[time-nuts] time-nuts Frequency Divider
Mike Monett
xde-l2g3 at myamail.com
Thu Apr 2 22:55:49 UTC 2009
> Message: 3
> Date: Fri, 03 Apr 2009 09:04:59 +1300
> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
> Subject: Re: [time-nuts] Frequency Divider
> 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.
> Hal
> 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.
> Bruce
The 74HC and 74AC input threshold tolerance is +/- 30%. This means
the threshold can vary from 1.5V to 3.5V with a Vcc of 5V.
This limits the maximum input signal to 3V p-p or +13.5dBm, and
leads to a very subtle flaw discovered in some amazing engineering
work by Martein Bakker, PA3AKE.
If the threshold is not controlled, it can cause AM noise to convert
to PM noise and degrade the jitter. This occurs in the Analog
Devices AD9910 1GHz DDS chip.
Martein Bakker discovered this in his noise analysis, and Kevin
Wheatly gave a nice entry in his blog on how to fix it:
http://www.m0khz.com/?p=589
Mike
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