[time-nuts] Zero-Crossing Detector Design?

[Magnus Danielson]

On 07/21/2012 01:41 AM, Hal Murray wrote:
>
> albertson.chris at gmail.com said:
>>> Hysteresis does nothing to eliminate jitter or temperature
>
>> Maybe, but it is absolutely needed if there is any noise on the signal.   A
>> perfect comparator with zero hysteresis would dither on every zero crossing.
>
> Hysteresis doesn't eliminate the dither from noise.
>
> Assume 5 volt CMOS logic with a switching point at 2.5 V.  With hysteresis,
> the low-to high direction switches at (say) 3 V and the high-to-low direction
> switches at 2 V.  If you have noise, whatever happens at 2.5 V without
> hysteresis happens at 3V with hysteresis.
>
> Actually, if you are working with a sine wave, shifting the switching point
> off center will make the problem worse since the slope will be reduced.
>
>
> Hysterssis will eliminate spikes or double pulses that are caused by noise on
> a signal with low rise time as long as the noise isn't too big.

... which is what he called dither.

Cheers,
Magnus