[time-nuts] Divider circuit for Rubidium Standard

[Bob Camp]

Hi


> On May 23, 2015, at 12:37 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
> 
> Bob wrote:
> 
>> The simple answer is that a biased fast CMOS gate will do a better job
>> ADEV wise than your signal sources will.
> 
> Maybe or maybe not, at tau ~1 second.  Trouble is, as tau gets larger, the gate performs *worse*.  The switching threshold of all MOSFET logic devices varies all over the place with temperature and supply voltage as well as random drift.  At tau >10 or 100 seconds, these effects become more and more pronounced and xDEV gets worse, even if you take pains to keep the circuitry out of drafts.  Gates are not a good way to square sine waves if you care about stability at longer tau.

Yes indeed, if you have a clock that goes below 1x10^-15 at 1 second and drops linearly with tau from there, you will have issues. If you do not have such a clock. The gate probably will do just fine. 

The delta on the gate turns out to be a delta time (as in delta ns / ps / fs). As you go out in tau, the impact (parts in 10^whatever) of that time delta drops linearly with tau. 

So: what sort of clock (that you have) are you proposing to look at?

Bob

> 
> Most of what has been said against comparators on this thread are indictments of mistakes made in applying them, NOT deficiencies of comparators per se.  I don't have the time nor energy to go into it in any depth right now, but: Properly applied, comparators can work better than pretty much anything else when the job is squaring a 1 to 100 MHz sine wave.
> 
> A few "Do's" and "Do not's":
> 
> Do use a comparator with split supplies for the input section, so you can use actual ground as the reference voltage.  Do not use inputs biased to mid-supply.  Most especially, do not use separate voltage dividers to bias the two inputs, because the divider noise is uncorrelated and adds.  If you must use inputs biased to mid-supply, use one good, low-noise voltage reference (LM329 or LM399) to bias both inputs so the bias noise is low and is common-mode (make sure to keep the time constants equal at the two inputs).  But just don't use inputs biased to mid-supply in the first place.
> 
> Do use a comparator with properly-designed internal hysteresis of a few mV (e.g., LT1719).  Do use a good, modern comparator (again, e.g., LT1719) that was designed since chip-level thermal flow analysis became standard practice, to avoid the mysterious drift, instabilities, and metastabilities that comparators from the bad old days (mid-'90s and earlier) were famous for.
> 
> Do not rely on a comparator to work with inputs from mV to 10s of volts.  You wouldn't expect that with a logic gate, why in the world would you expect it with a comparator?  Adjust the input level with amplifiers or attenuators to the optimum value for the comparator you are using at the frequency you are operating.
> 
> A 5 or 10Vp-p sine wave at 10MHz slews fast enough at zero-cross not to need bandwidth-limited clipping amplifiers (a la Dick and Collins).  Those techniques were designed for squaring audio-frequency sine waves, such as the mixer output(s) of a single- or double-mixer system.  If you feel the need, you can increase the zero-crossing slope of the input signal by starting with a larger input signal than is optimum for the comparator in use and using diode clamps to limit the peak amplitude.
> 
> There are many other best practices, but the ones above are enough to avoid the major application mistakes and have a reasonable chance of designing something that works to a high standard.
> 
> Best regards,
> 
> Charles
> 
> 
> 
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