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How to (ab)use a PIC ;-)

Discussion in 'Electronic Basics' started by Anthony Fremont, Mar 27, 2007.

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  1. First off, big thanks to everyone for the overwhelming amount of help I
    received.

    I've been playing around with LC oscillators and other RF like things lately
    and I decided to try my hand at making a superhet WWV receiver. Not being
    an analog kinda guy I was plagued with too much drift at 9.545 MHz, amongst
    other things. Being a PIC kinda guy, I thought it might be neat to apply
    one here. ;-)

    So I took a couple of variactor diodes and added them to the oscillator. I
    tapped the oscillator buffer output, fed it thru a Schmidt trigger to square
    it up and then into a PIC to clock TMR1. Over a 4mS (actually 4.096mS), I
    count pulses with the 16 bit timer. This gives a decent count without
    overflowing the 16 bit timer. Every 20mS I look at the counted frequency
    and then "dink" the duty cycle of the hardware PWM. The PWM output is fed
    thru a low-pass filter to smooth it out and then applied as reverse bias to
    the varactors.

    The end result is that the oscillator "locks" within a second or two of
    power on and then stays within 100-200Hz of the correct frequency from there
    on out. Not fantastic stability, but should be entirely usable for AM
    reception. Of course there is lots of room for improvement, but I thought
    it neat anyway. :) The PWM could be upped to 10bits and the frequency
    counter gate period increased. This would allow _much_ more precise
    frequency control than I presently have.

    Here is a pic of the circuit constructed on breadboard (800KB):
    http://i55.photobucket.com/albums/g143/afremont/DSC00885_edited.jpg

    The top left is the oscillator, just to the right of that is the buffer
    stage. Further to the right is the NE602 mixer that combines the signal
    from the antenna/preselector and the LO. The detector is not finished yet,
    but the receiver is hearing on 10MHz, just not very good yet. ;-) The
    bottom half of the board contains the PIC which is clocked by a 4MHz crystal
    and the 74HC14 gyrator...er..um hex inverting Schmidt trigger.

    Just thought some of you all might get a kick out of it. :) Thanks for
    looking.
     
  2. jasen

    jasen Guest

    It sounds kind-of like you've invented the PLL... :)
     
  3. Henry Kiefer

    Henry Kiefer Guest

    | First off, big thanks to everyone for the overwhelming amount of help I
    | received.
    |
    | I've been playing around with LC oscillators and other RF like things lately
    | and I decided to try my hand at making a superhet WWV receiver. Not being
    | an analog kinda guy I was plagued with too much drift at 9.545 MHz, amongst
    | other things. Being a PIC kinda guy, I thought it might be neat to apply
    | one here. ;-)
    |
    | So I took a couple of variactor diodes and added them to the oscillator. I
    | tapped the oscillator buffer output, fed it thru a Schmidt trigger to square
    | it up and then into a PIC to clock TMR1. Over a 4mS (actually 4.096mS), I
    | count pulses with the 16 bit timer. This gives a decent count without
    | overflowing the 16 bit timer. Every 20mS I look at the counted frequency
    | and then "dink" the duty cycle of the hardware PWM. The PWM output is fed
    | thru a low-pass filter to smooth it out and then applied as reverse bias to
    | the varactors.
    |
    | The end result is that the oscillator "locks" within a second or two of
    | power on and then stays within 100-200Hz of the correct frequency from there
    | on out. Not fantastic stability, but should be entirely usable for AM
    | reception. Of course there is lots of room for improvement, but I thought
    | it neat anyway. :) The PWM could be upped to 10bits and the frequency
    | counter gate period increased. This would allow _much_ more precise
    | frequency control than I presently have.
    |
    | Here is a pic of the circuit constructed on breadboard (800KB):
    | http://i55.photobucket.com/albums/g143/afremont/DSC00885_edited.jpg

    This loosely coupling in a pll is general known under the name huff-puff.

    regards -
    Henry
     
  4. john jardine

    john jardine Guest

    [...]
    Nice idea, neat layout!.

    On mine, I've 10u's at each end of the power rails acting as backup to the
    local decoupling.
    Wire links made on the spot from chopped up CAT5 (cat4-3-2-1?) twisted pair.
    The wire size seems perfect and can be colour themed.
    Sadly, I also have a little weep every time I have to chop the legs off
    stuff (eg inductors). Sort of like, anything I can remember how much it
    cost me to buy :)
     
  5. Thanks allot John. :) I sorta cheated, this was the second build since I
    ran out of room on the "single wide" proto-board and I also wanted to see if
    I could get rid of the horrid bounce.

    It was one of those projects that I just wanted to see if it would work. It
    actually works allot better than I expected, once I got the frequency
    counting lined out. :) Just one more stupid-PIC-trick in my collection.
    I'm always changing up the power jumper layouts from board to board. I'll
    add some of those and .01uFs to go with some 10uFs in the corners. When I'm
    using a new board, I often forget to put the little jumpers to connect the
    upper side rails with the lower ones. That's usually good for five or ten
    minutes of mystery. :-?
    I admit, I tend not to trim allot of the cap leads, but I do cut the other
    parts down to about half their original leg length. I'm pathetic, I buy the
    premade wires and use them most of the time. ;-) I can get them fairly
    cheap locally. I'm not real happy with my latest protoboard purchases.
    They're cheap, but I fold allot of wires over becuase of misallignment of
    the holes and metal contacts, and man I really hate that. :-/ I do find
    that as I use them, they become allot more friendly.
     
  6. Cool. I'm a bit surprised you didn't have any problems with using the
    breadboard at that frequency though.
    How is the new scope going? Have you posted your initial comments on
    it on the group yet? I may have missed it if you did.

    Dave.
     
  7. Joop

    Joop Guest

    If you are interested, here is a 10Hz "huff-puff" VFO stabilizer
    version by EI9GQ.
    http://homepage.eircom.net/~ei9gq/stab.html
    Perhaps you could combine his and your ideas.

    Joop
     
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