than 100m, with the meter you guys discussed in pg 19 or so, it shows an reading of 100mV. however there is a strange noise broadcast in the background, something that sound like 200Hz squarewave. Then all of a sudden it stopped working. I looked at everything and there are all fine, I had to remove Q2 to make it work again. Why is that. I though the tuned circuit with Q2 centered at 98Mhz gives good power over the entire band?

    Now I am thinking about building a mini amplifier for a commercial FM transmitter i brought, what about this circuit for amplification? they said the output of the transmitter is about 100uW on the manual.
    amp.JPG
     
  14. alexander3133

    alexander3133 New Member

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    I have some questions here for the mod4 FM transmitter:

    What is the type of configuration for the oscillator? Earlier post if not mistaken stated Colpitts? But how come it is different from the configuration I found on the net?

    With all the 2N3904 BJT transistor, is it possible to change all the existing one to FET type? as FETs are more energy efficient. If possible, any good suggestion which FET is suitable for that fm transmitter?

    I planned to inject RDS signal into this fm transmitter, any idea how can I make it work?

    Thanks.
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    There are a few ways to make a Colpitts oscillator. This way is used in most FM transmitter circuits.

    You could re-design the circuit to use a FET or two but the performance will be the same or with less output power.

    Try using a 2N3819 or MPF102 FET.

    Eliminate the pre-emphasis and use a data carrier frequency that is 20kHz. Re-design your radio to demodulate the 20kHz data.

    Thanks.[/QUOTE]
     
  16. alexander3133

    alexander3133 New Member

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    Thanks for the reply audioguru, I thought the RDS carrier frequency should be 57kHz (3 x 19kHz) instead of 20 kHz as you stated. Or am I missing something?

    Thanks.
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My Mod4 FM transmitter has a modulation limit of about 30kHz. It does not work as high as 57khz because it is designed for audio.

    If you re-design the FM oscillator to use a varactor diode for modulation then maybe it could work at 57kHz.
     
  18. alexander3133

    alexander3133 New Member

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    For the varactor diode, the design is it going to be something like this?

    http://electroschematics.com/558/fm-transmitter-with-smd/

    Or this?

    http://olec.8k.com/fm.pdf

    Also what is the function of both C2 and C5 in the FM transmitter?

    Thanks.
     
    Last edited: Oct 19, 2009
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    In my Mod4 FM transmitter, C2 stops the audio preamp transistor from amplifying the carrier frequency and C5 makes the base of the Colpitts oscillator at ground because it is a common-base amplifier for the RF frequency.
     
  20. sanjubluerock3

    sanjubluerock3 New Member

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    hello audio guru can u please send me the pcb circuit schematic (design) of FM transmitter which u have built coz i wanna etch it in pcb
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I have this pcb design but I never tried it.
     

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