CD4046 fun: the PLLedal and the Hacksaw

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CD4046 fun: the PLLedal and the Hacksaw

Apr 17, 2022 10 min read

Updated: Aug 22, 2022

Originally posted on the Freestompboxes forum

Among the CD family of chips, the 4046 surely stands out. It's not a simple, but still cool, logic gate or switch, it's a whole system under the name of “phase locked loop”! Of course, seen this way it doesn't make it any more clear how and why it's used in pedals, but after reading this app report it's easier to identify the building blocks and how they interact in what they do. In brief:

• We have two inputs to be compared, of which one is inverted. These get routed to not one but two phase comparators. Those compare both frequency and phase of the inputs and output a voltage proportional to how off they are. The two are different both in the way they are built and in how well they work in certain situations. Usually, PC2 is preferred for our use, although PC1 is still usable if unreliably, like in the Raygun youth fuzz. The issue, or the fun part, of PC1 is that little note about locking on to harmonics of the center frequency. Once that happens, good luck getting away from that arbitrary frequency.

• This voltage is then used to control a VCO, but first it needs to be smoothed just right. Usually this is done with an RC filter, but I want to highlight this document which recommends using an actual active integrator here, with design equations. I've tried it and it works, but I didn't like it as much. It's probably a good idea in other applications, but alas we only care about how it sounds here. Notice that since the integrator is inverting, the two inputs get swapped.

• You heard that right, VCO. This thing has an actual VCO inside, and you wouldn't be the first to think it can be exploited by itself. The most notable example is the X-4046, an incredibly usable voltage controlled synth which actually skips a step and controls the oscillator current, but some parasit pedals also offer a way to control this voltage and so the output frequency. Normally though, this is where the voltage from the phase comparator gets fed, and if you only wanted a square wave following the frequency of the input, you'd feed the output of the VCO to the “comparator” input of the PC. You now have a negative feedback loop, and by the might of feedback, it means that the VCO frequency will be adjusted as to be the same as the input and minimize the “error voltage” produced by the phase comparator.

• There are other usually neglected pins. The demodulator output is a source follower connected to the VCO input. The phase pulses goes low whenever the phase comparator 2 detects a phase difference (if you were to probe this pin you'd hear some buzz until the two signals lock together). You also have a zener to use for voltage regulation if you want.

The PLLedal

After this introduction, I wanted to make my own 4046 based circuit with the expressed goals of simplicity and usability. I've looked at previous circuits using this chip