Re: 7.1Hz, how the heck did Tesla succeed?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 12:21 PM 7/16/2005, you wrote:
> Original poster: William Beaty <billb@xxxxxxxxxx>
>
> On Mon, 11 Jul 2005, Tesla list wrote:
>
> > Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
> >
> > >fact it's high (but the peak moves around randomly which screws up the
> > >measurements.)
> >
> > Modern analysis techniques can pick out sub uHz signals many 10's of dB
> > down...  Full spectrum with known capture bandwidth...   Maybe the data is
> > very old...  But "now days" they just take 0 to say 100kHz bandwidth data
> > for a few weeks and feed it to a computer....  There is not much that can
> > go wrong,
>
> Really?  :)
>
> What goes wrong is that the received signal is way down in the noise,
> therefore exotic antenna techniques become useful.

If the signal is in the noise no exotic antenna will help you. What might 
help is receiving the same signal in multiple locations so the noise is 
decorrelated between sites. The SNR can then get the sqrt(N) improvement.


> Another problem is that the bandwidth of a detector varies in inverse
> proportion to sampling time, so a narrowband signal which wanders randomly
> will be wrongly interpreted as a wideband signal .  Regardless of whether
> the detection is performed live, or via sofware w/files, if (say) you
> sample at 1Hz but only for 0.1 second, the instrument will have chopped
> the signal and therefore falsely receives it as a wide band signal.


But, that's not that was suggested. what was suggested was sampling @ 100 % 
for a very Long time.Also it's not precisely true that the length of the 
sample epoch determines the measurement resolution. This is especially true 
it apriori information is known. Example would be if you know that the 
signal being measured is a sinewave and not some arbitrary waveform.

> To
> make narrowband measurements you have to make longterm measurements.  If
> the signal frequency is changing, then you can't measure it with
> narrowband filters unless you know just how it's changing.

Sure you Can. Simple example is a phase locked loop tracking a sine wave 
that's slowly varying.

>  That's why
> spread spectrum comm is used: the frequency hopping is a *huge* problem
> unless you know the code.
>
> > and exact Fo frequency "jumping around" is not problem at all...
>
> Totally wrong because the jumping around, combined with the narrowband
> filters, will chop the signal and add a wideband artifact.  Or do you have
> an explanation for how a spread spectrum signal which is deep down in the
> noise can be easily received when you don't know the random sequence of
> frequencies?  If frequency jumps caused no problems, then spread spectrum
> transmissions would give no security at all.  It's the same physics.

Let's be careful here. There's two distinct problems you've described. The 
first is just detecting a spread spectrum signal. The other is demodulating 
it. The former can be done with radiometric detection. That is, you just 
look for the added power/ energy in the band of interest. the second is a 
bit trickier, but can be done, but not easily. Suffice it to say that 
general SS systems are not cryptographically secure.



> >  Spectral analysis techniques are extremely well known,
>
> Not true.  Sutton and Spaniol were VLF researchers at NASA, and that
> particular paper is about techniques for increasing S/N ratio at the
> receiver.  There are no magical "Spectral analysis techniques" which
> eliminate noise, therefore it becomes important to reduce noise at the
> reciever.