A Basic Introduction to Phase Locked Loops

Phase-locked loops have been an electronics concept I've never quite understood.  Wikipedia of course has a good cursory introduction on the subject, but PLLs are so dynamic that it's hard to really grasp the concept regardless of how many articles you read on it.  Horowitz and Hill of course do a decent job in The Art of Electronics, and the ARRL Hardbook adds some insights on the subject with regards to analog applications, but I still didn't quite get it.  Jeri Ellsworth hosted a video on the subject, but even that didn't quite give me the intuitive understanding I was looking for.
The inevitable next step was to break out the breadboard, put together a simple PLL, and just play with it some to try and really get the concept.  Luckily, the classic 4000 logic series includes the 4046 PLL chip, which includes both a voltage controlled oscillator and both types of phase detectors.  After putting it all together, I figured I'd record a video and let you enjoy the play-by-play as I try and break it all down for you.


The PLL is an excellent example of your basic control loop.  Fin is the signal being fed into the loop, which needs to be recovered.  This is then compared to the VCO's frequency, and this comparison is used to speed up or slow down the VCO to bring the two frequencies in lock.  Of course, there are all sorts of extensions and alterations that can be made to this loop, so don't think that this is it.
The parts list isn't too bad; just two CD4046s and some basic resistors and capacitors.  The 4046 on the left of the schematic is being used just for its VCO as the input to the right one, which is actually wired up as a PLL.

 FM PLL Demodulation Video:

Of course, it's hard to understand why we're so excited about this phase-locked loop without a solid example, so as a follow-up to the first video, I made some alterations to use this VCO and PLL to use it to transmit frequency modulated audio.
 The needed modification is minor; I changed the VCO frequency controls to have it running at 100kHz instead of the 1000Hz I used for the first video.  Then I set up the VCO so it'll be modulated by an audio signal fed in, and amplified the VCO control voltage on the PLL.

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