[time-nuts] Distribution amps and slew rate

Peter Gottlieb nerd at verizon.net
Sun Nov 4 00:13:58 UTC 2012


Of course you can't have a perfect square wave!  That would imply zero 
transition time and since frequency is inverse to time that implies infinitely 
high frequency bandwidth is required to achieve that perfect square wave.  
Getting a "square" wave with a "fast enough" slew rate between high and low 
levels is certainly achievable and better than that perfect square wave.  Be 
careful what you ask for, because with a perfect square wave you would have such 
high frequency content that you would get induced noise everywhere.

Peter



On 11/3/2012 8:05 PM, Chris Albertson wrote:
> The below is correct but a simpler way to say it is this:
>
> "A square wave contains the fundamental frequency plus every odd harmonic
> up to infinity.  A sine wave contains only the fundamental frequency."
>
> It is the "up to infinity" part that causes all the trouble.  And yes it
> really does go to infinity, at least in theory.  but in real life you can't
> have frequencies so high so without them you can't and don't have a perfect
> square wave.  In other words perfect square wave can't esist in the real
> world but perfect sine wave, at least in theory could
>
>
> On Sat, Nov 3, 2012 at 4:39 PM, Charles P. Steinmetz <
> charles_steinmetz at lavabit.com> wrote:
>
>> david wrote:
>>
>>   Given that slew rate is so critical, why do we distribute sine waves and
>>> perform the zero-crossing detection at every target instrument?
>>>
>> Magnus made some good points in response to your question. To elaborate a
>> bit: it is much easier to provide a friendly transmission environment for a
>> sine wave (single frequency), and sine waves are less sensitive to
>> imperfections in the transmission environment (impedance discontinuities
>> and mismatches, noise ingress, etc.).  Reflections in the transmission
>> environment will put funny steps in what started life as clean square waves
>> or pulses, and differential phase shifts will also mis-shape square waves
>> or pulses.  This can even be a problem with sine waves -- see, for example,
>> the NIST paper on the timing effects of distortion in sine wave sources for
>> an example of the sensitivity of sine wave systems to harmonics (Walls and
>> Ascarrunz, The Effect of Harmonic Distortion on Phase Errors in Frequency
>> Distribution and Synthesis) -- but it is much worse with square waves or
>> pulses.
>>
>> Sine wave systems are also much less prone to radiating noise.  Anyone who
>> operates one or more frequency standards as well as sensitive RF receivers
>> can testify that sine waves are much less of a hassle.
>>
>> Best regards,
>>
>> Charles
>>
>>
>>
>>
>>
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