low frequency micropower vco needed

How about using a CD4053 as the inverters in a CMOS oscillator.

X1 +5V---O

The oscillator section of the CD4046, 74HC4046 PLL.

Mark Zenier snipped-for-privacy@eskimo.com Washington State resident

If that were true. However, the hysteresis is determined by the _strengths_ of MOS devices, so the hysteresis doesn't track Vcc very well.

...Jim Thompson

I'd think a schmitt trigger oscillator would be quite insensitive to channges in Vcc, as long as the hysteresis is a constant fraction of the supply voltage. That leaves out any effect of series resistance in teh output stage.

Thus I would suppose it largely depends on the chip used.

That said, what would it be like to build an oscilaltor and put a FET in the feedbackm, as a kind of 'variable resistor'? At least that gives you a defined direction of sensitivity.

Thomas

Hi Ken,

Saw your post in abse, thanks for the info.

I'm not sure whether I'll use the shmitt trigger based solution, or the ring oscillator you posted. Since I need a relatively small frequency variation, I'll probably go with the one that gives the smaller frequency excursion.

Thanks to all who participated in the discussion, I learned alot and appreciated all the suggestions!

Regards,

I think the '4046 that someone suggested will be the lowest power, and certainly simplest to set up.

...Jim Thompson

There is no more detailed circuit.

My provider doesn't carry that group.

My circuit gives a very wide frequency shift so I think you are better off with other circuits.

What's the lowest supply current op-amp quad (or??) that will operate nicely at your frequency? (I'm not up to date on micropower OAs.) If a suitable one exists, here's a possible.

There's a very simple circuit (one OA + 4 resistors) for an amplifier whose gain can be switched from +1 to -1. You would apply your control voltage to its input. A fet could be used for the switch.

A second OA would be used as a simple R-C integrator with the output of the previous applied to its input.

A two-level detector would be used to detect when the integrator output reached V1 and V2 and its output used to drive the switch.

The slope of the integrator output is directly proportional to the input - more voltage, steeper slope, less time to get from V1 to V2, higher frequency.

Ted