[time-nuts] Clock Driver Design

[Magnus Danielson]

On 09/28/2013 02:24 PM, Didier Juges wrote:
> As always, be careful with the hard and fast rules. Most crystal oscillators generate sine waves first, but if the oscillator is part of a GPSDO, it will have to be converted to square to be processed by the logic within the GPSDO, so even if the device has a sine output, there will be a square wave version of the signal within the device. Eventually, the sine has to be converted into a square wave at the other end of the cable either to drive a mixer or a logic gate. 
>
> Where you do that conversion (before the cable or after the cable), or whether you need to go to sine altogether is something that has to be considered as part of the overall system design. 
>
> In many applications, there is absolutely no need to go to sine if you have a clean square wave to begin with, for instance if the cable is short or you connect with well matched devices.
>
> For some info on what cable mismatch does to a square wave, look here: http://KO4BB.com/Test_Equipment/CoaxCableMatching.php
>
> As Magnus pointed out, there are additional considerations such as ease of  troubleshooting that come into play also.
>
> Finally, if your primary concern is frequency distribution to several instruments as opposed to precision timing, sine will probably be easier because multiple reflections and mismatches that would affect time of arrival don't matter if you are only interested in a stable and precise frequency for your radios.
Almost true. The missmatches depends on the coax electrical length, and
that vary with temperature, so if you are really deep down in stability,
it can affect you. Then again, most of the time the direct effect of
delay dominates.

So, there is no easy answer, there is a bunch of "it depends" and in the
process you learn that proper impedance match and termination is usually
a good thing. Choice of cable can alter your performance in many ways.
You signal and your input circuit needs to match for best performance.

Cheers,
Magnus