WTPA2, Current Sinks, and the 74HC4046

So once again somebody on the forum came through and found this guy’s page about designing a switched capacitor filter, where he talks about using a CURRENT SOURCE (sink, actually) instead of a resistor on one of the 4046’s frequency setting pins. This was a real forehead smacker. Thanks Kyle (erschlagener) for the heads up, and thanks to Tim for putting that out there in the first place!

Requisite googling found an EDN note about this, too:

Which claims a 1000:1 frequency range out of the 4046’s VCO!
It also confirms what I’d found on the breadboard before, namely that the 4046’s frequency output is not particularly linear outside of about 1v-4.75v, and that inside there you can expect to get about a decade of range. So I felt less dumb.

This note also has the bright idea of putting the sink in the feedback loop of an opamp, which almost certainly helps linearize the sink’s behavior at low voltages by getting rid of base current related weirdness.

Enter a quick trip to findchips for some pricing data. Turns out that the 4046 and a transistor are certainly cheaper than the LM331, although they take up more real estate. Even throwing in an cheap LM358 still costs less than the LM331, although the savings start to get less significant. Now, if there was some good use for that second op-amp in a ‘358, that’d be something…

This recalls some quote I heard and want to attribute to Jim Williams, though I can’t recall exactly where it’s from, about how in modern circuits like 80% of the thing is digital and takes 20% of the time to design, whereas the 20% that’s analog takes the 80%. Sometimes it does seem true, although it might just be (for me anyway) that the analog parts are a lot of fun :-)

Back to the breadboard!

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