[time-nuts] Anybody have suggestions for time related science fair projects?

Adrian Godwin artgodwin at gmail.com
Sat May 12 20:01:15 EDT 2018


How about a demonstration of how GPS works, substituting sound waves for
radio ?

Maybe three sound sources with harmonically-related frequencies, then
measure their phase difference on an oscilloscope.

Cheat a bit : you don't need to do cdma acquisition. Have one reference at
a low frequency, switch two more on alternately at a higher frequency.
Measure the phase difference between one pair at a time and calculate your
location relative to the stationary sources.


On Sun, May 13, 2018 at 12:31 AM, Dana Whitlow <k8yumdoober at gmail.com>
wrote:

> Indeed; however, with single mode fiber the limit is not too bad.  At
> Arecibo we routinely ran bandwidths in
> excess of 1 GHz through fibers of about 1500 ft length with no problems.
> For the science fair project a
> bandwidth of a few MHz should suffice for lengths of, say, 500 ft.  It's
> just that I don't know how bad the
> multimode dispersion problem would be when using shorter wavelengths, and
> I'm sure not equipped to
> make any measurements at home now that I'm retired and far away from the
> observatory.
>
> Dana
>
>
> On Sat, May 12, 2018 at 5:44 PM, Bruce Griffiths <
> bruce.griffiths at xtra.co.nz
> > wrote:
>
> > Even with single mode fiber its finite group delay dispersion will likely
> > restrict the usable light source bandwidth.
> >
> > Bruce
> > > On 13 May 2018 at 03:38 Dana Whitlow <k8yumdoober at gmail.com> wrote:
> > >
> > >
> > > It may  be that a nicely-written request to Corning could yield the
> loan
> > of
> > > a big spool of fiber
> > > for the duration of a science fair project.
> > >
> > > Another alternative, perhaps easier to implement, might be an
> > > electrically-driven light modulator
> > > at the detector end.  For the source, an LED or diode laser is easy to
> > > modulate at respectable
> > > rates.  This approach should allow use of such high frequencies that an
> > > open optical path using
> > > mirrors might even suffice.
> > >
> > > Or here's an intermediate scheme:
> > > If one were to use two modulated sources (or one with a beamsplitter),
> > with
> > > one path delayed
> > > by the long(ish) fiber and the other by a minimal-length local fiber,
> > > something resembling a streak
> > > camera (implemented with a rotating mirror) might permit use of
> > > substantially higher pulse rates
> > > than with a rotating disk, without incurring the need for anything very
> > > fancy in the way of mechanics.
> > > Only the modulated source should require a reasonably accurate drive
> > > frequency- the "detector"
> > > would be essentially self-calibrating.  A small mirror, say of cm size,
> > > could probably be safely
> > > rotated at Dremel speeds approaching 500 rev/s, and if 1 mrad angular
> > > resolution is attained,
> > > this would yield a resolution of ~160 ns.  So a fiber length of 500 ft
> > > (approx 750 ns one-way delay)
> > > should yield an angular separation of nearly five "dots" between
> delayed
> > > and undelayed dots.
> > > And if the sources are modulated at a rate such that a few pulse
> > > repetitions are visible in the
> > > field of view, the scheme is self-calibrating as long as the PRF and
> the
> > > velocity factor in the fiber
> > > are known.  Probably the only precision work would be the optics
> required
> > > to focus a reasonable
> > > amount of light from the source(s) onto the two fibers., and I believe
> > this
> > > requirement could be
> > > adequately met with microscope objectives borrowed from one's school's
> > > biology lab.
> > >
> > > A fly in the ointment is that if ordinary (read, inexpensive) IR fiber
> is
> > > used at convenient visible
> > > wavelengths, propagation will occur in more than one spatial mode, with
> > > different modes propagating
> > > at different speeds.   I don't know how much of a problem this would
> > > raise.  But it may be that if
> > > tweaking of the transmitting end illumination is done, both in angle
> and
> > > transverse position, most
> > > of the propagating light could be confined to a single mode.  I speak
> of
> > > visible wavelengths simply
> > > because using these avoids the cost of electronic detectors,
> > oscilloscopes,
> > > etc, potentially saving
> > > a lot on the cost of the experiment as well as making for a more
> > satisfying
> > > presentation.
> > >
> > > Dana
> > >
> > >
> > > On Sat, May 12, 2018 at 9:45 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> > >
> > > > Hi
> > > >
> > > >
> > > >
> > > > > On May 12, 2018, at 7:01 AM, jimlux <jimlux at earthlink.net> wrote:
> > > > >
> > > > > On 5/11/18 9:08 PM, Jeff Woolsey wrote:
> > > > >> David.vanhorn wrote:
> > > > >>> Measuring the speed of light (Fizeau or Michelson method? Other
> > ways)
> > > > >>>
> > > > >>>
> > > > >>> I saw a great demo of this at the Exploratorium in SF.  They had
> a
> > > > long spool of fiber optic, a disc with holes, and a light source.
> When
> > > > static, if the light shines through the hole in the disc into the
> > fiber,
> > > > then you can see the light coming out the other end of the fiber
> > through a
> > > > different hole.   When rotating, you increase speed and the fiber
> > output
> > > > gets dimmer and dimmer till it's gone.   At that point, the light
> going
> > > > into the fiber arrives when the other end is blocked, and vice versa.
> > High
> > > > tech, but simple.
> > > > >>>
> > > > >> My favorite exhibit that we never see anymore.   IIRC it was a
> > quarter
> > > > >> mile of fiber and a green laser.  And ISTR that the disc had one
> > hole on
> > > > >> one arm and two radially on the other, but I can't remember why.
> I
> > > > >> thought that the light would pass through the same hole twice,
> once
> > on
> > > > >> the way in and on the way out when that same hole rotated 180
> > degrees to
> > > > >> the other end of the fiber.  The disk spun somewhere around 50 rps
> > (60
> > > > >> with an AC motor?).
> > > > >
> > > > >
> > > > > 1km in free space would be 6 microseconds round trip. I'm not sure
> a
> > > > disk spinning at 3600 rpm would work.  you'd need to have the "hole
> > > > spacing" be on the order of 6 microseconds - and at 100 rps (6000
> > RPM), 10
> > > > ms/rev, you'd need the sending and receiving hole 6/10000 of a rev
> > apart
> > > > (about 0.2 degrees).
> > > > >
> > > > > if you had 10 km of fiber, it would be a bit easier.
> > > >
> > > > I think the term “long fiber” in this case should really be “very
> very
> > > > long”.  Exactly how the typical student
> > > > funds the acquisition of something in the “many miles” range, I have
> no
> > > > idea.
> > > >
> > > > You could use an optical grating of some sort as your “spinning
> disk”.
> > The
> > > > end of the fiber is going to be
> > > > mighty small. The spacing on the grating could be quite tight. Where
> > you
> > > > get a circular part like that ….
> > > > again no idea.
> > > >
> > > > Bob
> > > >
> > > >
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