50 kHz VCO w/sine output

/tek/fg502/

y

Thanks, I don't quite get the diode bridges. I was going to try Phil's tanh shaper idea. (logarthimic shaper in the AN263 app note.)

George H.

Nice! I think I'd like to try it using a Cypress PSoC to make a tiny DDS for the range of about .1 Hz to 10 kHz or so.

Thanks.

John S

Tiny? look at the Analog Devices AD9833. It's a 10 pin SOIC.

--
You can't have a sense of humor, if you have no sense.

Happy to have provided the pointer -- it's not my project but I did play around with it, back in the day. Fun stuff!

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Rich Webb     Norfolk, VA

3.3:1 in frequency is ~11:1 in capacitance. I'd go with one of the cheapy A-to-D approaches, although a sawtooth integrated twice is nice if you don't mind using some kind of leveling AGC. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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I love to cook with wine.     Sometimes I even put it in the food.

It's 3.3:1 in capacitance if you gang-tune a pair of capacitors in a Wien bridge. That's still impractically large, but that's what it is.

It's 11:1 if you're tuning against a fixed inductor.

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www.wescottdesign.com

e

=A0 =A0 ...Jim Thompson

=A0 =A0| =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

ide quoted text -

Thanks Jim, I've pretty much abandoned the above Wien-bridge thing. I'm pretty sure I could make it work... but I'd be disappointed with the distortion...I'd guess something like 1%.

I did order some of the 4017 decade counters. (Phil H's suggestion.) That design has a little digital in it. I'm picturing Monty Python...paraphrasing, "The Spam, eggs, bacon and Spam, only has a little Spam in it."

George H.

quoted text -

One cute sinewave source is a VCO that clocks a switched-capacitor filter at 100*f, and a 100:1 divider to make square wave "f". Use the filter to make the square wave into a sine. Lots of interesting variants and gotchas.

John

Last I heard you were doing well with your 300-1000kHz oscillator and

4017 sine generator.

But just for chuckles I thought I'd mention this: I just had occasion to breadboard a tunable low-pass filter for a project (it's pretending to be a $5000 device that, for some reason, my customer doesn't want to buy a second version of just to ship to me).

I ended up building a Sallen-Key filter, using resistors that were switched in and out of circuit with a 4066 at 1MHz and a variable duty cycle. The overall effect of the thing (aside from some 1MHz scrud 'cause it's built on a godforsaken proto board) is to smoothly tune from

10kHz to 20kHz under control of a voltage from a DAC.

It occurs to me that you could do this with a Wien bridge oscillator: the effective average resistance is R/rho, where rho is the duty cycle of the switch. This means that you can vary your frequency from some maximum down to zero, simply by varying the switch duty cycle. Moreover, you can use one of the unused switches in the package for AGC.

Not that you should change anything: I'm just having fun dinking with prototype circuits that never have to work outside the lab, and I thought I'd share...

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www.wescottdesign.com

Back in the 70's we used reverse biased LEDs instead of varactor diodes. They had substantial capacitance and worked reasonably well. However this was one-off stuff in the research labs and not anything designed for production. Art