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Uni Project FM TRnasmitter

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by meeral20, Jul 24, 2009.

  1. mneary

    mneary New Member

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    Four pages ago the O/P asked for help analyzing a circuit given by his professor and our conclusion is that the only correct approach is to replace it with your favorite circuit.

    Other circuits (including the one provided by the O/P) cannot possibly work properly, to the point of not being worth building or even simulating.

    Should we take a lesson away from this?
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The circuit might work, but without pre-emphasis it will sound awful.
    So why not find a better circuit? Like mine. It sounds perfect.
     
  3. Mikebits

    Mikebits Well-Known Member

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    Okay Nigel, according to forum rules, I think you need to give yourself one infraction. :)

    Edit----------------- This may be pushing it...
    http://www.youtube.com/watch?v=9efgLHgsBmM
     
    Last edited: Jul 28, 2009
  4. dave

    Dave New Member

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  5. transistor495

    transistor495 Member Forum Supporter

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    'muffled' too!:)


    Finally..!!

    The circuit looks good and it minimizes amplitude modulation by using varycap method. Why not add pre-empha..that thing and troubleshoot the problems. Let new circuits to come and newcomers get choices and happy shopping!
     
  6. marcbarker

    marcbarker New Member

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    edit: To be fair to NG, the word appear was used. :)

    Well I suppose in a way it doesn't matter who was right / wrong, as long as it it's debated in a gentleman-ly like manner and is a learning experience...

    Perhaps we could wrap this up Democratically with an Opinion Poll?

    1. Does the OP circuit (& Clapp Osc. posted) operate in "Class C" YES / NO

    2. Is the OP circuit's Varicap connected with correct polarity? YES / NO

    3. Is the OP a Good circuit? YES / NO

    Until ever sufficiently proven one way or another it seems there'll be no "right" or "wrong" answers to the above, so in the meantime feel free to vote democratically! :)
     
    Last edited: Jul 29, 2009
  7. marcbarker

    marcbarker New Member

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    Why BJT instead of FET? Just wondering :)

    About the proposed DC variable-controlled tuning, that might add too many extra parts. Otherwise, tuning is provided by bending L3

    This osc cct. (Clapp) is a development of the Colpitts.

    C9 and R4 might possibly be a crude 'pre-emphasis'. (more like a bass-cut)

    The FET Clapp osc might not be very voltage-dependent, or it might already be supplied by a voltage regulator.


    P.S. The OP Circuit has a features that could be borrowed to possibly improve the "AG Mod4 FM TX" design:


    Supply Rail 'decoupling' at each stage, 4 off, (C1, C2, C15, C16)

    Antenna load 'pi-matching' network & harmonic filter, adjustable. (C11+17, L2, C12+18)
     
    Last edited: Jul 28, 2009
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Pre-emphasis and bass-cut are two completely different animals.

    My FM transmitter has a 1000pF +5V supply bypass capacitor and another 1000pf +9V supply bypass capacitor.
    The capacitors are ceramic so are only 1.6 ohms at 100MHz.
    it also has a 100uF capacitor for +5V and another 100uF capacitor for +9V.

    Antenna load matching and filtering can be added but will not make a noticeable difference.
     
  9. marcbarker

    marcbarker New Member

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    Can this information be substantiated a little more?
     
  10. BrownOut

    BrownOut Banned

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    What I would have liked to hear from you is how this bias is produced. Another poster showed us a diagram where a diode rectified the RF and the charge was stored in the networks capacitors ( I guess ) Does the varactor do the same thing in the OP circuit? If you know how the bias is generated, share with us ( me at least ) I know how frustrating it can be when nobody wants to listen, but as Dr. Frasier Crane says, "I'm listening."
     
  11. mneary

    mneary New Member

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    I thought I said that in my simulation the negative bias was generated when the high AC voltage on the JFET gate went positive. The JFET gate conducts like any diode, and the capacitance of the varicap and the other caps store the charge. The varicap is not forward biased at any time (unless the audio is outrageous).

    The JFET circuit doesn't need a voltage regulator as badly as the BJT circuit because the JFET channel doesn't change capacitance like a BJT does.

    If this wasn't clear, it should be now.
     
  12. mneary

    mneary New Member

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    It won't make a noticeable difference if you don't care and you won't look.
     
  13. marcbarker

    marcbarker New Member

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    I know I said I was 'done with this thread' but please indulge me...

    First mentioned on #21. No one seemed to want to research it further, but instead make ill-informed judgements and generally pouring scorn on what I think is a good circuit, which I still defend.

    #28, #32 simulation of Clapp osc circuit suggested, but was responded to with more judgements and refusal to "waste time on something that doesn't work anyway". "Grid leak bias" was mentioned, and thread hung up on being "cathode bias"

    In #44 I stated this:
    "*the j-fet is only at maximum gain when the osc first starts up (class a). Start up begins by self-noise being amplified and fed back. Thereafter, the j-fet pinches itself off by the RF voltage it makes (& negative voltage for the Varicap). Because the j-fet only conducts during part of the oscillator cycle, is why it's Class C. "

    At the end of #59 http://www.electro-tech-online.com/threads/uni-project-fm-trnasmitter.95651/#post770466 I linked to some detailed analysis.
     
    Last edited: Jul 29, 2009
  14. BrownOut

    BrownOut Banned

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    I already acknowledge the judgements. I was hoping we could get past that and have a technical discussion for a change. I've seen this configuration many times in the past and always wondered how it works. When I tired to follow the analysis, I got a broken link. I'll try again and see if the discussion can answer my questions about the ckt. Hope you can get pase the emotions and discuss this. Really, it was only a couple people who didn't want to hear about it. The rest of us are actually interested in learning.
     
  15. marcbarker

    marcbarker New Member

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    One of the earliest and best descriptions is by a published paper of the person it's credited to: J. K. Clapp, "An inductance-capacitance oscillator of unusual frequency stability", Proc. IRE, vol. 367, pp. 356-358, Mar. 1948. I couldn't find a link for it though.

    In an oscillator with a stabilised amplitude, there's often a switch of some kind involved with the amplitude stabilising. Many years ago I did a precision low frequency oscillator, with bandgap-reference precision amplitude stability. It was based on a Wein oscillator, with a variable-gain element controlled by DC voltage. The amplitude peak level was detected by a 'class-c' switch, which fed narrow 'wave-tip' pulses into a leaky charge pump, then into the variable gain element to close the amplitude control loop. It was very simple, only one opamp, a bandgap reg and 3 transistors.

    Another variation of stabilised amplitude oscillator is the 'thermistor regulated' oscillator. The amplitude of the oscillator is regulated by the self heating of a thermistor in response to amplitude. (I tried this oscillator previously before I developed the bandgap-stabiliised circuit better and cheaper in the company's product)

    The J-FET circuit in this thread is somewhere in between these types. It's a bandpass tuned amplifier, with positive feedback. The gain of this amplifier is variable, according to a shaped curve: gate voltage/drain current. Generally, the more negative the gate voltage, the less gain the FET has. When the oscillator has stabilised, the LC tank resembles a clock pendulum, and the j-fet the escapement. There is a start up, that I'd described in an earlier posting.

    Like the bandgap-stabilised oscillator I described above, the j-fet conducts drain current only when the postive sinusoid biases the j-fet on. In practice, in a well-designed circuit, it will only conduct on the positive tips of the RF wave. Since this conduction period is considerably less than 50 % is why it's called a "class c oscillator".

    The start up is described* in: http://www.electro-tech-online.com/custompdfs/2009/07/322Lecture24-1.pdf apologies for the link sometimes not working, if it doesn't at the time you try it, try again later.

    *Thanks go to Keith W. Whites, Professor of Department of Electrical and Computer Engineering, The South Dakota School of Technology, Rapid City
    Keith W. Whites EE 322/322L Electronics II - Wireless Communication Electronics
     
    Last edited: Jul 29, 2009
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Instead of using a regulated reverse bias voltage for the vari-cap diode, the FET Clapp oscillator makes a reverse bias voltage for the FET and for the vari-cap diode depending on the particular spec's of the FET.
    I wonder how unstable is its frequency with temperature changes?
     
  17. marcbarker

    marcbarker New Member

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    That is a good point raised.

    If the FET pinch-off voltage alters with temperature, it will directly alter the frequency. That's probably 2.2 mV / deg C, whatever that equates to in frequency. Other 'particular specs of the fet' that affect the tuning stability, such as gm curves I suspect might be stable enough, I don't know for sure.

    The otherwise superior performance of this oscillator has been degraded somewhat, but again this trade-off probably acceptable in the application.

    It might be an idea to actually build and test one of these and see.
     
    Last edited: Jul 29, 2009
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My FM transmitter has a transistor oscillator with a voltage regulator to keep its supply (and its capacitance) from changing.
    The transistor clips so its output level is fixed but the harmonics are removed by the tuned circuit.

    I tuned it indoors where it is warm then tested it in the cold outside and its frequency did not change.

    It makes some AM in addition to its FM. But an FM radio ignors the AM.
     
  19. marcbarker

    marcbarker New Member

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    Just thought I ought to re-post the OP circuit, for easy reference.

    [​IMG]
     
  20. Hero999

    Hero999 Banned

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    You don't get any AM from a varicap oscillator, you can bias it to give maximum output so you don't have to worry about any clipping.

    EDIT:
    Here's an idea: why not RF couple the oscillator to the amplifier?

    The capacitance of Q1 will change with the audio level and C3 will have a very high impedance at audio so it won't cause any AM. The output impedance of Tr1 should be high enough not to load the RF stage too much.
     

    Attached Files:

    Last edited: Jul 30, 2009
  21. marcbarker

    marcbarker New Member

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    Here's a circuit that was posted on another thread
    Alan Yates' Laboratory - One Transistor Voice Transceiver

    It's a J-FET super-regenerative RF amplifier (when in recieve mode), which then switches to a Class-C RF oscillator (for transmit), re-using nearly all the same parts over again. The great thing about it is its minimalist simplicity.

    [​IMG]
     
    Last edited: Jul 31, 2009

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