The other issue that needs to be considered is the drift in temperature sensor characteristics when operated at a constant temperature (as is typical in a continuously operated crystal oven). High quality thermistors can achieve drifts of around 1mK/month. Its unlikely that something as complex as an AD590 will achieve a similar drift (1nA/month in a operating current of 300uA or so at 25C). High quality PRT sensors drift even less than thermistors when operating at constant temperature.
Bruce > > .On 05 June 2017 at 11:59 Attila Kinali <att...@kinali.ch> wrote: > > Moin, > > This discussion is kind of getting heated. > Let's put some facts in, to steer it away from > opinion based discussion. > > On Sun, 4 Jun 2017 08:44:33 -0700 > "Donald E. Pauly" <trojancow...@gmail.com> wrote: > > > > > > I stand by my remark that thermistors have been obsolete for over 40 > > years. The only exception that I know of is cesium beam tubes that > > must withstand a 350° C bakeout. Thermistors are unstable and > > manufactured with a witches brew straight out of MacBeth. Their > > output voltages are tiny and are they inconvenient to use at > > different > > temperatures. > > > > > > If you really mean thermistors, and not, as Bob suggested thermocouples, > then I have to disagree. The most stable temperature sensors are > platinum wire sensors. The standards class PRT's are the gold standard > when it comes to temperature measurement, for a quite wide range > (-260°C to +960°C) and are considered very stable. They offer (absolute) > accuracies in the order of 10mK in the temperature range below 400°C. > Even industrial grade PRT sensors give you an absolute accuracy better > than 0.1K up to 200-300°C. The "cheap" PT100 are more of the order of > 1-10°C > accuracy... all numbers just using a two-point calibration. > > For more information on this see [1] chapter 6 and [2] for industrial > sensors. > > NTC sensors have a higher variablity of their parameters in production > and are usually specified in % of temperature relative to their reference > point, which is usually 25°C. Typical values are 0.1% to 5%. Additionally > there is a deviation from the reference point, specified in °C, which > is usually in the order of 0.1°C to 1°C. > > The NTC sensors are less accurate than PT sensors, but offer the advantage > of higher resistance (thus lower self-heating), higher slope (thus better > precision). Biggest disadvantage is their non-linear curve. Their price > is also a fraction of PT sensors and due to that you can have them in > many different forms, from the 0201 SMD resistor, to a large stainless > steal pipe that goes into a chemical tank. NTCs are the workhorse in > todays temperature measurement and control designs. > > The next category are band-gap sensors like the AD590. Their biggest > advantage is that their 0 point is fix at 0K (and very accurately so). > Ie they can be used with single point calibration and achieve 1°C accuracy > this way. Their biggest drawback their large thermal mass and large > insulating case, because they are basically an standard, analog IC. > Ie their main use is in devices where there is a lot of convection and > slow temperature change. Due to their simple and and quite linear > characteristics, they are often used in purely analog temperature > control circuits, or where a linearization is not feasible. > But only if price isn't an issue (they cost 10-1000 times as > much as an PTC). Their biggest disadvantage, beside their slow > thermal raction time, is their large noise uncorrelated to the > supply voltage, and thus cannot be compensated by ratiometric measurement. > They are also more suceptible to mechanical stress than NTC's and PT's, > due to their construction. Similar to voltage references (which they > actually are), their aging is quite substantial and cannot be neglected > in precision application. > With a 3 point calibration, better than 0.5°C accuracy can be achieved > (modulo aging) within their operating temperature range, which is > rather limited, compared to the other sensor types. > > I don't know enough about thermocouples to say much about them, beside > that they are cumbersome to work with (e.g. the cold contact) and > produce a low voltage (several µV) output with quite high impedance, > which makes the analog electronics difficult to design as well. > > With todays electronics, the easiest sensors to work with are NTC and > PT100/PT1000 as most high resolution delta-sigma ADCs have direct support > for 3 and/or 4 wire measurement of those, including compensation for > reference voltage/current variation. Using a uC as control element > also opens up the possibility to linearize the curve of NTCs without > loss of accuracy. Usually measurement precision, with a state-of-the-art > circuit, is limited by noise coupling into the leads of the sensor > and noise in and around the ADC. (see [3-5]) > > > > > > Where did you get the idea to use a 1 k load for an AD590? > > > > > > Jim was refering to a circuit _he_ used in a satellite. Not to your > circuit. > > > > > > The room temperature coefficient of an AT crystal is -cd 100 ppb per > > reference cut angle in minutes. (-600 ppb/C° for standard crystal) > > The practical limit in a crystal designed for room temperature is > > about 0.1' cut accuracy or ±10 ppb/C°. If you have access to an > > atomic standard, you can use feed forward to get ±1 ppb/C°. If the > > temperature can be held to ±0.001° C, this is ±1 part per trillion. > > This kind of accuracy has never been heard of. > > > > > > It has been heard of. The 8607 was spec'ed to <2e-10 p-p deviation over > temperature range (-30°C to 60°C). Also, to hold the temperature stable to > 0.001K in a room temperature environment (let's say 10K variation), you need > a thermal gain of >10k. That's quite a bit and needs considerable > design effort. Most OCXO design's I am aware of are in the order of 100 > (the DIL14 designs) to a few 1000 for single ovens, to a few 10k for > double ovens. The only exception is the E1938 which achieves >1M. > But that design is not for the faint hearted. I don't remember seeing > any number, but i would guess the 8607 has a thermal gain in the > order of 100k to 1M as well, considering it being a double oven in > a dewar flask. > > Also, what do you mean by atomic standard and feed forward? > If you have an atomic standard you don't need to temperature > stabilize your quartz. You can just simply use a PLL to lock > it to your reference and achieve higher stability than any oven > design. > > > > > > Feed forward also > > allows you to incorporate the components of the oscillator into the > > thermal behavior. It does no good to have a perfect crystal if the > > oscillator components drift. > > > > > > Beyond tau=100s, the temperature and moisture sensitivity of the > electronics, combined with the aging of the electronics and the > crystal will be the limit of stability. Of course, this is under > the assumption that you achieved a thermal noise limited design > and thus the 1/f^a noise of the oscillator is negligible in the > time range considered. > > Attila Kinali > > [1] "Traceable Temperatures - An Introduction to Temperature Measurement > and Calibration", 2nd edition, by Nicholas and White, 2001 > > [2] "Thin-film platinum resistance thermometer for use at low temperatures > and in high magnetic fields", Haruyama, Yoshizaki, 1986 > > [3] "Completely Integrated 4-Wire RTD Measurement System Using a Low > Power, > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0381 > http://www.analog.com/CN0381 > > [4] "Completely Integrated 3-Wire RTD Measurement System Using a Low > Power, > Precision, 24-Bit, Sigma-Delta ADC", Analog Circuit Note CN-0383 > http://www.analog.com/CN0383 > > [5] "2- 3- 4- Wire RDT (Pt100 to PT1000)Temperature Measurement" > Ti Presentation > > http://www.ti.com/europe/downloads/2-%203-%204-Wire%20RTD%20Measurement.pdf > > -- > You know, the very powerful and the very stupid have one thing in common. > They don't alters their views to fit the facts, they alter the facts to > fit the views, which can be uncomfortable if you happen to be one of the > facts that needs altering. -- The Doctor > > _______________________________________________ > 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. > _______________________________________________ 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.