[time-nuts] Clock Driver Design

Bob Camp lists at rtty.us
Sat Sep 28 18:35:04 EDT 2013


Hi

If we're talking about a basement TimeNut setup there are some things that matter and some things that really don't:

Matters:

	Load isolation matters. You can get > 45 degrees (> 12 ns) quite easily with a load change
	ADEV matters. Your frequency counter will not be happy with a 1 ppm ADEV at 1 second. 
	Spurs and phase noise matter close in. Properly designed gear will not depend on sideband phase noise on the reference

Doesn't matter:

	Cable delay change. With < 20' of cable and a couple of degrees an hour, you won't see it.
	Wideband phase noise - see above
	Perfect match. You are unlikely to have APC-7 connectors on your setup. BNC's are only just so good.
	Harmonics past some point. It's > 20 dbc, but certainly < 60. Your change in load / change in timing isn't that big on a short cable.
	
Yes, it you have a dozen hydrogen masers in your basement, these rules probably don't apply. If you are a "hey, I've got a TBolt" sort of member, they very much do apply.

A few cautions:

	No, you don't drive your super duper phase noise mixer with the output of the distribution system. You use a dedicated source.
	Same is true for your super duper ADEV at parts in 10 to the zillion setup. You *need* to be isolated from everything for this. 

	If your "lab" swings temperature by 100C per day, that's a problem. It needs to be fixed. That swing will impact a lot of other stuff well before it nukes the distribution setup.

	If you daisy chain dozens of instruments with a forest of T connectors, you are headed for trouble. It's not the clock source that's the issue, it's the operator.
	Your best setup is one output to one instrument. 

With some common sense it all can work pretty well for very little money.

Bob




On Sep 28, 2013, at 5:21 PM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> 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
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.



More information about the time-nuts mailing list