[time-nuts] Divider circuit for Rubidium Standard

Richard (Rick) Karlquist richard at karlquist.com
Tue May 19 23:10:24 UTC 2015



On 5/8/2015 2:19 PM, Bob Camp wrote:

>> On May 7, 2015, at 11:09 AM, Attila Kinali <attila at kinali.ch> wrote:
>>
>> On Wed, 06 May 2015 18:09:03 -0700
>> "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
>>
>>>> A standard input on a frequency counter is not a very demanding thing in the hierarchy of
>>>> TimeNut signals. You can drive any of them with some pretty simple logic gate based
>>>> circuits. No need to spend a lot of money.
>>>
>>> Logic gate, yes.  Comparator, no.
>>
>> This reminds me a lot of a similar discussion a couple of weeks ago.
>> (where the issue boiled down to noise bandwidth)
>>
>> What is the problem with a comparator vs a logic gate?
>> What makes the logic gate supperior?
>>
>> 			Attila Kinali
>>

The comparator as a squarer circuit is folklore that unsophisticated
users want to believe in, because it is seemingly the easiest way
to get the job done.  Wouldn't it be wonderful to be able to put in
any signal from -30 dBm to +15 dBm and get a perfect square wave
out with no effort?  Unfortunately, what a comparator looks like
is a very high gain differential amplifier that is slew rate limited.
The threshold voltage input must be extremely low noise or it
will introduce jitter.  Even if the input pin is clean, there is
internal noise.  Driving it will a low level signal will produce
a jittery output for obvious reasons.  The trouble is that if you
drive it with a high level signal, the jitter doesn't go away because
the input stage is already in saturation.  Also, the effective noise
figure of the comparator is usually high.  Making the comparator
faster exacerbates the problem.  Read papers on "zero crossing 
detectors" such as John Dick's 1990 paper in PTTI and you will
see that a comparator is the exact opposite architecture from
the optimum one.  I hope that clears up the question.

Regarding logic gates:  it is not so much that there is something
magic about gates; actually ECL gates are lousy.  It is just that
comparators are so bad that almost anything else is better.

Rick Karlquist N6RK



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