[time-nuts] Zero-Crossing Detector Design?

[Chris Albertson]

On Thu, Jul 19, 2012 at 3:57 PM, Richard (Rick) Karlquist
<richard at karlquist.com> wrote:
> A fast comparator seems like a good idea, and it
> is simple, however it is actually the last thing
> you want to use.  High thermal sensitivity and high jitter.


The comparator will work but you need some positive feedback to create
hysteresis.   The problem is the hysteresis cause the output square
wave to be not quite 50% duty cycle.  But maybe you don't care if the
goal is to count cycles. or if you only look at (say) raising edges.

>
> Rick
>
> On 7/19/2012 1:35 PM, Dan Kemppainen wrote:
>>
>> Or use a fast comparator such as an ADCMP600 series. Much lower delays,
>> and faster rising/falling edges.
>> FYI, I've had good luck with this at 30Mhz. You could transformer couple
>> this one, or simply couple it through a cap.
>>
>> Dan
>>
>> On 7/19/2012 3:47 PM, time-nuts-request at febo.com wrote:
>>>
>>> This is sort of a FAQ: the argument was already discussed here. One of
>>> the
>>> most interesting idea (in my opinion) is to use an RS485 line receiver
>>> like
>>> the ST3485, MAX483, ADM485. They are actually transceivers so they
>>> must be
>>> tied permanently in RX. Since they are differential you can also put a
>>> 1:1
>>> (or a 1:4 to raise the level) transformer to isolate the input too.
>>
>>
>>
>>
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>
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-- 

Chris Albertson
Redondo Beach, California