[time-nuts] EFC info on Trimble 34310-T OXCO

[EWKehren at aol.com]

Charles 
I agree with every thing you wrote and I am implementing many of your  
recommendations. Forty years ago I bought a 15 foot Alu channel to make small  
frequency counter housings, always small, and at the time I did have access 
to a  machine shop so I made end plates. Still have five foot pieces now I 
cut then  off in 1 lb pieces and use them for tbolt, FE 405 B, FE 5650 and 
even a HP 10811  taken out of the can. As I said before am waiting for the 
small spheres and will  see what happens. Working on a GPSDO for the FE 5680A 
and the FE 405 B I did  find out the hard way what moving air will do. When AC 
season started my 405  tests showed the AC cycling it has a digital tuning 
resolution of 5.7 E-15.. The  nicely assembled packaged unit ended up in an 
other R&S chassis with bubble  pack on each end reduced AC influence but you 
can still see it. If you like to  see some data contact me off list file is 
to large to post. Picture of my Alu  channel is attached.
Bert Kehren
 
 
 
In a message dated 8/23/2014 10:20:19 P.M. Eastern Daylight Time,  
csteinmetz at yandex.com writes:

Ed  wrote:

>I agree with your statement regarding the determination of  the 
>optimum time constant, but, as Bob Camp mentioned, temperature  
>change has a significant impact on setting the value.  My 'lab'  is a 
>non-airconditioned bedroom.  My Tbolt doesn't have any  active 
>temperature control.  If I set the time constant to the  point that 
>Lady Heather thinks is optimum, I see large swings in PPS  offset 
>when I open the window and the temperature changes by a few  degrees 
>C.  If I leave the time constant at the default of 100  seconds, the 
>swimgs are drastically reduced.  Active temperature  control is on my 
>'round tuit' list.

Bert wrote:

>As  to Ed's and Bob's comments our projects are not able to compete 
>with  commercial products and I do not think that should be our 
>goals.  Having spend extensive time on temperature control, I limit 
>my self to  10 C and use fans on all Rb's and passive on OCXO's. 
>Concern about  vibration induced noise on the  OCXO made me remove 
>the fan on  the tbolt. Added a lot of mass and now ordered some foam 
>balls from  China to fill the enclosure as some one recommended.

Well, yeah, it  goes without saying (or at least I thought it would) 
that one must keep  the rate of change of temperature of the OCXO low 
enough that its oven can  keep the crystal temperature within design 
bounds at all times.  I  just assume that any time nut would do this, 
since it is extremely simple  and costs next to nothing (look in the 
archives for my previous posts  about "metal boxes," "metal 
enclosures," and "thermal capacitance" in  connection with 
OCXOs).  Active temperature control is NOT  necessary.  Which is not 
to say it's a bad idea, it's just not  necessary to stabilize any OCXO 
worth owning by a time nut.  (I'm not  sure the MV-89 qualifies, even 
if you are lucky enough to get a good  one.  There has been some 
discussion on this list about the  temperature control loop being 
quasi-stable and tending to oscillate or  even latch under some conditions.)

I also see no reason why amateur  efforts cannot surpass the 
performance of commercial products,  particularly if we assume that 
the environmental conditions are limited to  those encountered in 
living space, not a radio shelter exposed to the  elements at a remote 
tower.  That is why I've been critical of  designs that aim only to do 
"the best that can be done for $5," or "the  best that can be done 
with a small ARM and 3 transistors."  Given  good design, there is no 
reason why an inexpensive DIY GPSDO shouldn't  handily outperform a 
Thunderbolt (using the same OCXO), with two  conditions: (i) 
environmental conditions are limited to those encountered  in living 
space, and (ii) performance during holdover is  neglected.

The reasons why most DIY designs do not work as well as  commercial 
designs, even if they use OCXOs of equal quality, is that their  
designers evidently cannot design ADPLLs of sufficient performance to  
do justice to the OCXO.  (This includes implementing whatever means  
of phase comparison and sampling are chosen, the DSP loop filter,  
sawtooth correction, and the NCO or DAC/EFC design.)  Doing all of  
this right isn't particularly expensive, it just takes a designer who  
has the skills and is willing to devote the effort.  As a mentor once  
told me, "Good thinking isn't any more expensive than bad  thinking."

Some of the performance gain would be in reducing the rate  of 
temperature change seen by the OCXO, either passively as I have  
advocated and described before, or actively.  The other main  
improvement would be setting the PLL crossover out where it belongs,  
which becomes possible when the rate of change of temperature is  
controlled.  Avoiding a few common mistakes would provide some  
additional performance gains.

While the foam peanuts, which I  mentioned in a previous post, are 
helpful in some circumstances, I have  never seen the need for them in 
the case of an OCXO inside a cast aluminum  box.  In that post, I 
mentioned my gut feeling that spheres (balls)  likely pack too tightly 
to allow sufficient air circulation.  I think  irregularly-shaped 
pieces of foam (like packing peanuts), which leave much  more air 
space between them, are required.  The intent is NOT to  impede air 
flow, but to randomize it.

One point that I think gets  lost in many of these discussions:  The 
quality of individual OCXOs,  even of the same model, varies rather 
widely, and you often won't know how  good a particular OCXO is until 
you have run it continuously for at least  90 days (preferably 180 or 
more).  The job of any GPS discipline is  to gently keep the OCXO on 
frequency, without lowering its xDEV  performance at tau where the 
OCXO is better than GPS.

The most  effective thing you can do to construct a very stable GPSDO 
is to start  with a very stable OCXO.

Often, this means buying a bunch of OCXOs  (even if you have to do it 
one at a time for budgetary reasons), selecting  the best one(s), and 
moving the rest along.  This can take a long  time, since you need to 
run each new oscillator continuously for at least  90 days before you 
can know how stable it is.  The odds of finding a  good example are 
improved if you stick to models that, after long  experience, 
knowledgeable time researchers have found to be consistently  
good.  Alternatively, you can hope that your sample of the $20 ebay  
wonder of the week will live up to the anecdotal report someone  
posted, but the odds do not ride with you.

Best  regards,

Charles



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