[time-nuts] Temperature sensors and quartz crystals (was: HP5061B Versus HP5071 Cesium Line Frequencies)

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 4 20:23:14 EDT 2017

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.


>     .On 05 June 2017 at 11:59 Attila Kinali <attila at 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" <trojancowboy at 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
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