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FM transmitter WASP -oscillator working ,rf output stage not working

Discussion in 'Radio and Communications' started by savvej, Jan 14, 2012.

  1. savvej

    savvej Member

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    @JimB Sir,I did the same thing in the lab today.I have attached the pics of the Output I got today..
    While using spectrum analyser,I don't know meaning of "resolution bandwidth" and "reference level".Could you explain these terms in simple words?The value of reference level is displayed in units "dBm" ,how do I interpret it?
    Also I got pics of the datasheet concerning information regarding input overloading of the spectrum analyser.
    So what I observed today is that my BW is around 1Mhz and expands even more when I speak into the mike.But BW for regular FM is around 150Khz.So I am very well exceeding it.Also colin sir said that the frequency of oscillation changes according to power supply used.But the natural frequency of the oscillator is independent of the power supply.It depends only on L and C.Btw I forgot to check this on spectrum analyser after changing resolution BW. Will check and get back on tuesday(lab closed on weekends)

    The second pic was to get a nicer pic of the spectrum using average function(16 samples) from the trace option available on the scope.
    Also the frequency is changed today because I changed the spacing between coil turns to get the spectrum to 82.5Mhz
     
    Last edited: Mar 16, 2012
  2. savvej

    savvej Member

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    Last edited: Mar 16, 2012
  3. JimB

    JimB Super Moderator Most Helpful Member

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    The RBW is the bandwidth of the filter in the analyser. Usually measured at the 3db points on the response curve.

    What you are effectively displaying is the response of the filter because the audio components of the modulation are just a few khz.
    The modulation from the noise picked up by the microphone is what is causing the ripples on the response curve.
    Reduce the RBW to 1khz and try again and see what the thing looks like.

    Reference level.The power level when the trace is at the top linr of the screen.
    In your picture, the reference level is -10dbm. This corresponds to a power level of 0.1mW.

    Note.
    Your power measurement connection sucks!
    That connection is NOT the way to measure anything at VHF.

    No, your RBW is still far too wide to observe a signal with audio frequency modulation.

    Yes, but the voltage changes the characteristics of the transistor, the transistor has capacitance and so the frequency changes.

    JimB
     
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Most simple FM transmitters change their radio frequency as the battery voltage runs down.
    My FM transmitter circuit has the oscillator powered from a 5V low-dropout voltage regulator so its radio frequency does not change.
     
  6. savvej

    savvej Member

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    @JimB sir and auidioguru sir,could you comment on how should I connect the tranmsitter to spectrum analyser to measure output power(btw there is an option of PWR on the scope itself to measure power,should I use it?).. Also one more thing,even if I dont connect the probe to the antenna and keep it nearby,still the output of the scope does'nt change much.But the output is of my transmitter only,coz if I disconnect power to the transmitter the waveform collapses.

    Also the minimum RBW possible is 3Khz for the spectrum analyser..will it work to observe the signal?
     
    Last edited: Mar 16, 2012
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A simple FM transmitter is designed to drive an antenna. If your spectrum analyser does not have an input impedance that is the same as an antenna then the transmitter might not work properly.

    Usually an antenna is 50 ohms to 75 ohms. What is the input impedance of your spectrum analyser?
    If it is between 50 ohms and 75 ohms then use 50 ohms to 75 ohms coaxial cable to connect the output and ground from the transmitter to it.
     
  8. savvej

    savvej Member

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    the input impedance of spectrum analyser is 50 Ohm.Also I am using a co-axial wire connector having crocidile clips to connect ,one of the leads to the antenna and other to the ground(battery,you can check it out in pic-4 of the first post on this page)..
    Also sir could you explain how the length of the leads connecting the battery to the FM transmitter affect the output?
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Nobody uses Crocodile clips at VHF frequencies.

    Depending on the length and frequency, the inductance of a wire might have too much reactance.

    Please do not call everybody "SIR".
     
  10. colin55

    colin55 Well-Known Member

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    To savvej:

    In post #36 I told you to read all my FM transmitter projects.

    In post #47 you are still asking why the leads to the battery affect the output of a transmitter.

    In the WASP circuit, the oscillator transistor is also the output transistor.

    When the transistor is outputting, it is operating very similar to someone on a trampoline.

    If we have someone bouncing on a trampoline and get 4 people to life the trampoline off the ground, the person will not bounce as high as the frame of trampoline will move downwards at the wrong time.

    The same with an output transistor.
    We need the power rail(s) to be rigid so the transistor can ”push” the signal out the antenna.
    In this case it is the positive rail we need to be rigid.
    We do this by connecting it to the battery. The battery is very big and it cannot move up and down at 100MHz.
    If you have a lead any longer than 10mm to the battery, the positive rail will be able to move up and down and some of the signal will be lost.

    - The oscillator transistor can only handle about 5mA at 100MHz and it is pointless using a supply higher than 4v5.

    - You cannot connect the transmitter to a power supply as it will introduce hum.

    - The WASP can transmit up to 20 miles in line-of-sight and it all depends on the effectiveness of the antenna.
     
  11. savvej

    savvej Member

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    Thnx you collin55..
     

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