 |
| Open the image in a new tab for Full Size |
While the engineering feats of folks like Bob Moog, Terry Michaels and Serge Tcherepnin are remarkable, you don't always need fancy 1V/octave temperature-compensated VCOs to play around with audio and control voltage generator designs on a breadboard. The quirks of ultra simple VCOs and their sheer efficiency (plus low cost) make them really desirable when you want to fart around with chaotic recursive feedback networks that require a handful of oscillators or just want something NOW that will squeal. Everyone has different ways of doing their quick 'n dirty VCO. The most common ones I've seen use the venerable Integrator-Schmitt Trigger VCO from countless datasheets or 555s. Here's something that I like to use...
Above you'll see two VCOs based on the CD4046 Phase Locked Loop IC. These things feature a decent linear response rectangle wave VCO from the get-go that can be coerced into wider frequency ranges and exponential-ish CV response by using a NPN transistor current sink in place of a resistor to ground on pin 11*. You can also get a descending sawtooth wave (1 octave above the rectangle) by connecting the timing capacitor to a buffer via diodes**. Awesome. The additional logic functions that come from the two phase comparators and Enable logic input let you implement Hard sync, a XOR gate for pseudo ring modulator effects and a variety of other functions. It's worth cracking open a cold one and studying the datasheet along with chapter 7 of Don Lancaster's CMOS Cookbook***. If you really wanted to, you could even use the CD4046 as a PLL (har har).
Okay, so what the hell can you do with these VCOs? Make them whenever you're bored. Substitute them for more complicated VCOs found in other designs. Substitute them for manually adjusted oscillators or clocks. Wire them together in various ways. Have a ball. (More detailed applications to follow...)
*I've seen this method mentioned in many places. Thomas Henry is probably the most vocal practitioner. Todd Bailey also uses this method for a voltage-controlled clock in his WTPA samplers. The earliest mention may be in an article by Antonio Tagliavini. Please contact me if you know more about the history of this technique.
**Learned this trick from Grant Richter's amazing WOGGLEBUG.
***An incredible and accessible book that belongs in your library if you do anything with CMOS . Thanks Don!
