The continuing saga of the hyperbolic tangent

Ken Smith wrote:
In article <xr2dnTBmcZF_nAzfRVn-jA@adelphia.com>,
John Popelish <jpopelish@rica.net> wrote:
[...]

For now, the seismometer application is an excuse for me to explore
the world of nonlinear transfer functions and their uses. I have
built a seismometer mechanism, and am in the process of building the
amplifier and filter electronics. I happen to already have a 10 bit
A/D serial interface for a PC.


The earth seriously rolls off any frequencies above 1KHz. If you apply a
high frequency boost to the analog signal before the digitizer and a low
pass in software land, you can easily end up with more bits in the result
than the bits in the converter.

It gives you a dithering effect for free.

It seems that the Earth provides a 6 hertz rumble that is fairly
Click to expand...
independent of any earthquakes (a sort of crustal resonance). Picking
up this so called "microseism" is the test that any seismometer is
functioning. But I agree that if I can digitize faster than the 20 or
so samples needed to record the arriving waves, then I can use
multiple samples to increase my net A/D resolution.
 
John Popelish wrote:

It seems that the Earth provides a 6 hertz rumble that is fairly
independent of any earthquakes (a sort of crustal resonance). Picking
up this so called "microseism" is the test that any seismometer is
functioning. But I agree that if I can digitize faster than the 20 or
so samples needed to record the arriving waves, then I can use multiple
samples to increase my net A/D resolution.
Click to expand...
Make that 20 samples per second.
 
John Popelish wrote...
Mike Monett wrote:
Winfield Hill wrote:
John Popelish wrote...

So I am trying to come up with a way to make best use of a limited
resolution converter, as a learning experience. I may well throw
the whole thing over after a bit of play and use a 16 bit converter.

With 24-bit converters becoming commonplace, and selling for under
$10 to the big boys, why fool around with 16 bits, let alone 10?

The AD7791 claims 22 effective bits for $3.83/1k, with a very simple
serial interface. They allow two free samples:

http://www.analog.com/en/prod/0%2C2877%2CAD7791%2C00.html

The TI ADS1271 is $5.90/1k, offers high sample rate, three operating
modes, and a serial interface that can be daisy-chained. Also free
samples:

http://focus.ti.com/docs/prod/folders/print/ads1271.html
Click to expand...
I'll see your cheap-ass AD7791, ADS1271 converters, and raise you
an industry-standard high-performance AKM ak5394A or Cirrus Logic
cs5381 multi-bit delta-sigma 192kHz 24-bit two-channel converter,
the latter only $18.80 single-pieces at Newark. Or, if you insist
on cheap, cheap, go with the remnants of the older Crystal cs5360
24-bit 48kHz 2-channel converters, in stock at Newark, $8.68 each.
Or the tiny 8-pin 24-bit cs5333-BZ, $8.91. If cost is foremost,
try cs5340 24-bit "karaoke A/D" - factory-priced at $1.92 qty 10k.

These little high-performance A/Ds are amazing. For example, if
you want to make a small super-simple one-channel machine, try the
8-pin cs5511, $4.36 ea at Newark, $2.93 qty 1k (Cirrus Logic has
the datasheet). This fellow consumes a miserly 1.9mW and spews a
20-bit data stream at clock/612, e.g., 200Hz with a 122kHz clock.
Sheesh, forget 10-bit converters and an LM13700. :>)

You guys are trying to take all the fun out of this project.
Click to expand...
Your hardware project makes little sense to me, much as I liked
the good old OPAs, Gilbert-cell variants, etc. Now, it would be
interesting to take a clever high-performance 3-axis sensor with
signal conditioning plus cheap 3-channel 24-bit converters, and
add a digital tanh signal post-processing step to reduce the raw
long-term data-archiving requirements. 200 3-axis Sa/s = 18.6GSa/yr
= 75GB/yr = 750GB/decade of 32-bit words, not including time stamping
and engineering data. You could find clever algorithms to reduce
that burden significantly. And to analyze and display data.


--
Thanks,
- Win
 
In article <xPqdnU0C-tki1wzfRVn-oA@adelphia.com>,
John Popelish <jpopelish@rica.net> wrote:
[...]
It seems that the Earth provides a 6 hertz rumble that is fairly
independent of any earthquakes (a sort of crustal resonance). Picking
up this so called "microseism" is the test that any seismometer is
functioning. But I agree that if I can digitize faster than the 20 or
so samples needed to record the arriving waves, then I can use
multiple samples to increase my net A/D resolution.
Click to expand...

If you are near a road, beware that trucks make frequencies above your
10Hz Nyquist. IIRC, those soil compactors used for construction have a
peak in the 100Hz area, when seen from a distance of about 10 Meters.

If your geophone's characteristics are well damped and well known, you
should be able to correct the data to push the low end cut off down by
about a decade. A electromagnetic damping is usually built into a modern
geophone.



--
--
kensmith@rahul.net forging knowledge
 
Ken Smith wrote:

If you are near a road, beware that trucks make frequencies above your
10Hz Nyquist. IIRC, those soil compactors used for construction have a
peak in the 100Hz area, when seen from a distance of about 10 Meters.

If your geophone's characteristics are well damped and well known, you
should be able to correct the data to push the low end cut off down by
about a decade. A electromagnetic damping is usually built into a modern
geophone.
Click to expand...
Its worse than that. One of my neighbors is a black smith, and has
just installed a big pneumatic hammer. I expect to see that signature
quite often. I am not using a geophone, but am building my own design
of a Lehman (garden gate) pendulum.
http://psn.quake.net/lehman1.gif

I will have to characterize its electrical and mechanical frequency
response, but I expect to go easily down to 0.1 Hz. That is where the
energy is for quakes arriving from the far side of the planet. I
intend to come up with a fairly constant delay bandpass design (that
includes the mechanical poles caused by pendulum resonant frequency)
that covers the range of something like 0.1 to 10 Hz. When I get into
testing the filter design, I may post some results for comment and
suggestion.
 
John Popelish wrote...
I am not using a geophone, but am building my own design of a
Lehman (garden gate) pendulum. http://psn.quake.net/lehman1.gif

I will have to characterize its electrical and mechanical frequency
response, but I expect to go easily down to 0.1 Hz. That is where
the energy is for quakes arriving from the far side of the planet. I
intend to come up with a fairly constant delay bandpass design (that
includes the mechanical poles caused by pendulum resonant frequency)
that covers the range of something like 0.1 to 10 Hz. When I get
into testing the filter design, I may post some results for comment
and suggestion.
Click to expand...
I have a suggestion that'll simplify digitizing the data, and
give you frequency response down to DC in the process: Replace
the pickup coil with differential capacitance plates and use an
Analog Devices AD7746 24-bit capacitance-to-digital converter.
http://www.analog.com/en/prod/0%2C2877%2CAD7746%2C00.html



--
Thanks,
- Win
 
Winfield Hill wrote:
John Popelish wrote...

I am not using a geophone, but am building my own design of a
Lehman (garden gate) pendulum. http://psn.quake.net/lehman1.gif

I will have to characterize its electrical and mechanical frequency
response, but I expect to go easily down to 0.1 Hz. That is where
the energy is for quakes arriving from the far side of the planet. I
intend to come up with a fairly constant delay bandpass design (that
includes the mechanical poles caused by pendulum resonant frequency)
that covers the range of something like 0.1 to 10 Hz. When I get
into testing the filter design, I may post some results for comment
and suggestion.


I have a suggestion that'll simplify digitizing the data, and
give you frequency response down to DC in the process: Replace
the pickup coil with differential capacitance plates and use an
Analog Devices AD7746 24-bit capacitance-to-digital converter.
http://www.analog.com/en/prod/0%2C2877%2CAD7746%2C00.html
Click to expand...
I expect to eventually add a differential photometer output to the
pendulum to make it a tilt meter. I want to operate both sensors on
the same mechanism to compare their noise performance.
 
John Popelish wrote...
Winfield Hill wrote:
John Popelish wrote...

I am not using a geophone, but am building my own design of a
Lehman (garden gate) pendulum. http://psn.quake.net/lehman1.gif

I have a suggestion that'll simplify digitizing the data, and
give you frequency response down to DC in the process: Replace
the pickup coil with differential capacitance plates and use an
Analog Devices AD7746 24-bit capacitance-to-digital converter.
http://www.analog.com/en/prod/0%2C2877%2CAD7746%2C00.html

I expect to eventually add a differential photometer output to the
pendulum to make it a tilt meter. I want to operate both sensors
on the same mechanism to compare their noise performance.
Click to expand...
Check out Analog Devices' differential capacitance approach,
you'll find it can wipe the pants of a differential photometer.


--
Thanks,
- Win
 
Winfield Hill wrote:
John Popelish wrote...
(snip)
I expect to eventually add a differential photometer output to the
pendulum to make it a tilt meter. I want to operate both sensors
on the same mechanism to compare their noise performance.


Check out Analog Devices' differential capacitance approach,
you'll find it can wipe the pants of a differential photometer.
Click to expand...
Thanks. I'll take a look. The only charm of the photometer approach
is that an LED, a couple photo diodes and opamps are all that is needed.
 
In case anyone is interested in this continuing exploration of
arbitrary function generation, I discovered, today, that I get about
half as much ripple in the hyperbolic gain function that I am trying
to generate (equal % change in signal causes equal increment in
output) over the same range, if I cascade the two tanh generators
instead of paralleling them.

I am scratching my head to think of other ways to combine these two
tanh functions that might work even better.
 
On Sat, 21 May 2005 22:41:48 -0400, John Popelish <jpopelish@rica.net>
wrote:

Jim Thompson wrote:

Simple temperature compensation (with a PTAT) ONLY forces the same
slope (at zero) over temperature. The full-scale varies. The full
TANH compensation CAN be done on-chip, but it's a bitch to do with
off-the-shelf components.

Temperature control is probably the simplest solution, although I can
envision a possibility using thermistors.

I've just been commissioned to do a log IF (at 600MHz)... maybe that
task will provide the impetus to develop some methodology to flatten
the FS portion.

I will be watching if you come up with anything to share.
Click to expand...
Turns out to be pretty simple with a PTC thermistor, see....

Newsgroups: alt.binaries.schematics.electronic
Subject: TANH Curve Generation - TANH-Compressor.pdf
Message-ID: <p0q491dop3i15hhbs2e8b7d2k61t7hl2rh@4ax.com>

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
 
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