Design of an RF Local Oscillator

Discussion in 'The Projects Forum' started by Mc Bulge, Feb 4, 2014.

  1. Mc Bulge

    Thread Starter New Member

    Feb 4, 2014
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    I was asked to design a local oscillator. To be used in an AM receiver to demodulate the transmitted radio broadcast at 810KHz. The Local Oscillator should oscillate at 1265KHz. The signal should be sinusoidal with an amplitude of 100mV and have no DC offset. The oscillator should be tunable to allow other radio stations to be recieved. This means it should be able to oscillate at frequences from 1000KHz up to 1500KHz. It should be powered by a voltage DC in the range of 3-10V.

    I was given this project last week and have realized that it will involve a Bipolar Junction Transistor which will need to be biased. I am also aware that some sort of feedback should be put in place in order to have the input of the sinusoidal signal. Resistors, Capacitors and Inductors could be involved in this feedback process, some sort of noise should be created.

    Does anyone on here have an ideas regarding the feedback process which includes the components mentioned? Any comments will be gratefully received.
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    1,786
    It sounds like somebody tossed you in the pond expecting you to sink or swim. You haven't given us much in the way of "design requirements" so we can know what "done" means. You really need to start there.

    How stable does this oscillator need to be?
    Are there any restrictions on using higher level IC's like a phase locked loop?
    Are there particular transistors they want you to use?

    You might want to start by picking a basic oscillator configuration. In choosing components it is often easier to choose capacitors from a restricted set of standard values and then fabricate a custom inductor by choosing a core material, wire gauge, and number of turns to get the inductance you want.

    You'll also need access to some fancy test equipment if you want to do this right. An RF signal generator and an oscilloscope are essential. An LC bridge, an s-parameter test setup, a spectrum analyzer, and a VNA would be nice.

    I hope there is a mentor somewhere in the vicinity.
    Good Luck
     
  3. DickCappels

    Moderator

    Aug 21, 2008
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    A unijunction transistor, even if you can find one these days, would not do the job. Look up "Hartley Oscillator" and "Colpits Oscillator" and use one of these with a variable capacitor or a slug-tuned inductor.

    As an example:

    http://www.bobsdata.com/metal_detector/diagrammed.jpg

    Or make it crystal controlled for a single frequency:

    http://3.bp.blogspot.com/-NE6-WxL0cz4/UfzoOjkxVxI/AAAAAAAAEAc/qf4UZ_pkBGg/s200/untitled.JPG

    Come to think of it, how many people who don't know how to design an L.O. for AM radio are given this task? How many people in the world in 2014 are designing circuits for AM radio receivers? Smells like a homework assignment.
     
    Last edited: Feb 4, 2014
  4. TANDBERGEREN

    Member

    Jan 20, 2014
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    DickCappels likes this.
  5. Papabravo

    Expert

    Feb 24, 2006
    10,137
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    I didn't see the OP mention unijunction transistor -- did I miss something?
    I wasn't aware that they were ever suitable for RF work. Horizontal retrace for CRT maybe.

    If your not familiar with varactor diodes, you should be. They have largely replaced air variable capacitors for tuning.
     
    Last edited: Feb 4, 2014
  6. MrChips

    Moderator

    Oct 2, 2009
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    Every radio has a stable tunable local oscillator.

    It is like asking an automotive engineer to design a 4-cylinder internal combustion engine.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    ...and not many of them could succeed in their first week on the job without ALOT of support. That's why I'm a bit skeptical about the facts of the case.
     
  8. Mc Bulge

    Thread Starter New Member

    Feb 4, 2014
    3
    0
    I have a choice to use types of BJT's such as BCIO7, 2N2222, 2N3904.
    I have started to investigate a Hartley oscillator and a Colpits oscillator but not sure which would be best to use.

    Thanks for your comments.
     
  9. Mc Bulge

    Thread Starter New Member

    Feb 4, 2014
    3
    0
    I am also struggling to determine the reactance ratio of the tanks circuit to generate the feedback that I require.
    An oscillator which oscillates at 1265kHz
     
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