Comparator oscillation problem

[seyyah]

hc14 oscillator

I'm trying to realize this design ( **broken link removed** ) by using external components instead of pic like lm393 and lm311. But i couldn't be able to stabilize the output. With a high frequency and a given capacitor, the output is stable. But when the capacity is increased outputs begin to oscillate. I tried to add hysteresis and some other tricks but didn't work. What can you suggest me? It makes me crazy. Any help would be appreciated, thanks.

 

[master_picengineer]

3 gate oscillator comparator

Use a active Filter.

 

[seyyah]

lm393 oscillating

How?

 

[FvM]

oscillations comparator

The circuit is said to oscillate, cause it measures capacity by counting the frequency of oscillation. This should be no general difference either using the PIC internal comparator or external part.

 

[seyyah]

oscillator comparator

I didn't mean that kind of oscillation. The output is not stable. It doesn't settle on a stable frequency. It changes very rapidly in wide range of frequencies.

 

[FvM]

comparator oscillation

Hello,

I assume you know, how the standard-circuit for a comparator feedback-oscillator looks like. It actually can't operate without a hysterese. However, I think an adequate replacement is an oscillator from a CMOS inverter with schmitt-trigger,
eg. HC14. It has also lower part count.

Regards,
Frank

 

[seyyah]

pic comparator oscillator

Hmm it's an interesting idea FvM. It seems to run fine on simulation. Would it be more stable? I saw some designs with cmos inverter that they add gates in series and in parallel. What may be the reason? Adding more hysteresis or increasing supply amount from the 74hc14?

 

[seyyah]

comparator oscillation problem

I tried it on board and looks like it has the similar problem

 

[FvM]

oscillator design with a comparator

Two remarks:

1. The HC14 oscillator in general should better use a single gate than three cascaded (cause the part is already buffered internally, you're effectively chaining 9 inverters!), but probably that isn't the issue here.

2. The issue most likely is susceptibility for interferences from the enviroment, mainly low frequency AC voltage, but may be also strong RF fields. For an open capacitor, this problem is by design. You can't remove the interference, you can try to suppress the influence by an appropiate postprocessing of measured frequency.

A more sophisticated circuit could utilize parallel resonance with an inductor at the input to suppress low frequency coupling.