uC ADC resolution (was: Poor man's oven)
Another thing to watch out for on processor ADCs is their performance near the supply rails... the AVR ADCs are particularly entertaining below around 300 mV (with a 5V Vref).
One question for the control theory experts.
Assume me goal is to regulate temperer of an aluminum block to within 0.1C,
how good must my ADC be? Is an effective 6-bits good enough?
It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits. This is 1 part in 64. If I scale the input to the
ADC such that it os 1.0C from 0 to 63 counts then each cunt is 1/64 C
which is about 6 times better then my allowed error of 0.6 C.
My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits. Im
pretty sure I can get 8-bits by over sampling and filtering.
I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors. I thing I want them "a couple orders
of magnitude" smaller then the allied temperature range.
Of cose one could buy the best ADC on the market. But this is POOR MAN's
project. So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?
BTW, a related story. I'm on another couple lists that deal with vacuum
tube audio. We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter. So a fun project was proposed. Set a budget
of $200 to build a tube based stereo Hi Fi amplifier. Who can do the
best. Youhade to publish the BOM with prices and suppliers. Extra points
if you came in under budget. This eliminated all the suggestions to buy
high end hand made transformers from Sweden.
IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.
On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims holrum@hotmail.com wrote:
Another thing to watch out for on processor ADCs is their performance near
the supply rails... the AVR ADCs are particularly entertaining below
around 300 mV (with a 5V Vref).
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.
--
Chris Albertson
Redondo Beach, California
Hi
There is a gotcha with the initial assumption: You want the loop to be
quiet at a level well below 0.1C. If it is bouncing around that much,
the second order (rate defendant) tempco of a normal crystal will
become a pretty major issue.
Simple rule of thumb - add at least two bits past whatever the target is.
More or less, if you are after 0.1C and that comes out to 6 bits, you need
eight solid bits to get things to work properly.
Bob
On Jun 7, 2017, at 2:10 PM, Chris Albertson albertson.chris@gmail.com wrote:
One question for the control theory experts.
Assume me goal is to regulate temperer of an aluminum block to within 0.1C,
how good must my ADC be? Is an effective 6-bits good enough?
It seems to me the problem with fewer bits is only quantization noise.
Lets assume 6-bits. This is 1 part in 64. If I scale the input to the
ADC such that it os 1.0C from 0 to 63 counts then each cunt is 1/64 C
which is about 6 times better then my allowed error of 0.6 C.
My gut-feel is that this is marginal but could work ("work" is defined as
holds temperature within the range) but I'd be happier using 8 bits. Im
pretty sure I can get 8-bits by over sampling and filtering.
I don't know how to analyze this but I'm guessing with n-bits each each
sample has a 1/2 bit error so my I and D terms in the PID controller will
accumulate lots of 1/2 bit errors. I thing I want them "a couple orders
of magnitude" smaller then the allied temperature range.
Of cose one could buy the best ADC on the market. But this is POOR MAN's
project. So he asks, "What is the lowers performance/cost part that will
allow the system to meet its specification?
BTW, a related story. I'm on another couple lists that deal with vacuum
tube audio. We see the same things there people correctlypointing out how
to make something better but the question is always how much better and at
what cost an does it matter. So a fun project was proposed. Set a budget
of $200 to build a tube based stereo Hi Fi amplifier. Who can do the
best. Youhade to publish the BOM with prices and suppliers. Extra points
if you came in under budget. This eliminated all the suggestions to buy
high end hand made transformers from Sweden.
IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.
On Tue, Jun 6, 2017 at 2:39 PM, Mark Sims holrum@hotmail.com wrote:
Another thing to watch out for on processor ADCs is their performance near
the supply rails... the AVR ADCs are particularly entertaining below
around 300 mV (with a 5V Vref).
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.
--
Chris Albertson
Redondo Beach, California
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.
Chris wrote:
Assume me goal is to regulate temperer of an aluminum block to within
0.1C, how good must my ADC be? Is an effective 6-bits good enough?
* * *If I scale the input to the ADC such that it os 1.0C from 0 to 63
counts then each cunt is 1/64 C which is about 6 times better then my
allowed error of 0.6 C.
The required number of bits depends on the range it represents -- it is
all about scale, as your second paragraph above seems to recognize.
You are right, 6 bits is enough to represent a 1C span to the nearest
1/64C. But that is not the whole answer. This representation will be
mapped to 64 steps of oven output. If the oven has very little power,
these can be reasonably fine steps -- but recovery from large errors
will be painfully slow. On the other hand, if the oven has normal-ish
power, the steps will be much too large to control the oven temperature
finely.
You need both sufficient range and sufficient resolution to accomplish
the task at hand. In reality, 6 bits will be both too coarse for fine
control and will also have insufficient range. Experience has shown
that 12 bits (actual, not nominal) is barely to not really enough to do
the job with real crystal ovens if you want fine temperature control
over a useful range of temperatures.
Consider digital audio. 6 bits is plenty to digitize audio to the
nearest 30uV (typical LSB value for CD-quality audio) -- BUT, only over
a 1.9mV range. Actual CD-quality audio requires one to maintain this
same resolution over a range of 2V, which requires 16 bits.
Best regards,
Charles
On Wed, 7 Jun 2017 15:25:38 -0400
Charles Steinmetz csteinmetz@yandex.com wrote:
Chris wrote:
Assume me goal is to regulate temperer of an aluminum block to within
0.1C, how good must my ADC be? Is an effective 6-bits good enough?
* * *If I scale the input to the ADC such that it os 1.0C from 0 to 63
counts then each cunt is 1/64 C which is about 6 times better then my
allowed error of 0.6 C.
The required number of bits depends on the range it represents -- it is
all about scale, as your second paragraph above seems to recognize.
You are right, 6 bits is enough to represent a 1C span to the nearest
1/64C. But that is not the whole answer. This representation will be
mapped to 64 steps of oven output. If the oven has very little power,
these can be reasonably fine steps -- but recovery from large errors
will be painfully slow. On the other hand, if the oven has normal-ish
power, the steps will be much too large to control the oven temperature
finely.
Addedum to what Charles wrote:
If you want to build a temperature control for something similar
like an quartz oven, just get one of the modern delta-sigma ADCs.
You'll pay €10 for one, but it's really a hassle free way to
precisely measure temperature. As most of these have a large
number of channels, you can measure multiple sensors as well
at no additional cost (beside the thermistor).
Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven.
Such a design allows to have low temperature gradients within the flask
(due to the metal walls). The outer oven allows to optimize the inner
oven for stability without the need to deal with large temperature ranges.
And all together it is still quite cheap.
One drawback, though, is the large size of the flask. But for a hobby
project that does not need to fit into another product, this is fine.
Attila Kinali
--
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
On 7 June 2017 at 21:40, Attila Kinali attila@kinali.ch wrote:
Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven.
Might there be a problem that a thermos flask is TOO well insulated, and
even the minimal heat generated in the electronics would be more than could
escape through the vacuum, and so the unit would cook?
Hi
On Jun 7, 2017, at 4:40 PM, Attila Kinali attila@kinali.ch wrote:
On Wed, 7 Jun 2017 15:25:38 -0400
Charles Steinmetz csteinmetz@yandex.com wrote:
Chris wrote:
Assume me goal is to regulate temperer of an aluminum block to within
0.1C, how good must my ADC be? Is an effective 6-bits good enough?
* * *If I scale the input to the ADC such that it os 1.0C from 0 to 63
counts then each cunt is 1/64 C which is about 6 times better then my
allowed error of 0.6 C.
The required number of bits depends on the range it represents -- it is
all about scale, as your second paragraph above seems to recognize.
You are right, 6 bits is enough to represent a 1C span to the nearest
1/64C. But that is not the whole answer. This representation will be
mapped to 64 steps of oven output. If the oven has very little power,
these can be reasonably fine steps -- but recovery from large errors
will be painfully slow. On the other hand, if the oven has normal-ish
power, the steps will be much too large to control the oven temperature
finely.
Addedum to what Charles wrote:
If you want to build a temperature control for something similar
like an quartz oven, just get one of the modern delta-sigma ADCs.
You'll pay €10 for one, but it's really a hassle free way to
precisely measure temperature. As most of these have a large
number of channels, you can measure multiple sensors as well
at no additional cost (beside the thermistor).
Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven.
Such a design allows to have low temperature gradients within the flask
(due to the metal walls). The outer oven allows to optimize the inner
oven for stability without the need to deal with large temperature ranges.
And all together it is still quite cheap.
I certainly do not disagree with the idea of using a thermos or a cheap eBay
dewar in a home brew OCXO project.
It’s not all a free lunch. You still have issues. One somewhat counter intuitive one
is the gradient / cooling issue. The same super duper insulation that helps
you also hurts you. Your internal circuitry will have a finite power requirement.
Yes it can be pretty low, but it’s still there. You have very high thermal resistance.
Even small power sources will generate noticeable temperature rise(s).
No that observation is not original to me. It’s been passed down over the
generations of OCXO designers. I’d love to identify just who passed it on.
Unfortunately it’s been way to long for me to remember exactly who.
Bob
One drawback, though, is the large size of the flask. But for a hobby
project that does not need to fit into another product, this is fine.
Attila Kinali
--
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.
On Wed, 7 Jun 2017 22:20:50 +0100
Peter Vince petervince1952@gmail.com wrote:
Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven.
Might there be a problem that a thermos flask is TOO well insulated, and
even the minimal heat generated in the electronics would be more than could
escape through the vacuum, and so the unit would cook?
That's why the top of the flask is closed with an aluminium plate.
Assuming the plate on the top is around 5x5cm, that should give a
thermal resistance in the order of 20-40°C/W. That's not low, but
should be enough to support 100-400mW of heat generated inside
with a moderate temperature differential. If it turns out not to
be enough, making the plate bigger and attaching a few fins, or
having a small fan blowing lightly on it should solve the problem.
Attila Kinali
--
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
Hi
Consider that a lot of the heat flow is through the glass wall of the
vacuum gizmo. You want to tie your heater(s) to it in order to create
an iso-thermal “wall”. Often this is done by gluing the whole assembly
together.
In addition, you probably also want a foam plug in the end
of the beast to keep from creating an even more non-uniform (= very
directional) heat flow. Yes, the directivity issue is inherent in the design
approach. It’s not clear that you want to exaggerate it ….
Bob
On Jun 7, 2017, at 6:03 PM, Attila Kinali attila@kinali.ch wrote:
On Wed, 7 Jun 2017 22:20:50 +0100
Peter Vince petervince1952@gmail.com wrote:
Additionally: if I would set out to build my own OCXO today, I would
go and buy one of those lunch thermos flasks to house everything. Their
isolation is higher than anything you can easily build yourself,
especially at that size. I would place the (inner) oven at the bottom,
probably using a puck design similar to the E1938, place the electronics
on top of it and close the lid using an aluminium plate which forms
the outer oven.
Might there be a problem that a thermos flask is TOO well insulated, and
even the minimal heat generated in the electronics would be more than could
escape through the vacuum, and so the unit would cook?
That's why the top of the flask is closed with an aluminium plate.
Assuming the plate on the top is around 5x5cm, that should give a
thermal resistance in the order of 20-40°C/W. That's not low, but
should be enough to support 100-400mW of heat generated inside
with a moderate temperature differential. If it turns out not to
be enough, making the plate bigger and attaching a few fins, or
having a small fan blowing lightly on it should solve the problem.
Attila Kinali
--
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.
For under $20 you can get a OCXO with Si5351A multi output oscillator
chip from QRP-Labs.
http://www.qrp-labs.com/ocxokit.html
No idea just how good it is but this link gives insight into how it was
developed. Designed for WSPR radio transmissions. GPS discipline can
be added.
http://www.hanssummers.com/ocxosynth
Cheers,
Will
On 06/08/2017 06:10 AM, Chris Albertson wrote:
<snip>IT turrets out that you see MUCH more interesting designs when you lower
the budget. Anyone can make a high performance system even enough money.
They waste half the cost on useless stuff and the product costs double what
it should and is over complex but is works real, really well. That's
easy. Harder and more interesting is "Can you make something just as good
at 1/2 the price?" Answer is usually Yes. Then you say "what much do you
loose if I set the price to 1/4? The answer is surprisingly little if you
get smart about sourcing parts. Turns out about $180 is the minimum
for pretty decent quality HiFi vacuum tube.
An interesting graph would be Oven Specification vs. Price. What is the
minimum cost for keeping temperature to within 1.0 C, for 0.1C, 0.01 C?
Can you do 1.0C for under $5? or 0.1C for under $10. I bet yes.
I did an exercise a while back to see what is the minimum price and
complexity to build a GPSDO that was good enough only to drive the lab
bench instruments I have. I implements only 1/2 od Lars W's design and
cut his lines of code by about 90%. Turns outhe cost is the XO and about
$10. Compared to my Thunderbolt, performance was not nearly as good but
the ratio of performance over parts cost might be better.