[time-nuts] webcam app to watch for and time stamp changes

Bill Hawkins bill at iaxs.net
Mon Mar 4 21:52:21 EST 2013


Wouldn't it be easier to glue a fruit fly to the head of a pin and
attach electrodes to its visual ganglia to detect the change in
intensity of a segment of the display?

Excuse me, just back from surgery and the anesthetic may have
lingering effects. Discussions like this help to pass the time.

Bill Hawkins


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Sunday, March 03, 2013 1:10 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] webcam app to watch for and time stamp changes

On 3/3/13 10:47 AM, Chris Albertson wrote:
> It's not an e-field antenna.  The goal is the sense the current in the
LCD.
>
> My bet is no much of anything leaks out of that RSA device or a wrist
> watch either.   You have to figure that tiny battery lasts for over a
> year and even if ALL the energy in the battery went to making RF you'd
> divide the battery energy by the battery life time to get power.
> What's have some micro watts at most.   Then you figure most of the
> bettery power really goes into heat not RF.
>

detecting femtowatts isn't really an issue for RF.. 1 fW is -120dBm, 
which is a "strong" signal in a lot of applications.  Typical FM 
receivers have sensitivities around 0.2 microvolt into 50ohms, which is 
  about a femtowatt.   -150dBm is getting tougher, but is still 24 dB 
above the kTB noise floor in a 1 Hz BW.

However, what we're looking for here is most likely a changing magnetic 
field.  It's not being "radiated" away, the energy stays in the circuit 
(in the near field) for the most part.   (question, does putting a RSA 
fob in a lossy magnetic medium make the battery go dead faster?)

So the question really comes down to how small a repetitive change in 
magnetic field can be detected?

Or if you're using an efield probe, it's sort of the same thing. You're 
not concerned about far field radiation, which will be very small (the 
"antenna" is a tiny fraction of a wavelength.

Here's a similar thing.. Say you have a twisted pair carrying a signal: 
  very little radiates away.  But if you have a small probe (E or H) you 
can put it closer to one wire of the pair than the other, and 
(potentially) detect the E or H field from the wire.








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