On 07/20/2012 07:42 AM, Chris Albertson wrote:
On Thu, Jul 19, 2012 at 5:47 PM, Rick Karlquist<[email protected]> wrote:
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.
Yes, and this dither is due to the additive noise on the signal. The
slew-rate at and about the trigger point will determine how much of that
additive noise is converted into time-noise. The schmitt trigger is
there to make sure that you surpress the dither around each transition,
but it will not help you to remove the time polution, as the first time
the dither occurs, is bound to be early and bound to be controlled by
the noise. Those, the noise will shift the trigger point.
You can view the schmitt trigger detector as having a state, and when in
proximity of the trigger point, you let the noise control when the
trigger point occurs.
If you noise is pure gaussian noise, this is not so bad, since the
trigger point will be shifted by the noise RMS, but it will be noisy.
If you have say a sine signal, then the non-linearity of the trigger
point will act like a mixer and it will cause the time jitter to be
spread out, and the peak-to-peak amplitude of the signal will when
divided by the slew-rate of the trigger point convert to the
peak-to-peak time modulation at that frequency. The distribution has a
very steep bath-tub look, since the sine spend most of it times at its
extremes (where it's slew-rates are low) but very little time in the
middle (where it's slew-rate are high). The sine signal would modulate
the trigger point up and down on the slope it's at. The schmitt trigger
action doesn't help to protect this behaviour.
Schmitt trigger is a nice tool, but it can do you great harm if you do
not understand what it does help you with and what it doesn't help you with.
You need to gain yourself to slew-rates where a schmitt trigger would do
no harm, and when you are there it will do essentially no good either,
as you are looking at a high slew-rate square signal.
So, you *can* do better than a Schmitt trigger. A schmitt trigger can be
sufficiently good. A schmitt trigger can work well if you have filtering
in front of it to significantly reduce unwanted systematic noise.
Cheers,
Magnus
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