[time-nuts] ok, newbie questions

[Bob Camp]

Hi

A stand alone OCXO is good for telling you that where you are today is close to where you were yesterday. It's pretty much useless for telling you weather either of those places are the correct place to be. The GPS gizmo will do a  pretty good job of telling you that where you are is indeed where you want to be.

In the HP units the switching power supplies are inside the unit. With the TBolt,  the internal regulation is all linear. In many cases people run switchers external to the TBolt. Regardless of where they are switchers can create crud. 

Both the HP and the TBolt have CPU's in with the OCXO. In both cases there are indeed fun little RF things that the CPU can / will create. The same is true of your PC and likely a few dozen other things floating around your shack. 

The phase noise on both the TBolt and the HP's are very similar. If anything the TBolt has a slight edge over the HP. In both cases a properly designed microwave reference chain will have most of it's phase noise coming from sources other than the 10 MHz source. That makes it pretty much a toss up between the two units as far as noise is concerned. 

Bob


On Nov 26, 2010, at 7:36 PM, GandalfG8 at aol.com wrote:

> 
> In a message dated 26/11/2010 23:31:32 GMT Standard Time, w2hx at w2hx.com  
> writes:
> 
> Ahh.  Very interesting explanation. So is it somewhat correct to assume 
> (yes,
> I  know) that for a stationary (non-mobile) environment, these extra  sats
> don’t make much difference? This seems to be what the explanation is  
> saying.
> 
> Ok. So let me see. For a frequency standard for use in lab  equipment, it
> appears that short term, phase noise and other sources of  noise are the
> things to be concerned with  to get better results.  These seem to really be
> accomplished with a good oxco.  However, if I  want a very accurate
> time-of-day clock for long periods of time, then I  need long term stability
> which is where the GPS comes in.  Do I have  this right?
> 
> So if I want a really souped-up freq standard for my lab,  then I should
> concentrate on finding the best oxco I can (which may be  disciplined by the
> GPS or manually occasionally calibrated to GPS), and use  the best power
> supply I can find.  These seem to be what I should  concentrate on rather
> than more channels. 
> 
> I do believe that I read  some stuff on the internet that the HP GPS DO's do
> seem to have very good  power supplies (or converters) which contribute to
> low spurs. So it seems  the HP's do have a real advantage (not just  the
> name).
> 
> 
> 
> ---------------
> In searching for the best there is we may sometimes lost sight of  what we 
> were looking for to start with .....
> old Scottish proverb I just made up:-)
> 
> What gives "better" results so often depends on what you wish to  achieve.
> 
> You might not get the absolute minimum phase noise from something like a  
> Thunderbolt, but then you might also have to ask if you need it.
> 
> When it's working well, and some have been known not to in quite subtle  
> ways, the HP Z3801A, the only HP unit I can speak for, is an excellent  GPSDO.
> What you might not notice though without reading the very small print is  
> that it can take two to three weeks to settle after being repowered for  
> whatever reason.
> 
> On the other hand, the Thunderbolt, in my experience anyway, is generally  
> well settled in 15 to 30 minutes.
> 
> Because the Thunderbolt is so widely available it seems as though  it's can 
> also be easily dismissed at times but don't underestimate it, it's an  
> excellent GPSDO.
> 
> I would recommend that you look for one of the Chineses sellers on  Ebay 
> offering T'bolts from no earlier than 2005 and with revision E  software, 
> fluke.l is one good exapmple, and go for that.
> There may well be an issue with the temperature sensor, much more in the  
> archives on that, but replacements, if you really need one, cost pennies.
> 
> Once you have that, and it's stabilised, you can either use it as it stands 
> or use it to adjust your 10811 or whatever.
> 
> For some time now I've tended to treat my Z3801As as interesting high  
> quality museum pieces, whilst using three T'bolts as references/confidence  
> generators.
> I then use those to calibrate the rubidium modules  and 10811s that serve 
> as my lab standards, which works well for  me.
> 
> regards
> 
> Nigel
> 
> GM8PZR 
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