[time-nuts] Frequency Divider
bruce.griffiths at xtra.co.nz
Thu Apr 2 20:04:59 UTC 2009
Hal Murray wrote:
>> A large resistor connected between the input and output would
>> accommodate threshold variations better. Even better would be a
>> feedback loop that adjusts the input bias point to maintain the output
>> duty cycle at 50%.
> Isn't that resistor a feedback loop?
> I played with that setup in the lab many years ago. It didn't work as well
> as I was expecting. I didn't figure out why it didn't work better.
> Maybe some gain in the feedback path would help. Then we have to consider
> stability. Ugh.
Yes, a resistor connected between the input and output of an inverter is
a feedback loop but the loop gain is relatively low.
With a high amplitude input threshold variations from the nominal can
cause the input protection diodes to conduct.
Once these diodes conduct the output jitter may deteriorate
significantly (it does for HCMOS inverters).
Using a non inverting integrator in the feedback path can accurately
stabilise the duty cycle .
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