Connect with us

FM Oscillator

Discussion in 'Electronic Basics' started by camediaman, Apr 6, 2004.

Scroll to continue with content
  1. camediaman

    camediaman Guest

    Can someone explain to me the following fm oscillator

    http://www.geocities.com/myelectronicsrevision/fmosc.jpg

    No need to explain the audio section, just the oscillator, what
    kind of oscillator is it, where is the input/output/feedback, what
    are these thick lines - is it waveguide technology, what is the
    equivalent component, is it a transformer?
     
  2. John Larkin

    John Larkin Guest

    Some people draw inductors that way. If so, it's a Hartley.

    John
     
  3. Dbowey

    Dbowey Guest

    camediaman posted:

    << Can someone explain to me the following fm oscillator

    http://www.geocities.com/myelectronicsrevision/fmosc.jpg

    No need to explain the audio section, just the oscillator, what kind of
    oscillator is it, where is the input/output/feedback, what are these thick
    lines - is it waveguide technology, what is the equivalent component, is it a
    transformer?---

    NOT in order of the questions:

    • The type of oscillator is Hartley.

    • The thick lines are strip-line inductors
    cut into one side of a 2-sided board.

    The longer one is equivalent to a multi-turn
    coil, tapped at about 25% from the bottom.

    The smaller, is equivalent to a 1 turn coil
    (a "link") adjacent to the top end of the larger
    inductor.

    • Output is from the small inductor (the link)
    at the point designated "ANT"(enna).

    • Feedback is from the inductor's tap, to T1's source.

    • Is it waveguide technology? hmmm! tough question...
    Waveguides and other technologies cross each other
    at many junctures, and stripline was probably first
    in microwave circuits, some of which have striplines
    within waveguides, but I don't think I would consider
    striplines to be waveguide technology. It is too
    broadly used elsewhere.

    • The "input" is audio. I'd say the input is where R2
    connects to D1.

    • Equivalent component: Use coil inductors as described
    above. I estimate the larger inductor should be about
    0.25 uH end-to-end. All the other parts and values
    are fine.

    Don
     
  4. camediaman

    camediaman Guest

    . Feedback is from the inductor's tap, to T1's source.

    It seems as though T1 is not biased, can you see any
    DC current flowing? Vgs=0 should imply Id=0
    Sorry, I meant to say micro strip technology, but I think they
    are related to waveguide aren't they, they probably borrowed
    the technique to create the Hartley coils.

    Any ideas what are C6/D2 for ?
     
  5. Rich Grise

    Rich Grise Guest

    This is very misleading; I'm not trying to be a strip-line expert, since I'm
    not, but just by the seat of my pants, the conductor from the source of
    T1 to the tap would have more inductance than the whole tuned circuit!

    I'd be willing to bet real money that that's just the way that particular
    designer draws (bulk, ordinary, wound) inductors, judging from the styles
    of the other parts.

    Of course, there's just as much of a chance that the schematic doesn't
    really represent the mechanical layout, but if that's the case, then why
    provide a more pictorial version of just the one component?

    Cheers!
    Rich
     
  6. Dbowey

    Dbowey Guest

    Camidiaman posted, in part:
    << Any ideas what are C6/D2 for ? >>

    ---

    C6 brings the drain closer to ground for RF.

    D2: I think it's main purpose is to prevent the RF voltage from becoming high
    enough to damage T1. I haven't checked the spec on D2.... It could also be a
    polarity guard, but that is not an ideal location for that function.

    So.... when are you going to build it?

    For my first experience with stripline I built oscillators generating signals
    in the UHF TV band so I could find the signal with a frequency calibrated UHF
    TV tuner. This helped me determine the dialectric constant of the board
    materials I used. A Moto Tool makes rough cuts, but it got the job done.

    Don
     
  7. Dbowey

    Dbowey Guest

    Rich posted:

    This is very misleading; I'm not trying to be a strip-line expert, since I'm
    not, but just by the seat of my pants, the conductor from the source of
    T1 to the tap would have more inductance than the whole tuned circuit!
    <<

    It is definitely stripline - That is how they are shown schematicaly.
    Schematics rarely show the critical nature of layout. The source of T1 would
    have a very short lead-length to the "tap" point.

    designer draws (bulk, ordinary, wound) inductors, judging from the styles
    of the other parts.

    Of course, there's just as much of a chance that the schematic doesn't
    really represent the mechanical layout, but if that's the case, then why
    provide a more pictorial version of just the one component?
    Because it isn't a coil. All other components have their representative icon;
    so does stripline.

    Don
     
  8. This is a schematic for a Velleman "FM Oscillator Kit". The thick lines
    actually are the inductor, and are wound around on the PC board (which I
    actually have in front of me here)

    They are about 1mm thick, separated by a distance of 1mm. There is a
    separate 10cm antenna attached to one end of the coil.

    Regards,
    Bob Monsen
     
  9. camediaman

    camediaman Guest

    Thanks everyone, one more question, is the dc bias point
    saturated, seems like Vgs=0 which should result in Id=Idss
    and R3 is not going to offer any high current protection,
    this is strange, the transistor could burn ?
     
  10. Except the audio section is a huge waste of parts. They went to all the
    trouble to step the impedance down to drive a 1k pot, then feed that
    into a 220k resistor! Stoopid! Could've been done with a single
    transistor or FET, and a lot less parts.
     
  11. Rich Grise wrote:

    [snip]
    I agree with Rich. Those are regular coil wound inductors.
     
  12. Dbowey

    Dbowey Guest

    Watson posted:

    << Rich Grise wrote:

    [snip]
    I agree with Rich. Those are regular coil wound inductors.
    Now that we have heard from someone who has the actual board, I agree the board
    does not contain a stripline inductor. The schematic, however, does denote
    stripline.

    Don
     
  13. Sorry, I must have been confusing in my post. It IS a stripline inductor,
    the trace is wound around like this:

    -------------------------|
    | --------------------| |
    | | ###------------- | |
    | | ###2 | | |
    | | | | |
    | | ###1 | | |
    | | ###------------- | |
    | | ###--------------- |
    | |---------------------|
    | |
    | |
    | |
    | |
    | | 1 is the junction for a diode and variable cap
    | |
    | |
    | | 2 is jumpered over to another pad where the
    | | antenna is connected.
    | |
    | | The traces are 1mm, the distance between is 1mm
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    The scale is off, but this is the way the traces are layed out.

    Regards,
    Bob Monsen
     
  14. Rich Grise

    Rich Grise Guest

    Thanks - I guess I didn't really know what "stripline" is.
    But, I'm pretty sure that I've seen coil-wound inductors
    shown as a black bar like that, usually either in old or
    foreign schems.

    Or have I been hallucinating? ;-)
     
  15. But what makes that stripline?

    Is it double sided board, with the other side simply grounded?

    If not then it's not stripline.

    It's as you described it before, an inductor that etched right
    on the circuit board. It's no different than if one used wire
    to make up that coil. It likely is more mechanically stable
    etched right on the board, but if the board varies with some condition,
    such as temperature, that might affect the inductor.

    Such inductors are rarely seen, but they aren't new. They aren't
    practical unless the inductor value is small enough, and for that matter
    there is limited application where you need a small inductor and can
    live with it being out in the open.

    And having once seem a brief article in a hobby magazine about
    how to "wind" one of those inductors, details are either rare or
    hidden in professional material. And unlike winding a coil with
    wire, if you don't get it right you have to scrap the board and
    start again. Not so great when experimenting.

    Michael
     

  16. Its a single sided board. The components are on the other side, but
    there is no grounding layer. I can see through the board, so there
    isn't an internal grounding layer either.

    So, what is the definition of 'stripline'? I thought it meant a trace
    which acts like a circuit element (which is what these traces do.)

    Regards,
    Bob Monsen
     
  17. John Larkin

    John Larkin Guest

    Stripline is an inner-layer trace on a multilayer board, where
    adjacent layers are ground planes.

    Microstrip is a surface layer, where the opposite side (or an inner
    layer) is a ground plane.

    (Sometimes striplines and microstrips are used without ground planes,
    but that's not common.)

    It seems the OP's circuit is neither.

    Download Agilent's Appcad, select "passive circuits", and play around.

    John
     
  18. camediaman

    camediaman Guest

    I think (as someone pointed out) the circuit of this velleman kit
    (www.velleman.be) is a one side pcb, where these coils which
    are denoted as thick lines are in effect coils which are drawn
    on the track side of the pcb in a form of a sipral.

    I have another question about this circuit. It is about T1 and its
    dc bias configuration. It seems that there is no dc voltage source.
    My electronics book says that when Vgs=0 the transistor will
    be saturated (because it is a fet). Is this the correct analysis ?
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-