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