Friday, August 26, 2011

4046 vs 74hc4046

Got the COCOQUANTUS boards yesterday and I rushed to the workshop on Bentalou Street to test them out. All in all they work, no mystery parts (like last time), all the parts used are durable, stock items, that really work. Here's a picture of one of the unpopulated boards (populated ones are sitting in the workshop on Bentalou):


One interesting thing that came up: involving the 4046 digital VCO chip. Fansz of Nicolas Collins' book will note this is actually a PLL chip that can do some wonderful things, but note also that I only use this chip for its VCO section, for its unique topology of raw CMOS yields a sweetly symmetrical high frequency square wave. It is worth using this chip for its VCO, rather than homebrewing something, for the moment.

Furthermore, I am not even using it as a VCO, rather as a CCO! You see, I am tying its control-voltage input to pos5volts, and actually modulating its prime current input. Into this feed mine own arp-style exponential current generators and serge-style differential pair modulation inputs.

NOW the problem arose late last night, and I mean late, that the oscillator was able to run at non-relativistic velocities when given a modest control current. I kept upping the timer capacitor, up to almost 1 microfarad (!), and it still would run ssssuuuuppppeeeerrrr fast at the high end, but increasingly slower at the low end. Weird. I am still baffled how the silicon did this, but I did learn the fix, by comparing the old, Texas Instrument datasheet with a newer one, by NXP. What follows are pictures of how the chips respond to control voltages. First, the CD4046, note it is a smooth ramp, limited at its ends:



Now for the 74HC4046, it seems in updating the design for the high speed, 5V HC series, there is now some sort of new behavior above 4.1 volts, that the frequency of the VCO jumps up towards infinity (actually the number is 7Z96051.1 according to the datasheet):

So although I don't know how the silicon actually does this, I could go about creating a solid fix. I was driving VCO too high, it shouldn't be tied to positive 5 volts, but more like 4.1 volts. Resistor divider ipsus solutio est (4.7k 22k).

Still though, worth noting that this chip attempts to do hyper-relativistic speeds, and I noted quite a strange resonance on my oscilloscope as well as extensive interference on the FM band. Perhaps this can have some sort of noise radio usage, a phreaked out oscillator?

1 comment:

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