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

[Adrian Godwin]

Where do digital sensors (e.g. ds1820 and some more recent parts from TI)
fit into this ?


On Mon, Jun 5, 2017 at 12:59 AM, 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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