Hello PSN;
for you radio buffs.

Id think you'd make the capacitive sensor as part of some kind of oscillator
circuit like (colpits or heartly), not sure which.
or something like that then you'd
make a FM modulated transmitter at some fundamental frequency.
If you can do so without violating FCC law simply
transmit that to a receiver
where it is FM demodulated
to get your trace.
No wires between sensor and receiver.
The smaller the cap the higher the freq
for any given reactance value.
You must isolate the oscillator through buffering
so loads do not change frequency.
You must tightly regulate voltage also
directly to the oscillator. And it all must
be tightly shielded.

But in the USA you most probably are limited to what
frequencies you can use less than 5 milliwatts
into the oscillator itself.

With a quarter wave whip you get maybe 7dbv
of gain so at most 5mw effective radiated power
in the vertical plane.

It is quieter in the horizontal plane but then
with a three element yagi-uda antenna
you get more gain and better directional
capabilities.
At 100MHZ ( FM BAND) the driven element
is like 4 feet 10 and some odd inches long.
The reflector is like 100% speed of light
and the director is like 90% the speed of light
and like the spacing is like 0.2 wavelength
at the driven parameters like 98.5% light speed.

You need no test equipment just a good meter
bar with mm markings.

Gamma match it and a baluns into a
50 ohm coaxial cable and to the Antenna
maybe 50 ft away.

For you science peoples its probably more
complex than this.

I have heard of flea power going around the world
if you have a very good high class receiver.
Something the NSA might use.

I was told by a special forces cheif the fastest way to
die is to play with radio transmitters
so I guess a ham license may be a must
before you try such a thing in the USA.

Or so it seems.

geoff


----- Original Message ----- 
From: "Brett Nordgren" 
To: 
Sent: Saturday, August 22, 2009 5:30 PM
Subject: Re: Real time traces


Barry,

The main problem that I saw with using inductive sensors in a feedback 
design is that their excitation frequency tended to be relatively 
low.  After demodulating there is a lot of residual 2xcarrier which needs 
to be filtered.  To filter it out adds a great amount of phase shift in the 
loop, right near the gain crossover frequency, (the instrument's 
high-frequency corner) that annoys the feedback loop no end and makes it 
difficult or impossible to crank up the loop to really do its job.  If you 
try, it oscillates.

Capacitive sensors operate at much higher frequencies and so the residue is 
much easier to filter in a way that allows the feedback loop to work really 
well without oscillating.  Ours uses a quasi-square wave drive, so the 
filtering is also less than it might otherwise have to be.  I sort of agree 
with Chris, that a sine-wave drive seems like it would work much better, 
but Dave's design really works--a lot better than I could have imagined.

Capacitive sensors can work well with very small capacitors, ours is 
roughly 48pF and 2.5" x 3",  which is probably significantly larger than 
necessary.  When I look at the Nanometrics Trillium compact, which packs 
three force-balance sensors into a can slightly over 5" high and 3.5" dia, 
I have the feeling that good capacitive sensors can be made quite small.

Regards,
Brett


At 02:39 PM 8/22/2009 -0700, you wrote:
>Brett et al
>I think this discussion has been made before but... I was trying to decide 
>the best displacement sensor style. What was the problem with inductive 
>style sensors? I agree LVDT's can have clearance issues. VRTD's - I like, 
>but someone had a problem with it but I can't remember what the reason 
>was. I use it on a small version of STM style sensor. I do get an 
>occasional low frequency oscillation which I can't nail down (~0.01 hz). I 
>tentatively attribute to maybe my triple feedback is slightly off. I would 
>use a capacitive sensor but they seem so large to get a nominal 
>capacitance value. Especially when one is trying to design a small unit.
>Barry
>

Watch our wiggles
http://bnordgren.org/seismo/gif_images.htm

or watch some very very good wiggles
http://aslwww.cr.usgs.gov/Seismic_Data/telemetry_data/ANMO_24hr.html


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