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Thread: I Enjoyed A "Digitally Controlled VFO" Project

  1. #1
    Junior Member
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    Jul 2016
    Pineville, KY 40977

    I Enjoyed A "Digitally Controlled VFO" Project

    This was a project that gave me some contentment to work on though I did so with a meager level of lab equipment on the test bench.

    When I first became interested in Software Defined Radio, before present advances and low cost RTLSDR devices came out with software that can now sample many mega Hertz of spectrum. I was looking for a way to provide a means to tune SDR hardware with a VFO and with digital stability, with low phase noise and the typical spurs that a DDS synthesizer creates. This lead me to articles about the Huff & Puff VFO Stabilizer, edge triggered comparator circuits, that were like Phase Lock Loops, where instead of phase locking these circuits triggered themselves on the rising edge of RF wave forms, and with the use of a varicap diode in a VFO circuit would lock the VFO up with a 80 MHz or higher local oscillator reference (some circuits use reference or clock frequencies down in the LF frequency range and so some require HF or higher clocks, while other circuit designs use 70 kHz down to 30 kHz oscillators as the clock reference). This circuit provides rock solid stability without the spurious outputs that a PLL circuit or DDS circuit generates, though the DDS is superior with extremely low phase noise as compared to the PLL circuit.


    This circuit is my first basic concept, I have some more advanced circuits since this.

    This was very interesting for me to explore and hobby with, it required both bench test equipment and software to explore since there were no handbooks on this subject, just circuit diagrams that hobbyist had submitted.

    As time went along I decided to document what I was learning in a reference I could come back to in the future to expand on or revise as findings allowed me to better define the subject as well as make correction based upon new findings. That document is available but it is too large to attach here in this forum since its size exceeds the file size limit. I did manage to unlock the secrets of the circuit to be able to further it at a later date, plus I gather up all the math on the subject and derived a few useful equations of my own for future design use. To have compiled all the math and derived a few useful equations, as well as compile some graphics and text for future use, is my accomplishment here, it pleases me today when I look at the files which I have stored away in online archives to return to.

    Today I still find the subject fascinating and hope to return to testing these circuit more in depth as well as build those I explored with Spice software. I do have some notions about things that can be done to perhaps make these circuits into useful products not only for Software Defined Radio but for general use by radio hobbyist who like to build their own radios. Digitally controlled VFO kits, and radio kits with such a circuit, with a simple inexpensive frequency counter for the display are some of the ideas I have had for this circuit.

    I appreciate having had delved into the subject, since it provided me with a vision of a simple product that can be developed and would result in a low price simple to build kit, at least that's my hope. Fortunately I have kept copies of my Spice files too, I would hope to be able to collaborate with others on this subject in the future. I believe that in working on this for a few months again, once I have reestablished much of my test bench, will result in a product. I have had to rebuild my shop after some time of being out of electronics, but I have most of everything going again. Enough to work on some minor design projects. This project would require I obtain a NIST traceable frequency counter, or something near to its precision, my present frequency counter will not cut the mustard here. The chips for the project are fairly cheap and abundant. So once I have ordered a new breadboard and some parts I will be back into this project again, in between having to work on radios here at the shop.

    In the course of this project I also explored and designed IQ quadrature phased detectors and phased modulators for SDR radio use, and that too provided some enjoyment in exploring these things with LTSpice.
    Last edited by dansradio; 07-26-2016 at 07:11 PM. Reason: correction

  2. #2
    I investigated with Spice programming. Essay Writing Service I do have a few thoughts about things that should be possible to maybe make these circuits into valuable items for Software Defined Radio as well as for general use by radio specialist who get a kick out of the chance to construct their own radios. Carefully controlled VFO packs, and radio units with such a circuit, with a straightforward cheap recurrence counter for the show are a portion of the thoughts I have had for this circuit.

  3. #3
    Excellent post. Thank you very much!!

  4. #4
    What a good approach to the subject, also think so.
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