A Great Guide To Software PLLs

There are some things that you think you know quite well because you learned them in your youth and you understand their principles of operation. Then along comes a link in your morning feed that reminds you of the limits of your knowledge, and you realize that there is a whole new level of understanding to be reached.

Take Phase Locked Loops (PLLs) for example. You learn how they work, you use them for frequency synthesis, and you know they can do other things like recover noisy clock lines and do FM demodulation. But then you read [Paul Lutus’] Understanding Phase-Locked Loops page, and a whole new vista opens.

He’s discussing PLLs in the context of software, as part of a weather fax decoder project, and this allows a perspective that was unavailable to those of us who learned about them through the medium of hardware such as the venerable 4046 CMOS chip. We can easily look at different PLLs with varying parameters, for example their use with a narrowband loop filter to retrieve signals buried in the noise, all through some straightforward code tweaks rather than extensive circuitry. It’s a page that’s a few years old now, but resources like this one do not age.

If PLLs are entirely new to you then you need to reat last year’s excellent PLL primer by Hackaday’s own [Al Williams].

[via Hacker News]

[PLL diagram: Chetvorno CC0]

16 thoughts on “A Great Guide To Software PLLs

        1. And here I though math made designing the hardware possible, including the hardware necessary to construct SDR equipment. While SDR is great and all, but it isn’t noting special, while the is an attempt to make it sound greater than what it is. Software facilitating the marriage of RF hardware and computer hardware. Shit that was started shortly after relatively inexpensive computers made it to the benches of amateur radio operators, and has been maturing ever since.

      1. I often feel like our having say the Realtek SDR hack was completely unintentional and like so many amazing hackable things(think Nokia Maemo phones) is more of an accident or an oversight. Were it not for the electronic wild west in Senjen China I doubt we would still have the DVB-T sticks for so cheap either without a hardware revision removing the i/q sampling mode hack form ordinary users because reasons.

  1. > ” … a whole new vista opens.”.
    You must be operating in ‘Positive Feedback Mode”.

    I can’t count how many times I’ve been somewhere, even worked there a short while, and their connections are inverted; their Hardware (Brain) is Software and their Software (Ideas) are Hardware (they learned the ropes).

    Fact is that they learnt somewhere, from ‘someguy’ at someplace, they learnt something; and now having worked at their current location over a half dozen years (some, even decades) they ‘know about this stuff’, and if you know to listen you’ll be lucky, they’ll share one of their Tips.

    To that I respond “pffft”, spin the Dial to some ridiculous setting, and outperform their weak effort by several times – boom.

    In Positive Feedback Mode they know they’ve learned something and been making the same mistake all their life, in Negative Feedback Mode they get a short circuit and know they’ve got another card to play – like you’ve laid down 4 Aces and they can lay down a few small cards and a 10, that must be better.

    Rule to remember is sometimes the Rules get rewritten.

    Anyone want to hear what I learned about the Planet Pluto when I was in School, you know the rest, or not.

    SDR is one of the greatest things to come to Radio. New Modulation schemes can extract more from a given bandwidth and beamform the signal. Predistortion can compensate for errors in the Path and Receiver producing a perfect reproduction of the original. It’s even possible to spread the Spectrum making it immune to interference and increasing the range with a given power budget.

    The Hardware’s hardwiring was it’s shortcoming, satiric.

  2. Recovering clocks from noise – that’s up there with DSP correlation and other such voodoo.
    I first learned about pulling signals out of noise on an old radar system, where it was called “Sub Clutter Visibility” or SCV.

  3. Cool stuff! Paul Lutus created Applewriter, the very slick much-more-than a word processor for the Apple II family. In fact, Don Lancaster, the “original hacker”, wrote his books with Applewriter and used the embedded scripting to put the PostScript for his illustrations right under the text, and a lot of other tricks with embedded snippets.

    The included code (iPython Notebook?) invites some experimentation, which may take up some spare time for a couple weeks :-)

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