doppler problem
 

how accurate can we get with doppler shift when we have a digitally
modulated signal as its source?compared to a unmodulated CW signals
which has a tone frequency,whats the reliability of an FSK signal?

doppler problem
 

On 7 Mar 2006 06:04:13 -0800, "mazerom"
wrote:

how accurate can we get with doppler shift when we have a digitally
modulated signal as its source?compared to a unmodulated CW signals
which has a tone frequency,whats the reliability of an FSK signal?

Hi OM,

You've got too many factors running through this. For one, digitally
modulated is not digital data mode (FSK). There are a world of
modulation types and some are strictly derived from a bit stream (not
all bits equal in width either). Sometimes even the carriers are
digitally derived (not a sine wave).

Doppler is going to give rise to dispersion (often a term confined
these days to fiber optics, but whose derivation arose from waveguides
- LF Optics).

73's
Richard Clark, KB7QHC

doppler problem
 

exactly sir but im referring to the spectral content of the signal its
carrier and sidebands for that matter.how significant will be the
doppler shift of its spectral components other than its carrier
signal?thanks

doppler problem
 

What's the formula for Doppler shift? Why would you think it does not
apply exactly to all the spectral components of your signal? Do you
think you are dealing with a nonlinear system when you combine doppler
shift with a complex signal?

Cheers,
Tom


mazerom wrote:
exactly sir but im referring to the spectral content of the signal its
carrier and sidebands for that matter.how significant will be the
doppler shift of its spectral components other than its carrier
signal?thanks

doppler problem
 

mazerom wrote:

exactly sir but im referring to the spectral content of the signal its
carrier and sidebands for that matter.how significant will be the
doppler shift of its spectral components other than its carrier
signal?thanks


Doppler shift is applicable to all frequency components of a signal.

It is a percentage change of the base frequenc[ies] as a function of the
rate of closure or divergence between an observer and the originator of
the signal.

So, for FM it applies to the carrier frequency at ONE doppler shifted
value. It applies to the individual frequency [secondary] components as
another doppler shifted component for each frequency. In practice, the
doppler shifted frequenc[ies] among the secondary components are
extremely small when compared to the prime frequency.

For spread spectrum, it is applicable to ALL frequencies.
For AM, it is applicable to ALL frequencies.
For SSB, it is applicable to ALL frequencies.
For CW it is applicable to ALL frequencies
For FM/NBFM/WBFM, it is applicable to ALL frequencies.
For unmodulated carriers, it is applicable to ALL frequencies.
For audio, it is applicable to ALL frequencies.
For ...., it is applicable to ALL frequencies.

doppler problem
 

On 7 Mar 2006 16:29:45 -0800, "mazerom"
wrote:

.how significant will be the
doppler shift of its spectral components other than its carrier
signal?thanks

How much dispersion can you tolerate? "Significance" is an emotional
measure (which is to say it isn't a measure at all). If you cannot
quantify what you mean by this, no one can offer how significant it
is. I've a career of measuring very small differences out to 12
places. What was significant to me was a matter of utter indifference
to 99.999999% of the population - if that many.

73's
Richard Clark, KB7QHC

doppler problem
 

how can you say that seconday components are "EXTREMELY SMALL"
compared to the prime frequency when your signal is in fact a
broadband?
and you forgot one;
For UWB,it's applicable to ALLLLLL frequencies

doppler problem
 

On 8 Mar 2006 05:21:07 -0800, "mazerom"
wrote:

...any comment on this or
suggestion or objection sir?thanks

Perhaps later, first, how much dispersion can you tolerate?

73's
Richard Clark, KB7QHC

doppler problem
 

On 8 Mar 2006 07:37:07 -0800, "mazerom"
wrote:

another thing...you're not answering the question..im not asking if its
possible for two EM source to produce doppler shift when at least one
is moving relative to the other because a 5th grader knows that.
what im asking is how significant.....

dont answer if u intend to insult!

Would you mind quoting what it is you are repling to?

doppler problem
 

kb7qhc wrote, "Doppler is going to give rise to dispersion..."

Doppler alone? Care to elaborate on that?

doppler problem
 

On 8 Mar 2006 12:04:22 -0800, "K7ITM" wrote:

kb7qhc wrote, "Doppler is going to give rise to dispersion..."

Doppler alone? Care to elaborate on that?

What do you mean by doppler alone?

doppler problem
 

Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?

doppler problem
 

K7ITM wrote:

Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?


There is a small dispersion caused by the modulation scheme. The
modulating frequencies are slightly different from the carrier frequency
and therefore have slightly different doppler shift.

Restated: the Doppler shift on a 144.500 MHz carrier is different from
the Doppler shift on a 15 KHZ FM signal. How much? That's an exercise
for the student ... so sez my Prof!!

The doppler shift of 144.500 MHz is different from the doppler shift of
a 144.515 MHz sub carrier. If the FM moves the sub carrier doppler moves.

doppler problem
 

OK, Amos, I see what you wrote as a set of changes of frequency, all in
the same proportion, exactly as I'd expect from the Doppler formulas
I've seen, including relativistic ones. But how does that relate to
dispersion? Perhaps kb7qhc will offer us a definition of dispersion...

doppler problem
 

On 8 Mar 2006 12:24:13 -0800, "K7ITM" wrote:

Consider the case of an EM source moving away from you at constant
velocity, with nothing but you with your measurement system, the EM
source, and freespace. What gives rise to dispersion?

Doppler does. However, you should follow the nature of the thread in
that it is discussing a spectrum of emission, not just one frequency.
When you have more than one associated frequency, and the source(s)
are in a moving reference plane, these frequencies are all shifted by
the proportion of their frequency in relation to their speed. This
gives rise to corruption of waveshape in modulations - dispersion.

doppler problem
 

You can probably get lots of comments from users of Doppler DF antenna
systems about the performance to be expected. (There's a "fox hunting"
email list that could get you into a group with lots of
direction-finding experience quickly.) Of course, since indicated
direction depends on the detected phase of the received Doppler-derived
FM modulation, relative to the antenna "rotation," it is important to
use a detection path that maintains a very stable phase shift. It's
generally useful in practice to have a very narrow filter on the
Doppler-derived FM, so that other modulation on the signal does not
interfere with your detection. You can do that with switched-capacitor
"analog" filters, or with digital filters, locked to the
antenna-switching subsystem. Expect multipath to mess up your
indication just as it can with any other direction-finding technique.
Many practical factors affect how well your antenna system will work.
Beware of the mutual impedance among all the antenna elements: you
will probably want to insure that elements which are de-selected are
non-resonant. If you don't need very fast indication of bearing, you
may do better with a rotating directional antenna, ESPECIALLY in terms
of minimum detectable signal. If you want to know just how well this
Doppler DF scheme is going to work with a particular signal (with
perhaps broadband complex modulation), I'd suggest that it's a pretty
easy experiment to set up, either with the antenna and receiver you
plan to use, or even by generating a version of the source signal with
FM added. I have this vision of you trying to detect something that
looks like 5MHz wide random noise modulated by a 50kHz-deviation FM,
using a receiver with a 50kHz or 100kHz bandwidth, and perhaps being
very disappointed in performance.

Finally, why didn't you indicate in your original posting that it was a
particular direction-finding application that you were interested in?
That probably would have gotten to the point a bit quicker.

Cheers,
Tom

doppler problem
 

OK, so all wavelengths shift by the same ratio. In what way does that
change the "waveshape in modulations," other than to simply change the
time scale by that same ratio?

Or--does Doppler shift (alone) give rise to different propagation
velocities at different frequencies? Does Doppler shift change an
impulse to a chirp (or the right chirp to an impulse)?

doppler problem
 

On 8 Mar 2006 14:15:20 -0800, "K7ITM" wrote:

OK, so all wavelengths shift by the same ratio.

Yes, in fact they do. I mistakenly ascribed a disproportionality to
the relation of the doppler sidebands and the doppler carrier.

Net answer, no dispersion.

doppler problem
 

Thanks, Richard. I'd have been happy to learn that indeed Doppler
shift does lead to dispersion -- "any day I learn something new is a
good day." But coming to agreement was also good. Now, for the next
part of the story, Doppler shift _in_combination_with_ multipath indeed
does lead to dispersion, I believe. That's why I was careful in my
original question to say, "Doppler alone?" See for example
http://www.eleceng.adelaide.edu.au/s...grad/Yu05.html. I
get the impression that Ms. Yu is a pretty bright young woman.

Cheers,
Tom

doppler problem
 

On 8 Mar 2006 15:16:05 -0800, "K7ITM" wrote:

I'd have been happy to learn that indeed Doppler shift does lead to dispersion

Hi Tom,

I'd gotten it in my mind that the higher sideband would have increased
more in relation to the carrier than the lower sideband would have (or
versa vice) hence dispersion. Scribbling out the math in response to
your questions resolved that error.

73's
Richard Clark, KB7QHC

doppler problem
 

hi tom,
very nice and masterly suggestions and insights...thanks