Connect with us

The 27 MHz Tx

Discussion in 'General Electronics Discussion' started by samy555, Apr 21, 2012.

Scroll to continue with content
  1. samy555

    samy555

    63
    0
    May 11, 2010
    I have some simple questions about this circuit and I hope to get answers
    [​IMG]
    from: http://www.talkingelectronics.com/projects/27MHz Transmitters/27MHzLinks-1.html
    (1) Where is the positive feedback needed to sustain oscillation?
    (2)
    How will I know that the frequency is adjusted? What is the simple way to do that?
    (3)
    The crystal grounded the base at 27MHz, so the transistor is configured as a common base amplifier?
    (4)
    I have calculated the reactance of that 82p cap and it was almost equal to 72 ohm. Is not a large value? Why not choose largest value of the capacitor so that it becomes less reactive?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,350
    2,255
    Nov 17, 2011
    The link is inop.


    Harald
     
  3. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    works ok for me
     
  4. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    samy

    its a Colpitts oscillator -- you can do some google searching on that type of osc and do some learning :)

    You will need a frequency counter so you can monitor the change in freq as you adjust

    once again read up on the osc as said above..

    cheers
    Dave
     
  5. samy555

    samy555

    63
    0
    May 11, 2010
    Thank you very much for this good advice
     
  6. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    The field strength meter circuit that has been shown for you to build is a good little project on its own

    [​IMG]

    This wont tell you what frequency you are on but it will tell you when maximum power is being produced by the transmitter

    even better than the 8cm lead would be a telescopic whip antenna off an old radio receiver so it can be pulled out to full length or pushed in for storage. and at 27MHz a 30cm or more length for the whip will work so much better

    Dave
     

    Attached Files:

    Last edited: Apr 21, 2012
  7. samy555

    samy555

    63
    0
    May 11, 2010
    Dave

    How do you know that it is its a Colpitts oscillator?
    thanks
     
  8. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    There's 2 oscillators that are very common for crystal oscillators
    The Colpitts and the Hartley, if anything it is a little closer to the Hartley
    but their operation and layout is very similar. And there are often little variations
    that are played around with.

    the circuit takes very little changes to make it a Pierce or a Millar osc

    Dave
     
  9. samy555

    samy555

    63
    0
    May 11, 2010
  10. davenn

    davenn Moderator

    13,654
    1,888
    Sep 5, 2009
    There's some good info in that link
    there's a few reasonable tutorials online that you will find using google

    gosh when I was learning all this stuff, the internet didnt exist
    very basic computers were just starting to appear. It all came out of mainly amateur radio books like the ARRL Handbook, for me

    Dave
     
  11. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    The feeback for oscillation comes from the capacitance in the transistor, transistor type may therefore be critical.

    The crystal does not ground the base, it will have a parallel and series resonance and the circuit will operate somewhere between these.

    If I remember correctly, the collector circuit has to be tuned to be inductive to oscillate. Just tune to maximum output.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

    10,350
    2,255
    Nov 17, 2011
    Dave: It now works for me, too. It didn't in the first place. But you explained everything.

    Harald
     
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

-