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

Bob kb8tq kb8tq at n1k.org
Mon Jun 5 06:53:38 EDT 2017


If your objective is a resolution of < 0.001 C at something < 1 second, the current crop of 
digital sensors don’t quite do what you need to do. They are a terrific way to do wide range 
measurements that might feed into some sort of correction algorithm. A conventional 
thermistor bridge falls apart if you try to run it -55 to +125. The range of resistances 
involved results in significantly lowered resolution at the end(s) of the range. 


> On Jun 4, 2017, at 8:18 PM, Adrian Godwin <artgodwin at gmail.com> wrote:
> 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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