Help: FM oscillator 2W

Discussion in 'Wireless & RF Design' started by hazim, Mar 4, 2010.

  1. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    I have an "FM oscillator 2W (88... 108 Mhz)" kit from "GENIUS ELECTRONICS"
    I have some questions about it. First, may you make a look on the circuit and give me your view if it will work fine or what?
    I compared the connections in the circuit and the real connections on the PCB (as shown in the PCB layout in the picture), I found a difference.. a mistake in the connections. The connection mistake is more probably to be with the circuit diagram and not with the PCB since the kit is being in the market from a long time.
    Here is the problem: According to the circuit diagram, the positive terminal of the battery is connected to the emitter of T2 and to C5... while on the pcb it is connected to the second side of R5, it is connected to the emitter of T1 and C4, R4, and the +ve terminal of the battery is connected to one of the terminals of the mic.

    This is the most important thing to know for now but I want to ask two other questions:
    L1 is printed on the pcb as shown in the picture, T2=sf118 have a metal case as bc109/bc107 but its size is about 3 times that of bc109 (I didn't found its datasheet). Do you think this fm 'oscillator' is really 2W? if it's 2W its range should be 3KM or even more right? There is no supplied information about the antenna.... what do you think?

    Point 'A' (on the pcb) it connected to ground, what do you think it is?

    [​IMG]
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Be aware the transistors are both PNP types.
    I do not know the SF118.
    Point A is the connection for the antenna.
    A wire of about 75 cm will do well as antenna.

    Greetings,
    Bertus
     
  3. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    Yes it's obvious that they are PNP.. maybe I don't know how this affects the connections (+ connected to mic?...)
    Ok but which connection is the right one? that of the circuit diagram or the PCB? and is it ok for the antenna to be connected from the ground?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The + on the PCB seems more logical than the + in the schematic.
    What kind of mic do you want to use?
    There is no power connection to use a electret mic.

    Greetings,
    Bertus
     
  5. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    Hello,
    I know about the connections/powering of the electret mic, it doesn't work with this kit!
    What about the antenna? is it ok to be connected from the ground?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The antenna only must be connected to this point A.
    When you want the antenna away form the PCB then you can use a 75 Ohm coax.
    The core is connected to point A the sleeve is connected to the ground.
    You can make a dipole antenna using two wires of 75 cm on the coax.

    Greetings,
    Bertus
     
  7. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    bertus, I can understand what you are saying well, A (antenna) should be that point at the collector of T2 and VC and L right? Please look and examine where point A is connected on the PCB. It is obviously connected directly with the ground.

    regards
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Point A is about halfway the curved trace.
    This curved trace is the coil.

    Here is where point A is in the schematic:

    [​IMG]

    Greetings,
    Bertus
     
  9. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    Yes you are right. Do you think it transmits clear voice for 2KM?
    All the FM transmitters with coil on PCB I saw, transmits for few hundred meters maximum
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    As the power is generated by the oscillator itself, I do not expect much of the range.
    It will be larger if the power supply is higher (more power).
    The stability will also be not so good as heating of the T2 will give a drift.

    Greetings,
    Bertus
     
  11. hazim

    Thread Starter Active Member

    Jan 3, 2008
    419
    13
    Thank you very much bertus. I bought this kit to use in a project I'm building. It is labeled 2W and 2W fm transmitter fits my need. Now I noticed that this kit can't transmit a clear signal for a distance of about 2KM.. I want a simple circuit to do this job. I got this one:

    [​IMG]
    This is a well designed basic FM transmitter that you can easily recive the signals transmitted from this transmitter in a 1-2km range with using a normal FM reciever. Another property of this circuit is that the bobin is placed on the printed circuit board. The input sound’s amplitude can be adjusted by using the P1 potentiometer.

    Parts List:
    R1 220K
    R2 4.7K
    R3,R4 10K
    R5 100ohm
    C1,C2 4.7uF electrolytic
    C3,C4 1nF
    C5 2-15pF
    C6 3.3pF
    Q1 BC547C
    Q2 2N2219A
    P1 25K
    MIC ecm series

    And here is the datasheet of the 2N2219A transistor:
    http://www.pci-card.com/2n2219a.pdf

    I just want to know your view if this one is good and should work properly.

    Thanks again

    Regards,
    Hazim
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    It still is NOT well designed.
    With an 800 mW transistor you will never get 2 W output.
    I think the output will be 300 mW (400 mW at max).

    For a good design, the oscillator is seperate from the endstage powertransistor.
    Here is a little better design:
    http://home.zonnet.nl/vdheeden_ls/Zender.htm

    It has s FET as oscillator with three transistors for the amplification of the transmitting signal.

    Greetings,
    Bertus
     
  13. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    For this circuit http://english.cxem.net/radiomic/bug22.php
    If I replaced T2=2N2219 with 2N3553 (65V, 1A, PQ=2,5W(175MHz)), I think the transmitting range will increase much more and so I solve my problem. What do you think?
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Again the claiming the 2n2219 can deliver 4 watts, 400 mW at most I would say.
    The 2n3553 will deliver 2.5 watts at most when powered on 28 Volts.

    Again the oscillator has no stabelized powersource.

    In the provided schematic the oscillator is powered via a 9 V zener.
    I must admit the given power of 5 Watts is not correct:

    SD1127 =MRF237, BFQ43 3W VHF TO5 beter dan de MRF

    Greetings,
    Bertus
     
  15. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    I confess that RF defeated me :). Actually, I tried a lot to build a powerful transmitters earlier, my theory background was insignificant, but now it's better.. I still have problems with rf circuits even though I love it. One of the problems is that I hate working with coils and I find it hard many times. The main problem is with the parts needed, wires for coils with different thicknesses and-ferrite core-rf transistors...etc because I live in a remote village far from electronic components stores...etc

    I will try the last simple circuit I provided and another one, I'll try to edit them and see...

    Anyway thank you Bertus for your significant help :)
     
  16. bertus

    Administrator

    Apr 5, 2008
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  17. hazim

    Thread Starter Active Member

    Jan 3, 2008
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    Thank you :)
     
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