In a message dated 2008/02/15, Brett3mr@............. writes:

> >>Numerical integration looks interesting.  What I think I need to make it
> >>work is a D/A with something like 24-bit resolution and correspondingly 
> low
> >>noise.  Haven't looked too hard, and haven't found any.
> >
> >        There are some about.
> 
> Any suggestions as to what manufacturers to check?

       Texas, Burr-Brown They do 20 bit single channel, DAC1220E. Most of the 
24 bit ones seem to be audio codecs. I don't know what their error or 
stability are like.

> >        The STS-2 goes to this. Particular versions of the STS-1 would go 
> > out to 1,000 seconds. It is a very hard way to get this performance!
> 
> Given the fundamental noise issues in any vertical, I think it's the only 
> way.

       Only because you are using a particular feedback loop method involving 
velocity feedback. If you measure the position and relate the movement 
digitally, you should not have problems over period.

> >        There is a lot of earth noise down to the Eigenmodes, which are 
> > interesting in themselves. Transient signals occur which look very like 
> > quake precursors.
> 
> Those transients worry me just a little.

       What worries you about them? I find the prospect both interesting and 
exciting! The crust of the Earth is being continually cyclically flexed by the 
Sun and the Moon. It would be very surprising if there were NO transients! 
You also get Earth Hum.

> >>Not sure how you are proposing to reduce the restoring force.  If you are
> >>suggesting feedback, it actually doesn't act in that way.
> >
> >    Positive feedback does and it will increase the period.
> 
> Sounds like an oscillator to me.

       Then you may be misunderstanding something. There is no reason why you 
should not provide a weaker spring / seek to reduce the force required to 
move the mass by feedback. You can certainly get an oscillator this way, but only 
if you generate a net positive force, not reduce a negative one.

> >        You are using a position sensor, which will have a measurement 
> > range and a noise level which limits what you can sense. I am enquiring 
> > what resolution you can get. The practical limit is likely to be set 
> > above this by thermal variations.
> 
> Using the same C/D device, a little better than the SDC, maybe 5-10x the 
> displacement sensitivity depending on the plate size, so 5-10x S/N.  I 
> have been scratching my head as to how to characterize C/D quantization 
> noise relative to feedback.  I'm sure as you apply feedback, reducing the 
> sensitivity, the displacement corresponding to one C/D step also reduces, 
> so S/N from that source shouldn't get worse.  I need to think about this 
> more.

       If you apply strong feedback, the detector will not 'know' anything 
about it, but the mass movement for a given quake amplitude will be decreased. 
You will be requiring increased resolution and this conflicts with stability / 
drift. 
       You can fit fixed capacitors to the system and then measure the actual 
output noise that you get. This is what we did with Barzilai's circuit and 
the sine wave circuit. I don't see how you can calculate it. The digital method 
is inferior due to it's sensitivity to tiny variations in timing.

> >> >      A capacitative position sensor system can have a very high
> >> > linearity. What other system nonlinearities were you considering that
> >> > could be relevant?
> >>
> >>Primarily the position sensor system.  That would include, of course, the
> >>C/D converter as well as the capacitor.  When you say very high linearity
> >>are you implying 1%, 0.1%, 0.01%....?  Have any measurements been made?
> >
> >     My guesstimate would be in the 0.1% region, but probably better. It 
> will 
> > depend mostly on the precision of the physical sensor construction. The 
> > linearity over a small range will be extreme.
> 
> OK, sometime I'll play around with numbers in that range and see what 
> happens.

       Regards,

       Chris   
In a me=
ssage dated 2008/02/15, Brett3mr@............. writes:



>>Numerical integration l=
ooks interesting.  What I think I need to make it

>>work is a D/A with something like 24-bit resolution and correspondin=
gly low

>>noise.  Haven't looked too hard, and haven't found any.

>

>        There are some about.



Any suggestions as to what manufacturers to check?




       Texas, Burr-Brown They do 20 bit singl=
e channel, DAC1220E. Most of the 24 bit ones seem to be audio codecs. I don'=
t know what their error or stability are like.



>    &nb=
sp;   The STS-2 goes to this. Particular versions of the STS-1 wou=
ld go 

> out to 1,000 seconds. It is a very hard way to get this performance!


Given the fundamental noise issues in any vertical, I think it's the only wa=
y.




       Only because you are using a particula=
r feedback loop method involving velocity feedback. If you measure the posit=
ion and relate the movement digitally, you should not have problems over per=
iod.



>    &nb=
sp;   There is a lot of earth noise down to the Eigenmodes, which=20=
are 

> interesting in themselves. Transient signals occur which look very like=
 

> quake precursors.



Those transients worry me just a little.




       What worries you about them? I find th=
e prospect both interesting and exciting! The crust of the Earth is being co=
ntinually cyclically flexed by the Sun and the Moon. It would be very surpri=
sing if there were NO transients! You also get Earth Hum.



>>Not sure how you are pr=
oposing to reduce the restoring force.  If you are

>>suggesting feedback, it actually doesn't act in that way.

>

>    Positive feedback does and it will increase the perio=
d.



Sounds like an oscillator to me.




       Then you may be misunderstanding somet=
hing. There is no reason why you should not provide a weaker spring / seek t=
o reduce the force required to move the mass by feedback. You can certainly=20=
get an oscillator this way, but only if you generate a net positive force, n=
ot reduce a negative one.



>    &nb=
sp;   You are using a position sensor, which will have a measureme=
nt 

> range and a noise level which limits what you can sense. I am enquiring=
 

> what resolution you can get. The practical limit is likely to be set 
> above this by thermal variations.



Using the same C/D device, a little better than the SDC, maybe 5-10x the 
displacement sensitivity depending on the plate size, so 5-10x S/N.  I=20=


have been scratching my head as to how to characterize C/D quantization 

noise relative to feedback.  I'm sure as you apply feedback, reducing t=
he 

sensitivity, the displacement corresponding to one C/D step also reduces, 
so S/N from that source shouldn't get worse.  I need to think about thi=
s more.




       If you apply strong feedback, the dete=
ctor will not 'know' anything about it, but the mass movement for a given qu=
ake amplitude will be decreased. You will be requiring increased resolution=20=
and this conflicts with stability / drift. 

       You can fit fixed capacitors to the sys=
tem and then measure the actual output noise that you get. This is what we d=
id with Barzilai's circuit and the sine wave circuit. I don't see how you ca=
n calculate it. The digital method is inferior due to it's sensitivity to ti=
ny variations in timing.



>> >   =
   A capacitative position sensor system can have a very high

>> > linearity. What other system nonlinearities were you consideri=
ng that

>> > could be relevant?

>>

>>Primarily the position sensor system.  That would include, of c=
ourse, the

>>C/D converter as well as the capacitor.  When you say very high=
 linearity

>>are you implying 1%, 0.1%, 0.01%....?  Have any measurements be=
en made?

>

>     My guesstimate would be in the 0.1% region, but=
 probably better. It will 

> depend mostly on the precision of the physical sensor construction. The=
 

> linearity over a small range will be extreme.



OK, sometime I'll play around with numbers in that range and see what happen=
s.




       Regards,



       Chris