Re: [time-nuts] Distributed DDS
On Sun, 30 Jan 2011 23:47:10 +0100 Magnus Danielson mag...@rubidium.dyndns.org wrote: On 30/01/11 23:08, ehydra wrote: I'm not sure if I overview the problem correctly. Hm, why not run the cable back and measure the round time? Half this time und you know the delay of one way. Then you can shift this with a PLL away. A similar scheme is used in almost all modern PC clock distribution chips. A bunch of PLLs on the chip. Severals are available. For example Cypress Semi. The LHC at Cern and associated accelerators build up cable distance. They also have a myriad of different timing requirements. Essentially they try to do away with them once and for all. I'm not convinced they can do all of them in this round... but can't blame them for trying. Only then they will learn the real requirements and problem areas. Another Problem they have at CERN is that basically anything that goes into the vicinity of the detectors has to radiation-hard to some very high levels, roughly up to a few hundred MegaRads and 1e15 neutrons/cm^2 for the next upgrade in order to survive for the planned time spans. Have no idea whether this also goes for the clock distribution. But this limits the choice of vendors quite a bit. Just my 2 Cents, Florian ___ 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.
Re: [time-nuts] Gamma-ray and jitter
On Sun, 14 Nov 2010 01:27:10 +0100 Magnus Danielson mag...@rubidium.dyndns.org wrote: Don't recall from the top of my head the effects of gamma-rays on crystals, but I do recall there is effects there. However, I do suspect they are rather poor gamma-ray detectors and loads of other sources to hide any correlation from direct observation. IIRC, Crystals usually are made up from silicon dioxide in a more or less pure crystalline form. I usually deal with more or less amorphuous silicon dioxide as in gate oxides, and i'm not sure whether or not this makes a difference. Anyways, gamma rays can produce electron-hole pairs in SiO2. And while electrons are fairly mobile (at least compared to the holes), this can depending on DC bias conditions lead to a net positive charge inside the crystal. This charge could be annealed though by applying high temperatures on the order of several hundred °C to the crystal. No idea what some electric field could do jitter-wise though. Might have to do with some sort of mechanical stress inside the crystal due to electric forces. I think there are more mechanisms, but here i'm not sure. Am just learning about that stuff. But I would be willing to look it up at work ;-) HTH, Florian ___ 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.
Re: [time-nuts] Temperature sensors and bridge amps
On Thu, 11 Nov 2010 21:38:39 -0800 (PST) Perry Sandeen sandee...@yahoo.com wrote: List, My following comments are am exploratory thought process of which I don’t profess to know the answers. Perhaps in the future experiments will provide some. So here we start. The Ni1000 SOT temperature sensor is a nickel based unit that has a basic resistance of 1K ohms at 20 degrees C and a 6+ ohms (approx) change per degree. The sensitivity of a standard platinum 100 ohm sensor is a nominal 0.385 ohm/°C. Well, to make comparison a fair game, i'd suggest to take the relative resitance change instead of the absolute one. For one, one could take a PT1000 instead of a PT100 so both the Nickel and the Platinum start out at the same initial value of resistance. By that, the platinum resistor gets to 3.85 Ohms/°C, roughly half the sensitivity of nickel. Wrote: Nickel sensors are more stable than thermistors, but not as stable as platinum. The cost is more attractive than Pt, tho. Agreed. The platinum price I was able to find was about $30 each. So the Ni1000 is one tenth the price. Surely one could find nickel resistors for less than 3 bucks each, but platinum isn't that expensive either. Depending on how they're built (bare resistor or housed in stainless steel, cables attached or not), one can find them for about 6 USD / 4 EUR at well-known sources and for something like 10 USD / 7 EUR packs of five off eBay. For plain resistors, that is. On the other hand, at quick glance i haven't found nickel resistors at all. Temperature control modules that can work with them, sure, but no resistors. Strange... Wrote: I'd consider staying analog with a DC bridge and a PID control op-amp. You don't need a highly accurate voltage source for the bridge because null is null, whatever the excitation voltage. Of course, you'll want a stable null for the op-amp, too. I don’t know what a PID is but I agree about using a bridge circuit. Wrote: need a highly stable set of bridge resistors for a stable temperature. In the old days, precision, stable resistors were wound on ceramic forms by soldering a loop of e.g. constantan wire to the lead wires at each end of the form. Then you pull the loop at the center so that you can wind it on the core in a non-inductive manner. Snipped I have a number of them salvage from older test equipment. Using them in a small enclosed temperature control module is really impractical. One can easily buy 50 PPM/ degree metal film resistors. Probably sorting the two other branch resistors from a batch of 10 with a 4 1/2 digit or greater resolution DVM can provide an extremely well matched set. Let’s assume for example we want 80 C. for our oscillator. The Ni1000 is rated as 1482.5 ohms at 80C and 1489.1 ohms at 81C resulting in a change of about.0066 ohms per milli-degree. As stated earlier, the standard platinum 100 ohm sensor is a nominal 0.385 ohm/°C. or .000385 ohms per milli-degree. Although the platinum sensor is superior can such a low value of change be used practically in a bridge circuit made by us time-nuts? In a bridge circuit, you don't measure resistance directly, but use the voltage that appears across the bridge. So for a 100 ohms element, you'd usually have ten times the current flowing in that branch compared to a 1kohm element. That again works out to the same voltage swing. And as said, there are platinum sensors around with 1kOhm, also have seen 2k and IIRC 5k. On the other hand, the high-value platinum sensors generally aren't available in the highest accuracy bins. Another question is are we over-engineering a regulating circuit for the crystal, as in how sharp is the turning point? Will this be gold plating a Yugo? I have no idea. I’m bringing this up for discussion. Umm, is there such a thing as over-engineering to a time-nut? ;-) How sharp the turning point is strongly depends on how the crystal is cut. IIRC, SC-cut crystals tend to have a quite flat turning point somewhere around 50-80°C. Can't give you numbers on that though. But i'd guess to get to the point of zero tempco of the crystal and stay there, one would need to get within less than one degree C sonsistently. So you'll want to know the temperature with accuracy of at least 0.2°C. The thermostat will need fine-tuning anyways in order to accomodate all the unknowns like exact turnover temperature of your crystal, tempco of the other resistors, whatever. Wrote: Don't even think of using any kind of variable resistor to adjust the bridge null. What you want is a stable temperature near the value that gives the least crystal tempco. Agreed. But I have a question. If one was using the Ni1000 couldn’t one use say a 20 turn 10 ohm ceramic trimpot swamped with a 10 ohm resistor or a low value Beckmen 10 turn pot to find the center of the turning point? In my opinion, for finding the turning point a trimpot is okay. But then replace the
Re: [time-nuts] Temperature sensors and bridge amps
On Fri, 12 Nov 2010 10:36:15 + Poul-Henning Kamp p...@phk.freebsd.dk wrote: In message 20101112110627.488cb...@vz127.worldserver.net, Florian E. Teply writes: In a bridge circuit, you don't measure resistance directly, but use the voltage that appears across the bridge. So for a 100 ohms element, you'd usually have ten times the current flowing in that branch compared to a 1kohm element. This was one of the things that I wondered about: How large currents are used ? Can't be too much because that would lead to self-heating... Exactly. Usually you want to avoid self-heating. There are two possibilites: a) Use low currents. Still you want to read your voltage precisely enough to make use fo the superior properties of platinum, so with 1 milliamp of current (roughly 100 uW power in a PT100), the .385 Ohms/°C make for 385 microvolts of signal for a 1 degree of temperature change. Yet the power dissipation in the resistor might still be too much, especially in very sensitive circuits. In a PT1000, 100 microamps will yield the same voltage swing in your signal but only 10 uW of power dissipation in your sensor. Generally speaking, in precision circuitry one would use as much current as reasonably possible to maximize the signal. The upper limit to the current is posed by the power dissipation in the sensor. For one, the system must not be disturbed by the power introduced by the sensor. And second, the difference between the platinum resistor internal temperature (that one sets the resistance) and the temperature to be controlled must be small enough. On the other hand, you can't get too low with current in order to not swamp the signal with noise (both thermal noise in the bridge resistors and noise imposed by the amplifying circuitry). You could still go lower with the current, but make sure the signal you want to see, say 20 microvolts for a 0.05 °C temperature change, is significantly above the noise. So, basically your amplification circuitry may not introduce more than very few microvolts of noise referred to the input. Plan b): Duty-cycling the measurement. That perfectly lends itself to digital circuitry, but is a tad more complicated in the analog domain. Basically what you do is power the bridge, measure the voltage and unpower it again. So if you measure the temperature once every second for, say, 10 milliseconds, you get only 1 percent of the power dissipation in the resistor on average. Given the thermal inertia of typical systems, once every second should be enough. But of course, you'll have to be careful not to introduce switching noise into your oscillator. The circuitry will be a lot more complex though. In both cases, given the slow thermal behaviour of the systems, you can low-pass filter the signal heavily. HTH, Florian ___ 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.
