Hi Jim,

On 06/24/2013 01:03 AM, Jim Palfreyman wrote:

Using a laser that reflects and a detector, create a resonator, such
that the received signal is amplified and modulated on the laser.
The delay through the circuit needs to be calibrated, but once you have
done that, you remove it from the measured period of this oscillator,
and the remaining time will be the flight-time. Using a TIC like the
5370B you can measure this period with a fairly high resolution,
averaging and all. Recall that the flight-time will for twice the
distance. This setup isn't perfect in many senses, but you can do it.
The 3325B might come handy in calibration.

By intentionally insert extra delay (which we assume is stable and
known) you can reduce the range of frequency and hence phase shift
variation needed to be calibrated, which can increase the precision to
some point.

This is a rough method of doing it, but it may serve your needs
sufficiently good. I could probably hack this up with scraps lying
around my lab, possibly lacking the optics for the laser.

What is your actual application?

Cheers,
Magnus

Hi

Pulse -> (fast) LED
LED -> splitter
splitter -> 2 (fast) photo diodes

Start with the diodes in the same plane, calibrate out the systematic delays. Likely with a deliberate offset (coax cable length).

Move one detector and note the change in time. Distance would be highly dependent on how fast your LED is and how fast your detectors are.

Bob

On Jun 23, 2013, at 7:03 PM, Jim Palfreyman jim77742@gmail.com wrote:

Sell the 3325B, and buy a calibrated meter stick.

Jim Palfreyman

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My actual application is as a quick cool demo showing what I can do with
this gear in my garage when people go "why"?

:-)

On 24 June 2013 09:22, Magnus Danielson magnus@rubidium.dyndns.org wrote:

On 06/24/2013 01:59 AM, Jim Palfreyman wrote:

Ah... :-)

You need to prepare yourself to answer the question "Why don't you just
use a measuring stick?" :-)

Cheers,
Magnus

Jim,

Yes, it makes a very nice demo (I did this as an experiment in college, using a hp 5245L).

Set up LED/laser diode, mirror (or other optics), and photo detector so that you create an oscillator (it will be many MHz); each pulse received generates one pulse out. Measure the frequency. Then simply move the mirror by a few dX cm. Again measure the frequency. From this you can calculate c.

The beauty of this method is that only numbers you need are dF (=F1-F2) and dX. Within reason, all the rest of the factors cancel out; no measurement or calibration is required.

For added fun, start with a similar pulse-echo-oscillator using sound. Almost everyone of any age knows about canyon echoes, PA system feedback, or counting seconds between lightning and thunder. So (like a trombone) move the speaker/microphone a few dX feet. Again, all you need is dF and dX to calculate the speed of sound.

The speeds differ by almost exactly a factor of a million (sound travels about one foot per millisecond; light travels about one foot per nanosecond). It should make a stunning audio & visual demo.

/tvb

----- Original Message -----
From: "Jim Palfreyman" jim77742@gmail.com
To: "Discussion of precise time and frequency measurement" time-nuts@febo.com
Sent: Sunday, June 23, 2013 4:03 PM
Subject: [time-nuts] Measuring speed of light or reproducing a metre

I think one classic way to measure the speed of light is to build a
resonant microwave cavity.  Measure the physical dimensions VERY accurately
and then measure the resonant frequency.

On Sun, Jun 23, 2013 at 4:41 PM, Bob Camp lists@rtty.us wrote:

--

Chris Albertson
Redondo Beach, California

Something a bit similar was first published by Nick Hood in 2007.

Here is a copy:

http://www.sciencebuddies.org/science-fair-projects/project_ideas/Phys_p056.
shtml

Here is Nick's website:

http://cullaloe.com/

Some people use marshmallows.

Dave

In message CALH-g5boAbvTcgtbtdr8uoAv0Mwwhg5vfBrK8K-gDJ7NdYC9Dw@mail.gmail.com
, Jim Palfreyman writes:

Run a couple of meters bare wire across your table, terminate one
end in 50 Ohm and feed the other end with a sharp square-wave.

Hook the 5370B up with two oscilloscope probes and set it to TI mode,
AVG=1000 or so.

Start with both probes the same place, SET REF on 5370B to cancel
out the differential delay of the probes.

Now slide the probes along the wire, record distance between them and
measurements, have student plot them on {white|black}board and you
have a pretty good approximation of speed of light.

Even the most battle-scarred physics-teachers will look astonished,
when you nail the speed of light down on their table like that...

However, the meter you get is not long enough, due to the
lack of vacuum and excess of copper atoms.

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